From: Brian Lo <dragon@csulb.edu>
Newsgroups: rec.gardens
Subject: FAQ: More on Onions II
Date: Wed, 1 Mar 1995 14:57:29 -0800
ONIONS - PEARL, SET & BOILER
Allium cepa
Onion bulb size is influenced by a number of factors. Variety, plant
density, photoperiod and temperature are several important ones.
Small bulb onions are usually produced in the Northwest by using
techniques that capitalize on the onion's response to day length, plant
density and temperature. Pearl onions are short-day varieties grown under the
Northwest's long-day summer conditions and planted at very high plant popula-
tions to produce the necessary yields. Onions for "sets" or "boiler" produc-
tion may be short, intermediate, or long-day types also grown at high popula-
tions and using planting dates that capitalize on optimum warm summer tempera-
tures at bulbing.
Technically, all onions are "long-day" plants, in that bulbing begins as
day length increases. Short-day varieties are those requiring only 10 to 12
hours of day length for bulbing to occur. These are grown in southern states
generally below the 35 th. parallel, and are not suitable for bulb production
in the northwest, except as pearl or boiler varieties, or for over-winter
production.
Temperatures and light intensity and light quality (red:far red) can
modify onion bulbing response. High temperatures and bright days can "compen-
sate" for some day length, causing onions to bulb sooner than they would
otherwise. Overcast, cool temperatures, delay bulbing. Time of bulbing is an
important factor in determining onion bulb size. Early bulbing contributes to
small bulb size, with delayed bulbing resulting in larger size.
Selecting the correct variety for your production area and understanding
the climatic and other environmental factors that affect its performance is
extremely important.
VARIETIES
Pearl or cocktail onions: Crystal Wax (also known as White Bermuda),
Eclipse (L303), Barletta (or White Pearl), White Mexican (or El Toro). All
these are short-day varieties that bulb shortly after emergence to produce the
desired bulb size at the lattitudes of Pacific Northwest production areas.
Set onions: Generally long day varieties such as Stuttgart, Yellow
Ebenezer, Yellow Rock, White Ebenezer, and Red Weathersfield are used.
Boiler onions: Small bulbs (1-1 3/4 inches) of any of the varieties
listed above may be used. When planting specifically for boilers, Southport
White Globe or the yellow types such as Stuttgart, Australian Brown or Yellow
Ebenezer are commonly used.
SEED COMPANIES
Asgrow Seed Company, POB 5038, Salinas CA 93915
Crookham Co. Caldwell, ID 83606
Ferry-Morse Seed Co. POB 4938, 555 Codoni Ave. Modesto, CA 95352
Harris Moran Seed Co. 3670 Buffalo Rd. Rochester, NY 14624
Petoseed Co. Inc. POB 4206, Saticoy, CA 93303
Sunseeds, 2320 Technology Pkwy. Hollister, CA 95023
Zenner Bros. Seed Co., Inc. 1311 S.E. Gideon St. Portland, OR 97202
SOIL TEMPERATURE
The optimum temperature range for germination is 48 to 90 F. March and
April plantings in western and central Oregon and late February to April in
eastern Oregon and the Columbia Basin.
SOILS
In western Oregon, onions are grown on peat soils or silt loams and
sandy loams. For pearl or cocktail onions, soils must be friable to allow
rapid mechanical separation of onions from the soil at harvest. Loamy sands
or similar sandy soils are best.
SEEDING
Onion seed numbers approximately 9,500 per ounce. These onions are all
direct seeded at high seeding rates using specially designed planters that
allow for uniform scattering of the seed and close spacings between rows.
This is usually done using gangs of planters mounted on multiple tool bars.
Depth of seeding has an effect on bulb shape since the onion stem plate
(the base of the onion bulb) forms at the point where the seed germinates.
Shallow planting results in flatter bulbs, while deeper seed placement results
in taller, and sometimes top-shaped bulbs.
Plant density also has an effect on onion shape, with onions tending to
be taller at high density. When round onions are desired at the high densi-
ties used for pearl onions, use varieties that tend to be flat-round in the
southern U.S. production areas where they are normally grown as fresh market
bulb onions.
All these onions are spring seeded. Onions may be seeded as soon as the
land can be made ready, but April is considered ideal. Seeding should be
completed by May l in Eastern Oregon and May 5 in Western Oregon.
Pearl onions are seeded at 80 to 110 lbs of seed per acre depending on
the size distribution of the onions desired. The lower densities provide a
higher percentage of "boiler" size onions. A Planet Jr. seeder plate number
23 to 26 will provide approximately 80-110 lbs of seed per acre on a total
broadcast basis, at a planting speed of about 3 MPH. To accomplish total
broadcast plantings, gangs of planters are mounted on several tool bars in a
staggered arrangement and 4 inch scatter shoes are used to cover the desired
bed width.
Onions for sets are seeded at about 55-75 lbs. per acre. Distribute
seed in a wide band so that plants are approximately 1/2 inch apart. Uniform
distribution of seed is important in obtaining good yields and size distribu-
tion. When planting in rows, growers in western Oregon use custom built
planting units that meter six rows at a time. These are spaced to plant 18
rows accross a 40 inch bed. These planters use metering units such as those
used in dispensing granular materials. These are fitted with flexible tubes
that direct the seed to scatter shoes mounted on Planer Jr. or similar seeder
frames.
When planting primarily for boiler onions, a seeding rate of about 20 to
30 lbs. per acre is used. Seeds are 3/4 to 1 inch apart. They may be planted
as described for pearl or set onions above.
FERTILIZER
Western Oregon Mineral Soils:
A soil test is the most accurate guide to fertilizer requirements.
Good management practices are essential if optimum fertilizer responses
are to be realized. These practices include use of recommended varieties,
selection of adapted soils, weed control, disease and insect control, good
seed bed preparation, proper seeding methods, and timely harvest.
Because of the influence of soil type, climatic conditions, and other
cultural practices, crop responses from fertilizer may not always by accurate-
ly predicted. Soil test results, field experience, and knowledge of specific
crop requirements help determine the nutrients needed and the rate of applica-
tion. The fertilizer application for onions should insure adequate levels of
all nutrients. Optimum fertilization is essential for top quality and yields.
NITROGEN (N):
N rates of 80-100 lbs/A should be broadcast and worked into the seedbed
before planting, using fertilizers such as ammonium sulfate, 16-20-0 or a
mixed fertilizer containing sulfate sulfur. Caution, when mild-flavored
onions are desired, reduce sulfur applications to the minimum recommended for
proper plant growth since sulfur applications have been shown to increase
onion pungency (see sections on "sulfur" below).
If necessary, additional nitrogen may be applied during the growing
season.
Sidedressed nitrogen applications or nitrogen applied in irrigation
water can be an effective means of providing supplemental nitrogen to the crop
during the season. DO NOT use aqua or anhydrous ammonia in sprinkler irriga-
tion.
Shallow incorporation of high rates of topdressed or broadcast fertiliz-
er N can seriously reduce onion stands.
Nitrogen liquid fertilizer formulations having herbicidal effects:
AN-20, Ammonium Thiosulfate and mixtures of these two materials can be
used in a number of crops. Monocarbamide Dihydrogensulfate (Monourea sulfuric
acid) is used in Alliums such as onion, shallots, leeks and garlic. The
amount of nitrogen per gallon of material, the rate at which it is applied,
and the phyto-toxicity of the material determines what crops these can be used
on.
Monocarbamide Dihydrogensulfate (Monourea sulfuric acid) is also sold as
an herbicide under different trade names (see below). ALWAYS FOLLOW THE LABEL
WHEN USING THIS MATERIAL AS AN HERBICIDE.
All of these materials, when applied at the proper time and manner have
the added benefit of ancillary control of many small (less than 3 inches)
broadleaf weeds such as groundsell, pineapple weed, shepherdspurse, mallow,
pigweed, hairy nightshade chickweed and many others. Poor, or no control is
obtained on lambs quarters, purslane, henbit, black nightshade, knotweed, sow
thistle, malva goosefoot and grasses.
When using these nitrogen sources in foliar applications, follow cau-
tions listed. Severe crop injury may occur if cautions are not observed.
General cautions that apply to nitrogen liquid fertilizer formulations are:
* Apply to onions only after the first true leaf is as tall as the flag
leaf and before onions are 8 inches tall.
* Do not apply within 4 days of a period of cool, cloudy or rainy
weather, or if free water is present on the leaves.
* Discontinue application if spray droplets do not roll off or "bead-
up" on the leaves. This indicates a deficiency of adequate waxy cuticle, and
may be a temporary condition due to rapid growth, cloudy or rainy weather, or
more seriously, the lack of adaptability of the variety.
* Do not apply within 4 days after a foliar pesticide has been applied,
or with any wetting agent.
* Use large nozzles to minimize misting.
AN-20 fertilizer formulation:
This is a manufactured 20-0-0 formulation (containing 20% N) that may be
applied at 50 to 70 gallons of product per acre as a foliar spray. This form-
ulation weighs 10.55 lbs per gallon and contains 2.11 lbs N/gallon of product
and would provide approximately 100 to 140 lbs N/acre (but some N is lost
through volatilization). Do not irrigate for 48 hours after application, but
irrigate then to move the fertilizer into the root zone.
When this treatment is used, subsequent nitrogen side dressings may be
reduced by a portion of the amount of N applied in the treatment.
Note: Mixtures of An-20 and Ammonium thiosulfate (usually 1:1) can be
made which will reduce the amount of nitrogen applied, provide some sulfur (if
AN-20 only would have been applied, and reducing the sulfur if only Ammonium
thiosulfate were to be used), and be as effective as either material alone.
Application rates are still held at 50 to 70 gallons of the mixture per acre.
Note: This formulation is different from a solution that could be
prepared by dissolving ammonium nitrate in water. Such a solution would not
have the same concentration or herbicidal effect.
Also, To prevent crop injury, follow the cautions listed above.
*****
Ammonium thiosulfate:
This is a manufactured 12-0-0-26 formulation (containing 12% N and 26%
sulfur) and weighs 9.33 lbs per gallon. It may be applied at 50 to 70 gallons
of product per acre as a foliar spray. This solution contains 1.12 lbs nitro-
gen and 2.4 lbs sulfur per gallon of product and would provide approximately
55 to 75 lbs N/acre (some N may be lost through volatilization). At the
application gallonage mentioned, about 150 to 168 lbs of S are also applied.
Do not irrigate for 48 hours after application, but irrigate then to move the
fertilizer into the root zone.
When this treatment is used, subsequent nitrogen side dressings may be
reduced by a portion of the amount of N applied in the treatment.
Note: Mixtures of An-20 and Ammonium thiosulfate (usually 1:1) can be
made which will reduce the amount of nitrogen applied, provide some sulfur (if
AN-20 only would have been applied, and reducing the sulfur if only Ammonium
thiosulfate were to be used), and be as effective as either material alone.
Application rates are still held at 50 to 70 gallons of the mixture per acre.
Also, To prevent crop injury, follow the cautions listed above.
*****
Monocarbamide Dihydrogensulfate (Monourea sulfuric acid) 15-0-0-16 (n-pHuric
or ENQUIK):
This product is also registered as an herbicide to be applied at 10 to
30 gallons of product per acre as a foliar spray. FOLLOW THE LABEL INST-
RUCTIONS WHEN USING IT AS A NITROGEN SOURCE OR AS AN HERBICIDE. It weighs
12.65 lb/gallon and contains 1.9 lbs of nitrogen and 2.0 lbs sulfur per gallon
of product. It is most generally used at about 15 to 20 gallons per acre pro-
viding approximately 28 to 38 lbs of N per acre (but some N is lost through
volatilization). Apply in sufficient water to get good foliar coverage
(approximately 1:1 dilution). Do not irrigate for 48 hours after application,
but irrigate then to move the fertilizer into the root zone.
When this treatment is used, subsequent nitrogen side dressings may be
reduced by a portion of the amount of N applied in the treatment.
CAUTION: This solution is corrosive, and can injure skin and damage
spray equipment. Use proper safety precautions and suitable spray equipment
(stainless steel).
Also, To prevent crop injury, follow the cautions listed above, and
FOLLOW ALL LABEL INSTRUCTIONS.
*****
PHOSPHORUS (P):
Phosphorus is essential for vigorous early growth of seedlings.
If the OSU soil test Apply this amount of
for P reads (ppm): phosphate (P2 O5) (lbs/A):
0-40 170-200
40-100 140-170
over 100 100-140
POTASSIUM (K):
All K should be applied and worked into the seedbed before planting.
Onions require medium levels of available soil potassium. The K soil test is
based on K present in the surface soil (0-12").
If the OSU soil test Apply this amount of
for K reads (ppm): potash (k2 o) (lbs/A)
0-100 150-200
100-200 100-150
200-350 50-100
Over 350 0-50
MAGNESIUM (Mg):
When the soil test value is below 1.0 meq Mg/100g, broadcast 20 to 25 lb
Mg/A and work into the seedbed before planting. If deficiency symptoms
appear, spray with 10 lb Epsom salts in 100 gal water/A.
Mg can also be supplied in dolomite, which is a liming material and
reduces soil acidity to about the same degree as ground limestone. Dolomite
should be mixed into the seed bed in the fall.
BORON (B):
No consistent responses of onions to the application of B have been
observed in western Oregon. Where the soil test for B is below 0.25 ppm
growers may wish to make a trial application of 2 lb B/A. Too much B fertili-
zer can be highly toxic and suggested rates of B application should not be
exceeded. B should be evenly distributed over the field and not banded.
ZINC (Zn):
The application of Zn has increased the yield of onions where soils have
been deficient. When the soil test for Zn is below 2 ppm broadcast 10 lb Zn/A
before planting or include 3-4 lb Zn/A in the fertilizer band. A broadcast
application of 10 lb Zn/A should supply Zn needs for 2 or 3 years.
OTHER NUTRIENTS:
Responses of onions to nutrients other than those discussed in this
guide have not been observed in western Oregon.
LIME:
Onions are classified as being "very sensitive to soil acidity" when
grown on mineral soils. Lime applications should be considered when mineral
soil pH is 6.0 or below. Lime should be mixed into the soil in the fall for
best results. Keep mineral soil pH between 6.0 and 6.8.
For acid soils low in magnesium (Mg) (less than 1.0 meq Mg/100 g of
soil) 1 Ton/A of dolomite lime can be used as a Mg source. Dolomite and
ground limestone have about the same ability to neutralize soil acidity.
Western Oregon Muck Soils:
A soil test is the most accurate guide to fertilizer requirements.
Good management practices are essential if optimum fertilizer responses
are to be realized. These practices include use of recommended varieties,
selection of adapted soils, weed control, disease and insect control, good
seed bed preparation, proper seeding methods, and timely harvest.
Because of the influence of soil type, climatic conditions, and other
cultural practices, crop responses from fertilizer may not always by accurate-
ly predicted. Soil test results, field experience, and knowledge of specific
crop requirements help determine the nutrients needed and the rate of applica-
tion. The fertilizer application for onions should insure adequate levels of
all nutrients. Optimum fertilization is essential for top quality and yields.
NITROGEN (N):
See also the comments in the section "western Oregon mineral soils -
nitrogen regarding "liquid nitrogen fertilizer solutions having herbicidal
effects" as you consider nitrogen applications on western Oregon muck soils.
N rates of 60-80 lbs/A should be broadcast and worked into the seedbed
before planting, using fertilizers such as ammonium sulfate, 16-20-0 or a
mixed fertilizer containing sulfate sulfur. Caution, when mild-flavored
onions are desired, reduce sulfur applications to the minimum recommended for
proper plant growth since sulfur applications have been shown to increase
onion pungency (see section on "sulfur" below).
If necessary, additional nitrogen may be applied during the growing
season.
Refer also to the section above "western Oregon mineral soils -
nitrogen" regarding "liquid nitrogen fertilizer solutions having herbicidal
effects" as you consider nitrogen fertilization.
PHOSPHORUS (P):
P is essential for vigorous early growth of seedlings.
If the OSU soil test Apply this amount of
for P reads (ppm): phosphate (P2 O5) (lbs/A):
0-40 120-150
40-100 80-120
over 100 60-80
POTASSIUM (K):
K should be applied and worked into the seedbed before planting. Onions
require medium levels of available soil potassium. The K soil test is based
on K present in the surface soil (0-12").
If the OSU soil test Apply this amount of
for K reads (ppm): potash (k2 O) (lbs/A):
0-100 150-200
100-200 100-150
200-350 50-100
Over 350 0-50
MAGNESIUM (Mg):
When the soil test value is below 1.0 meq Mg/100g, broadcast 20 to 25 lb
Mg/A and work into the seedbed before planting. If deficiency symptoms
appear, spray with 10 lb Epsom salts in 100 gal water/A.
Mg can also be supplied in dolomite, which is a liming material and
reduces soil acidity to about the same degree as ground limestone. Dolomite
should be mixed into the seed bed in the fall.
BORON (B):
No consistent responses of onions to the application of B have been
observed in western Oregon. Where the soil test for B is below 0.25 ppm
growers may wish to make a trial application of 2 lb B/A. Too much B fertili-
zer can be highly toxic and suggested rates of B application should not be
exceeded. B should be evenly distributed over the field and not banded.
ZINC (Zn):
The application of Zn has increased the yield of onions where soils have
been deficient. When the soil test for Zn is below 2 ppm broadcast 10 lb Zn/A
before planting or include 3-4 lb Zn/A in the fertilizer band. A broadcast
application of 10 lb Zn/A should supply Zn needs for 2 or 3 years.
COPPER (Cu):
Muck soils tend to be deficient in Cu and "fix" Cu. If Cu has not been
applied for 3 years , apply 15-25 lb Cu/A to the soil before planting, OR two
foliar applications of 1-2 lb Cu in 100 gal water /A may be applied to onion
leaves. Soil Cu applications need not be repeated every year. The best indi-
cator for copper is leaf analysis. Consider copper applications when leaf
copper levels are below 4 ppm.
OTHER NUTRIENTS:
Responses of onions to nutrients other than those discussed in this
guide have not been observed in western Oregon.
LIME:
Experimental work on the muck soils of western Oregon has shown that
onions will produce good yields over a fairly wide range of muck soil acidity.
Lime applications should be considered when the soil pH is 5.5 or below. Keep
muck soil pH between 5.6 and 6.0.
The liming rate is based on 100-score lime. A lime application is ef-
fective over several years.
If the OSU soil test Apply this amount
for pH reads: of lime (Tons/A):
below 5.0 4-5
5.0-5.5 2-4
over 5.5 0
Lime should be mixed into the soil in the fall.
For acid soils low in magnesium (Mg) (less than 1.0 meq Mg/100 g of
soil) 1 Ton/A of dolomite lime can be used as a Mg source. Dolomite and
ground limestone have about the same ability to neutralize soil acidity.
Do not apply lime when the muck soil pH is above 6.0. Decreased yields
have resulted in Willamette Valley experiments when liming raised the soil pH
to 6.5.
Eastern Oregon Mineral Soils:
NITROGEN:
See also the comments in the section "western Oregon mineral soils -
nitrogen regarding "liquid nitrogen fertilizer solutions having herbicidal
effects" as you consider nitrogen applications on eastern Oregon mineral
soils.
Onions require a good supply of available N however excess N applica-
tions can result in late maturity and large necks that are difficult to cure,
soft onion bulbs, and poor storage quality.
Sidedressed nitrogen applications or nitrogen applied in irrigation
water can be an effective means of providing supplemental nitrogen to the crop
during the season. DO NOT use aqua or anhydrous ammonia in sprinkler irriga-
tion.
Shallow incorporation of high rates of topdressed or broadcast fertiliz-
er N can seriously reduce onion stands.
Excessive nitrogen available to onions, particularly later in the season
(mid-July), can delay onion maturity and make field curing of onions more
difficult.
The amount of N fertilizer required depends on the nitrogen carry-over
from the previous crop and the amount and type of residue to be plowed under.
The amount of residual N in the soil varies considerably. A soil test
for nitrate-N (NO3-N) helps in evaluation the N carry-over from the previous
crops in the case of mineral soils with organic matter content below 2%.
Soil samples for NO3-N should be taken in the spring prior to the appli-
cation of N fertilizer.
Soil samples should be taken from the 0-12 inch soil depth.
OSU soil test results for N are reported in ppm. One ppm N in a 12 inch
depth of soil equals about 4 lbs N/acre.
Nitrate-N Soil Test (ppm)
(in 12 inch depth) N application (lbs/A)
0 150
10 125
20 100
30 75
over 30 75
PHOSPHOROUS (P):
Onions require relatively high levels of available P. The P soil test
is based on P present in the surface soil (0-12").
If the OSU soil test Apply this amount of
for P reads (ppm): phosphate (P2 O5) (lbs/A):
0-5 170-200
5-10 140-170
10-15 110-140
15-20 80-110
Over 20 50-80
POTASSIUM (K):
Onions require medium levels of available soil potassium. The K soil
test is based on K present in the surface soil (0-12").
If the OSU soil test Apply this amount of
for K reads (ppm): potash (k2 o) (lbs/A)
0-100 150-200
100-200 100-150
200-350 50-100
Over 350 0-50
SULFUR (S):
Onions require relatively high levels of available S. Sulfur fertilizer
requirements will vary with soil texture, leaching losses, and S content of
the soil. S is frequently contained in fertilizers used to supply other
nutrients such as N, P, and K and may be present in irrigation water, which
can be tested for S content.
Plants absorb S in the form of sulfate. Fertilizer materials supply S
in the form of sulfate and elemental S. Elemental S must convert to sulfate
in the soil before the S becomes available to plants. The conversion of
elemental S to sulfate is usually rapid for fine ground (less than 40 mesh)
material in warm, moist soil.
Elemental S should be applied the year preceding the crop, using finely
ground material. A soil test for sulfate-S is not recommended if elemental S
was applied the preceding year.
If S levels in the soil are not known, apply 40-60 lbs S/A. Caution,
when mild-flavored onions are desired, reduce sulfur applications to the
minimum rate recommended for proper plant growth since sulfur applications
have been shown to increase onion pungency. Onion pungency is also strongly
dependent on the variety being used.
Elemental S is a strong acidifier. S in the sulfate form can be applied
at planting time.
ZINC (Zn):
An application of Zn is suggested when the Zn soil test value is below
0.8 ppm.
Where Zn is required, either 10 lb/A of Zn should be broadcast and
worked into the soil prior to planting or 3-4 lb/A of Zn should be banded with
the fertilizer at planting time. An application of 10 lb Zn/A should supply
Zn needs for 2 or 3 years.
To correct Zn deficiency during the growing season, thoroughly wet
plants with a solution containing 1 lb Zn in 50-100 gallons of water.
Banding an acidifying fertilizer such as ammonium sulfate can reduce Zn
deficiency. Zn deficiency is most likely to occur on calcareous soils where
land leveling has exposed a calcareous subsoil.
OTHER NUTRIENTS:
Responses of onions to other nutrients and lime have not been observed
on mineral soils in eastern Oregon.
IRRIGATION
Onions are shallow-rooted, and require a constant supply of moisture.
Maintaining moisture near the surface, at the onion stemplate is important in
root generation. Onion roots generate at the stemplate only when moisture is
present. Proper moisture management is important in alleviating pink root
problems, general root health, and therefore bulb growth vigor.
Excessive irrigation causes nitrogen movement beyond the root system and
reduces nitrogen available to the plants.
Soil type does not affect the amount of total water needed, but does
dictate frequency of water application. Lighter soils need more frequent
water applications, but less water applied per application.
HARVESTING, HANDLING, CURING, AND STORAGE
Although small in size, yields of pearl onions can be about 20 tons/acre
when the correct plant population and other cultural practices are realized.
Harvesting begins after tops have fallen and dried thoroughly. Beds of
pearl onions intended for processing are topped using flail choppers or mowers
just prior (about 24 hours) to digging the onions. This allows the tips to
dry yet does not allow the onions to green or scald. In western Oregon, "set"
onions may be harvested and temporarily stored without removing the tops.
Tops are removed prior to packaging. When tops are not to be removed, "set"
onions are undercut and allowed to cure in the field. Undercutting us
accomplished in such a way as to lay the tops over the windrow to protect the
onions from sunscald. "Set" onions must not have been treated with sprout
inhibitors commonly used in bulb onion production.
When pearl onions are harvested for processing, they are undercut to
remove as much of the root system as possible and harvested using specialized
equipment designed specifically for pearl onion production. Harvesters lift
the entire bed and convey the onions over closely spaced rod conveyers to bins
or bulk trucks for transport to the processing plant.
Onions intended for storage should be adequately cured in the field, in
open sheds, or by artificial means before or in storage. Adequate curing in
the field or in open sheds may require 2 to 4 weeks, depending on the weather.
The best temperatures are 75 to 90 F.
STORAGE (modified from USDA Ag. Handbook #66):
Pearl, set and boiler onions may be held in either common or cold
storage. Storage quality is influenced by cultivar and by the conditions
under which they are grown.
Onions are damaged by freezing, the damage appearing as water soaked
scales when the thawed onions are cut. Onions only slightly frozen may
recover with little perceptible injury if allowed to thaw slowly and without
handling.
Store at 32 F and 65 to 70 % relative humidity. The temperature and
humidity requirements for onion sets, used primarily as planting stock for
early green onions, as well as pearl and boiler onions are essentially the
same as those for large dry onions. Because of their small size, these onions
tend to pack closely, so they should not be placed into deep piles. They are
usually held in ventilated storage in shallow, slatted trays rather than in
bags or crates.
Set onions are handled in mesh or Kraft paper bags for marketing. Low
relative humidity and low temperature are important to keep the sets sound and
dormant and free from sprouting and rooting. At humidities much above 70 %
and at warmer temperatures (40 to 50 F), more of the sets will sprout, develop
roots and decay. Onion sets should be stacked to allow good air circulation.
A storage life of 6 to 8 months is possible for good quality sets.
Onions should not be stored with other products that tend to absorb
odors. They may be stored with garlic.
Set onions are handled in mesh or Kraft paper bags for marketing. Low
relative humidity and low temperature are important to keep the sets sound and
dormant and free from sprouting and rooting. At humidities much above 70 %
and at warmer temperatures (40 to 50 F), more of the sets will sprout, develop
roots and decay. Onion sets should be stacked to allow good air circulation.
A storage life of 6 to 8 months is possible for good quality sets.
Onions should not be stored with other products that tend to absorb
odors. They may be stored with garlic.
PEST CONTROL FOR ONIONS, PEARL, SET & BOILER
THE PESTICIDES LISTED BELOW, TAKEN FROM THE PACIFIC NORTHWEST PEST
CONTROL HANDBOOKS, ARE FOR INFORMATION ONLY, AND ARE REVISED ONLY ANNUALLY.
BECAUSE OF CONSTANTLY CHANGING LABELS, LAWS, AND REGULATIONS, OREGON STATE
UNIVERSITY CAN ASSUME NO LIABILITY FOR THE CONSEQUENCES OF USE OF CHEMICALS
SUGGESTED HERE. IN ALL CASES, READ AND FOLLOW THE DIRECTIONS AND PRECAUTION-
ARY STATEMENTS ON THE SPECIFIC PESTICIDE PRODUCT LABEL.
USE PESTICIDES SAFELY!
Wear protective clothing and safety devices as recommended on the label.
Bathe or shower after each use.
Read the pesticide label--even if you've used the pesticide before. Follow
closely the instructions on the label (and any other directions you have).
Be cautious when you apply pesticides. Know your legal responsibility as a
pesticide applicator. You may be liable for injury or damage resulting from
pesticide use.
WEED CONTROL
Cultivate as often as necessary when weeds are small. Proper cultiva-
tion, field selection and rotations can reduce or eliminate the need for
chemical weed control.
THE WEED TREATMENTS LISTED BELOW ARE TAKEN FROM THE PACIFIC NORTHWEST
WEED CONTROL HANDBOOK, AND ARE FOR INFORMATION ONLY. CONSULT PRODUCT LABELS
FOR CURRENTLY LEGAL REGISTRATIONS, RATES, AND COMPLETE INSTRUCTIONS.
WEED MANAGEMENT GENERAL STRATEGY
Onions are a noncompetitive crop with short, vertical leaves that fail to form
a canopy that suppresses weeds. Weeds are managed with selective herbicides
combined with frequent cultivation. Winter cover crops or crop rotations
enhance both soil and weed management.
______________________________________________________________________________
STALE SEEDBEDS FOR VEGETABLE PRODUCTION
Stale seedbeds are sometimes used for vegetable production when other selec-
tive weed control practices are limited or unavailable. The concept depends
on controlling the first flush of emerged weeds before planting or emergence
of the crop followed by minimal soil disturbance which reduces subsequent weed
flushes during establishment of the crop. Following establishment, other weed
control practices including cultivation or use of other herbicides can be
used.
1. Prepare a seedbed, preferably 2 to 3 weeks before planting to achieve
maximum weed seed germination near the soil surface. Soil temperatures
and moisture must be reasonable or results will be erratic.
2. Plant the crop with minimum soil disturbance. Otherwise, new weed seed
will be exposed to favorable germinating conditions.
3. Just before or after planting but before crop emergence, treat the
field by flaming or with herbicide to kill all germinated or exposed
weeds.
Weed Treatment Active Ingredient Per Acre and Application Comments
------------------------------------------------------------------------------
Flaming
Use liquid propane or similar fuel-burning equipment to quickly sear the
weeds, but not consume them with flame.
------------------------------------------------------------------------
paraquat 0.625 to 0.94 lb ai/A
Gramoxone Extra
(see remarks)
Restricted-use pesticide. Do not ingest or inhale spray mist. Wearing
protective face shields, respirators, and clothing is advised. Apply
during or after planting, but before crop emergence. Add a nonionic
surfactant or crop oil concentrate according to label specifications,
taking care to avoid anionic formulations that react in the tank to form
insoluble precipitates. Exposed crop plants will be killed. Do not
apply preplant or preemergence to soils lacking clay minerals, such as
peat, muck, pure sand, or artificial planting media. (Acts as contact;
absorbs energy produced by photosynthesis forming peroxides that disrupt
living cells.)
------------------------------------------------------------------------
glyphosate Consult Labels
Roundup
Roundup RT
(E. Washington and
Idaho only)
or Honcho +
nonionic surfactant
Apply to actively growing weeds either preplant in conjunction with site
preparation or preemergence crops listed on the label. Consult label
for specific rates, times of application, and perennial species control-
led. Generally 1 to 1.5 lbs ai/A should be used for annual weeds, while
perennial weeds require higher rates. Addition of a surfactant or
ammonium sulfate improves activity when weeds are stressed. Do not
apply more than 8 lbs. ai/A per year, nor plant nonlabeled crops within
12 months following application. (Inhibits production of 3 amino acids
and protein synthesis.)
______________________________________________________________________________
ONIONS--MINERAL SOILS
Weed Treatment Active Ingredient Per Acre and Application Comments
------------------------------------------------------------------------------
bensulide 3 to 6 lb ai/A
Prefar
(spring planted
bulb onions only)
Idaho, Oregon and Washington special local needs (SLN-ID840011 and
OR840041 and WA880015) only. Apply 3 lb rate in fall in 10-12 inch
band and bed-up by throwing soil from furrows over the sprayed band, or
broadcast 6 lb rate and incorporate before bedding up in the fall. In
spring, drag-off bed tops with spiketooth harrow being careful not to
expose soil beneath the treated layer. Plant in center of beds.
(Inhibits roots of seedlings).
------------------------------------------------------------------------
DCPA 6 to 9 lb ai/A
Dacthal
Apply after planting to moist soil surface before weeds and onions
emerge. In eastern Oregon when rainfall is sparse, shallow incorpora-
tion using a nailboard has improved weed control. Results are improved
with overhead irrigation immediately following application. Consult
label for planting sensitive crops within 8 months. (Inhibits mitosis).
------------------------------------------------------------------------
oxyfluorfen 0.12 to 0.25 lb ai/A
Goal
Causes skin irritation--Wear protective gear. Apply postemergence to
onions with 2 fully developed true leaves while broadleaf weeds have
less than 4 true leaves. Multiple treatments may be required as new
weeds emerge, but do not exceed 0.5 lb ai/A. Use a clean sprayer and do
not mix with oils, surfactants, or other agricultural chemicals. Do not
apply to onions under any stress. Slight leaf curl may occur and young
plants sometimes become prostrate on the soil surface for 1 to 2 days,
but yields have not been reduced. Do not apply within 60 days of
harvest. (Acts as a selective contact that disrupts cell membranes).
Special local needs (SLNOR-910026 and WA-910012) allows irrigation
through sprinkler irrigation systems. Read and follow label instruc-
tions precisely regarding irrigation interval and all chemigation
specifications to minimize environmental hazards.
------------------------------------------------------------------------
monocarbamide 10 to 30 gal product/A
dihydrogensulfate
Enquik
Apply in sufficient water volume for full coverage when first true leaf
is as tall as flag leaf. Repeat applications may be necessary. Do
not apply in presence of free moisture or within 5 days after a foliar
pesticide application. Repeat as required until crop is 8 inches tall;
then direct sprays between rows. Do not apply to green onions after
they reach 8 inches to avoid scarring. Adjust fertilizer rates for
nitrogen and sulfur. (Acts as contact).
------------------------------------------------------------------------
bromoxynil 0.25 to 0.38 lb ai/A
Buctril
Do not ingest or inhale spray mist and prevent contact with skin.
Wearing protective face shields, respirators, and clothing is advised.
For use in Idaho and east of the Cascades in Oregon and Washington
only. Apply in 50 to 70 gal water for thorough coverage when onions
have 2 to 5 true leaves. Apply on sunny days when plants are dry and
humidity is low. Do not apply west of the Cascades or when the protec-
tive waxy cuticle on onion leaves may be thin or damaged. Application
can be preemergence but irrigation or rainfall within 2 days of applica-
tion may result in crop injury. Do not add adjuvants. Injury can some
times occur even under ideal conditions. (Precise action unknown; acts
as a selective contact that disrupts cell membranes).
------------------------------------------------------------------------
paraquat 0.625 to 0.94 lb ai/A
Gramoxone Extra
A restricted-use pesticide. Do not ingest or inhale spray mist.
Wearing protective face shields, respirators, and clothing is advised.
Apply before, during, or after planting, but preemergence to crop to
control emerged weed seedlings only. Add a nonionic surfactant or crop
oil concentrate according to label specifications, taking care to avoid
anionic formulations that react in the tank to form insoluble precipi-
tates. Preharvest interval is 60 days. (Acts as contact; absorbs energy
produced by photosynthesis forming peroxides that disrupt living cells.)
------------------------------------------------------------------------
sethoxydim 0.19 to 0.28 lb ai/A
Poast
Identify susceptible grasses and apply at optimum growth stage listed on
the label. Add 2 pints/A nonphytotoxic crop oil concentrate to improve
leaf absorption. Erratic control often occurs when grasses are stunted
or stressed from drought, high temperatures, or low fertility. Resis-
tant grasses include annual bluegrass and all fine fescues, whereas
quackgrass can be suppressed. Do not apply within 30 days of harvest,
nor exceed 4.5 pints/A per season. (Inhibits fatty acid production,
cell membranes, and new growth.)
______________________________________________________________________________
ONIONS--MUCK OR PEAT SOILS
Weed Treatment Active Ingredient Per Acre and Application Comments
------------------------------------------------------------------------------
oxyfluorfen 0.12 to 0.25 lb ai/A
Goal
Causes skin irritation--Wear protective gear. Apply postemergence to
onions with 2 fully developed true leaves while broadleaf weeds have
less than 4 true leaves. Multiple treatments may be required as new
weeds emerge, but do not exceed 0.5 lb ai/A. Use a clean sprayer and do
not mix with oils, surfactants, or other agricultural chemicals. Do not
apply to onions under any stress. Slight leaf curl may occur and young
plants sometimes become prostrate on the soil surface for 1 to 2 days,
but yields have not been reduced. Do not apply within 60 days of
harvest. (Acts as a selective contact that disrupts cell membranes).
------------------------------------------------------------------------
monocarbamide 10 to 30 gal product/A
dihydrogensulfate
Enquik
Oregon special local needs (SNL-OR850029) label. Apply in sufficient
water volume for full coverage when first true leaf is as tall as flag
leaf. Repeat applications may be necessary. Do not apply in presence
of free moisture, nor within 3 days after a foliar pesticide ap-
plication. Do not apply to green onions after they reach 8 inches to
avoid scarring. Adjust fertilizer rates for nitrogen and sulfur.
(Acts as contact).
------------------------------------------------------------------------
sethoxydim 0.19 to 0.28 lb ai/A
Poast
Identify susceptible grasses and apply at optimum growth stage listed on
the label. Add 2 pints/A nonphytotoxic crop oil concentrate to improve
leaf absorption. Erratic control often occurs when grasses are stunted
or stressed from drought, high temperatures, or low fertility. Resistant
grasses include annual bluegrass and all fine fescues, whereas quack-
grass can be suppressed. Do not apply within 30 days of harvest, nor
exceed 4.5 pints/A per season. (Inhibits fatty acid production, cell
membranes, and new growth.)
------------------------------------------------------------------------
paraquat 0.625 to 0.94 lb ai/A
Gramoxone Extra
A restricted-use pesticide. Do not ingest or inhale spray mist.
Wearing protective faceshields, respirators, and clothing is advised.
Apply before, during, or after planting, but preemergence to crop to
control emerged weed seedlings only. Add a nonionic surfactant or crop
oil concentrate according to label specifications, taking care to avoid
anionic formulations that react in the tank to form insoluble precipi-
tates. Preharvest interval is 60 days. (Acts as contact; absorbs energy
produced by photosynthesis forming peroxides that disrupt living cells.)
______________________________________________________________________________
ONIONS-SPROUT INHIBITION
Weed Treatment Active Ingredients Per Acre and Application Comments
------------------------------------------------------------------------------
maleic hydrazide 2 lb ai/A
Royal MH-30
Apply when bulbs are fully with soft necks and 5 to 8 green leaves, or
when approximately 50% of the tops have fallen, but are still green.
Should be applied at temperatures below 80 to 85 F to avoid crystaliz-
ation on leaf surfaces. Use of a spray adjuvant is suggested in arid
regions west of the Rocky Mountains. Avoid early sprays before maturity
to reduce spongy onions. Do not treat seed onions.
______________________________________________________________________________
______________________________________________________________________________
INSECT CONTROL
Proper rotations and field selection can minimize problems with insects.
THE PESTICIDES LISTED BELOW ARE TAKEN FROM THE PACIFIC NORTHWEST INSECT
CONTROL HANDBOOK, AND ARE FOR INFORMATION ONLY. CONSULT PRODUCT LABELS ARE
CURRENTLY LEGAL REGISTRATIONS, RATES AND COMPLETE INSTRUCTIONS.
Insect and Description Control, Active Ingredient Per Acre
---------------------------------------------------------------------------
Cutworms and Armyworms Lannate - 0.45 to 0.9 lb
Different species
Bacillus thuringiensis - 1 to 2
Small to large larvae that chew qt. Use a spreader-sticker to
holes in bulbs and leaves. enhance control.
Phosdrin - 0.25 to 0.5 lb
Climbing cutworms only.
permethrin - 0.1 to 0.3 lb
---------------------------------------------------------------------------
Brown wheat mite
Petrobia latens
Small, light tan to brown active
mite. Damages leaves.
---------------------------------------------------------------------------
Bulb mites soil fumigation
Rhyzoglyphus sp.
Small, white mites capable of causing
damage to developing bulbs and intro-
ducing pathogenic fungi. Damage can
continue in storage.
--------------------------------------------------------------------------
Onion thrips azinphosmethyl -
Thrips tabaci 0.5 to 0.75 lb
Adults are small, pale yellow to malathion - 1 to 2 lb
light brown in color with feath-
ery wings. Young are pale yel- Phosdrin - 0.5 lb
lowish-green and wingless.
They feed on onion foliage and diazinon - 0.5 lb
cause it to become white, kill-
ing the tops and reducing Penncap-M - 0.5 lb Apply
yields. only to dry onions. Do not
use on green onions.
WARNING: Microencapsulated methyl
parathion (Penncap-M) should never methyl parathion - 0.5 to
be sprayed on blooming weeds in 0.83 lb
onion fields or adjacent fields.
If misused, this formulation is permethrin - 0.15 to 0.3 lb
especially hazardous to honey bees
and other pollinating insects be- Metasystox-R - 0.75 lb Use
cause of its strong tendency to only on Spanish bulb onions.
contaminate pollen.
Vydate L - 0.25 to 0.5 lb
Note: Dasanit applied in the seed Do not harvest tops.
furrow for onion maggot control sup-
presses thrips populations in the Lannate - 0.45 lb
early season. This may allow
growers to reduce foliar sprays.
Oregon, Idaho, and Washington only,
24 c. Caution: Dasanit may reduce
onion stands in sandy/light soils.
---------------------------------------------------------------------------
Onion maggot diazinon - 2 to 4 lb
Delia antiqua
Dyfonate - 1 lb Use Dyfonate
Adult is a fly, pale to dark gray only where organic matter
in color, less than inch in exceeds 10 percent.
size. Larvae are legless, blunt
white maggots that kill seed- diazinon - 1 lb
lings and damage bulbs of Furrow applications only.
older plants.
malathion - 1.5 to 2 lb
Lorsban 4E - 0.034 lb ai/1000
row ft. Apply as in-furrow
treatment on direct-seeded dry
bulb onions only.
Lorsban 15G - 0.034 lb ai/1000
row ft. Apply as in-furrow
treatment on direct-seeded dry
bulb onions only.
---------------------------------------------------------------------------
Pea leafminer diazinon - 0.5 lb
Liriomyza huidobrensis
Larvae of tiny fly. Mine in
leaves. When abundant, cause
leaves to wither from tip downward.
---------------------------------------------------------------------------
Wireworms diazinon - 3 to 4 lb
Limonius spp.
Telone II - preplant
Brown, jointed larvae of click
beetles. Kill young plants, Telone C-17 - preplant
weaken older ones.
______________________________________________________________________________
______________________________________________________________________________
DISEASE CONTROL
Proper rotations, field selection, sanitation, spacings, fertilizer and
irrigation practices can reduce the risk of many diseases. Fields can be
tested for presence of harmful nematodes. Using seed from reputable seed
sources reduces risk from seed born diseases.
THE PESTICIDES LISTED BELOW ARE TAKEN FROM THE PACIFIC NORTHWEST DISEASE
CONTROL HANDBOOK, AND ARE FOR INFORMATION ONLY. CONSULT PRODUCT LABELS FOR
CURRENTLY LEGAL REGISTRATIONS, RATES AND COMPLETE INSTRUCTIONS.
ONION -- BASAL ROT
Cause: Fusarium sp., soilborne fungi.
Symptoms: Yellowing, wilting, and lack of vigorous growth. Red-brown rot where
roots are attached to the base. The rot and discoloration usually affect all
of the base and upwards into the scales. Sometimes a weft of mycelium
develops over the surface of the base or between the affected scales. Rot may
progress in storage under situations where elevated moisture levels exist.
Infection is often associated with pink root, maggots, or other insect injury.
~Control:
1. Protect plants from insect, fertilizer, or other injury.
2. Rotate crops use well-drained soil.
3. Some soil chemicals have been tried but at present none are being recom-
mended.
In a test in Malheur County, the following varieties had less basal rot when
compared to a number of other varieties: `Golden Cascade', `Cima', `Oro
Grande', and `Cashe'. Other onion varieties described as tolerant but not
tested in Oregon are `Bronze Reserve', `North Star', `Sassy Brassy', and
`Sentinel'.
---------------------------------------------------------------------------
ONION -- DOWNY MILDEW
Cause: Peronospora destructor, a fungus. This organism survives the winter on
infected plants or on old onion plant material in the field.
Symptoms: Bluish, downy spots appear on the leaves. Plants may be dwarfed,
distorted, and pale green. Spots develop first with a water-soaked appearance,
later taking on a purplish shade. In dry climates, the leaf spots may be white
in appearance. As the spots enlarge, often girdling the leaves, a whitish mold
develops on the surface of the spot, later turning black. Secondary leaves may
develop on infected plants, retarding normal bulb development. Plants are not
often killed, but bulb quality is poor and often spongy.
~Control:
1. Avoid poorly-drained soils.
2. Practice a 3-year crop rotation where possible.
3. Destroy onion plant refuse and cull piles.
4. Eradicate perennial onions or wild onions.
5. There are no resistant varieties of the types grown in Oregon.
6. Spray with one of the following:
a. Mancozeb (dry onions only) at 1.6 to 2.4 lb ai/100 gal/A.
b. Maneb at 1.6 to 2.4 lb ai/A.
c. Ridomil MZ58 at 1.5 to 2 lb product/A.
d. Ridomil/Bravo 8l W at 1.5 to 2 lb product/A.
e. Fosetyl-Al (Aliette) at 1.6 to 2.4 lb ai/A.
f. Fixed copper.
Residues:
Mancozeb - Tolerance 0.5 ppm. Do not apply within 7 days of harvest.
Do not apply to exposed bulbs.
Maneb - Do not apply more than 16.8 lb ai/A/season. Do not apply within
7 days of harvest.
Fosetyl-Al -- Do not apply within 7 days of harvest.
Ridomil/Bravo 81W - Do not apply to dry bulb onions within 7 days of
harvest or to green onions within 21 days of harvest or more
than 4 times on green onions. See product label for rota-
tion restrictions.
---------------------------------------------------------------------------
ONION -- NECK ROT
Cause: Botrytis allii, a fungus. This organism survives the winter on dead
plant parts in the soil and on infected bulbs. The most common causes for
severe losses are (1) use of excessive amounts of nitrogen which delay
maturity of the crops, (2) irrigation and/or rainfall late in the season, (3)
inadequate or improper curing, and (4) improper storage.
Symptoms: The organism appears on maturing bulbs just prior to harvest and
during harvest, with infection occurring through the neck tissue or through
wounds in the bulbs. Frequently infection enters the neck as a result of
topping the onions prior to proper drying of soft susceptible top tissues. The
fungus moves rapidly into the succulent onion bulb in the neck region,
producing a water-soaked appearance. Yellow discoloration often appears.
Between the onion scales a gray mold develops, later producing small, black
bodies (sclerotia) which develop as a solid layer around the neck. Ultimately
the bulbs break down to a soft mass, and secondary infections follow.
~Control: The tops should be allowed to mature well, and the onions should then
be lifted or undercut. If normal dry weather prevails, the onions should then
cure on the ground for 6 to 10 days. Bruising and mechanical injury in topping
and storing should be kept to a minimum, and the onions should be stored in
well-ventilated houses at temperatures of 32 F or just slightly higher. Higher
temperatures should be used when humidity cannot be held below 75 percent RH.
There are no fungicides that consistantly control neck rot even though EPA has
granted several labels for this use.
---------------------------------------------------------------------------
ONION -- NEMATODE (Stubby Root)
Cause: Paratrichodorus allius. An ectoparasitic nematode that overwinters in
the soil. The largest populations of nematodes and conspicuous plant symptoms
occur during cool, damp springs.
Symptoms: Roots are extremely short with yellow to brownish cast plants are
stunted usually in spots in the field.
~Control:
1. Telone II at 4.5 to 36 gal/A (follow label recommendations). Fumigate in
the fall.
OR
2. An in-furrow spray of 1.5 to 2 gal Vydate L (broadcast basis) in 20 to 50
gal of water or .75 to 1 gal Vydate L (broadcast basis) as an in-furrow drench
in 100 to 150 gal of water are usually adequate to control the problem. In
situations with extremely high nematode presence, two or three postemergence
applications may be made. See label--Oregon only (SLN OR 810033).
Residues: Telone II. Preplant soil treatment. Seal soil for 7 to 14 days then
aerate 1 week for each 10 gal/A applied--longer in case of heavy rains or
temperatures below 60 F--until the odor of the fumigant is gone. Do not treat
extremely heavy soils. Do not apply near living plants. Considered to be a
nonfood use.
Soil fumigants. If fall fumigation is done, beds will be free of fumigant by
spring planting.
Vydate. Limit 14 days before harvest. Such late applications will not be
effective in any case.
Sampling: Stubby-root nematodes are migratory ectoparasites (see Nematodes-
Life History), and are found only in the soil. Before planting crops suscep-
tible to stubby-root nematodes, soil samples should be taken with sufficient
lead time to implement management procedures if necessary. If this is soil
fumigation, it may be several weeks before crops can be planted. Fall
sampling for crops to be planted the following spring is an excellent strate-
gy. For diagnosis, submit both soil and onions containing roots.
---------------------------------------------------------------------------
ONION -- PINK ROOT
Cause: Pyrenochaeta terrestris, a fungus. This fungus may live in the soil
saprophytically for several years.
Symptoms: The disease may appear either in young seedlings or later throughout
the growth of the onion. There is a yellowing of roots followed by a
pinkish-colored root development. These take on a bright-pink shade and die.
New roots produced thereafter become diseased and also die. As this process
continues through the growing season, the bulbs lack normal development and
are reduced in size and vigor with stunted tops.
~Control:
1. A crop rotation in which onions are planted only every 5 years will reduce
the severity of infection however, some plants will be invaded by the fungus.
2. Use adequate fertilizer, irrigation, cultivation, etc., to stimulate rapid
bulb growth.
3. Some partially resistant varieties of the sweet spanish type are avail-
able.
4. Preplant soil fumigation with Telone C-17 is effective in control of the
disease on mineral soils at the rate of 5.1 to 41 gal/A. Not effective on peat
soils. Note: Fall fumigation is more effective than spring.
5. The varieties `El Capitan', `Avalanche', `Bullseye', `Bravado', `Merit',
`Durango', `Yula', `Armada', `Valdez', and `Fiesta' were bred for resistance.
`Yellow Globe Danvers' types are resistant.
Caution: Since the fumigation must be done prior to planting, it is important
that the planting row be directly over the fumigated soil when only the
planting row is treated. Variations of a few inches can cause complete loss of
control. Not effective on peat soils.
---------------------------------------------------------------------------
ONION -- PURPLE BLOTCH
Cause: Alternaria porri, a fungus. The organism overwinters on dead, diseased
onion trash in the field and on infected bulbs. Infection often follows injury
caused by other fungi, e.g., Botrytis, or by sand or dust during windstorms.
The spores require rain or persistent dew to cause infection. The optimum
temperatures are 77o to 81o F. Almost no infection occurs below 55 F. The
disease is sometimes severe in irrigated onion growing areas of eastern
Oregon.
Symptoms: On the leaves a small, water-soaked area develops, which soon turns
brown. As the spot enlarges, zones may appear, and it turns purplish. White
margins often encircle the purplish centers. When the weather is moist, the
surface of the spot usually becomes covered with a brownish-black, powdery
fungus growth. Leaves with large spots turn yellow and are blown over by the
wind. Infection of the leaves may be noticed by the presence of leaf spots
very soon after infection has occurred, sometimes within 2 days.
Bulb decay may occur during and after harvest. It shows first as a watery rot
starting around the neck and is particularly noticeable because of the
yellowish to wine-red discoloration in the neck region. As the fungus moves
through the onion tissues, they dry and take on a papery texture toward the
outside. `Sweet Spanish' onions are very susceptible to purple blotch.
~Control: The disease does not occur in Oregon with sufficient frequency to
warrant an annual spray program. Spraying as soon as the disease occurs will
prevent further spread of the disease. The variety `Fiesta' has tolerance.
Other control practices which will reduce losses are:
1. Destroy old onion pulp piles.
2. Handle and harvest with care, destroying any diseased onions.
3. Proper curing of bulbs in field--lift and allow to dry several days before
topping.
Spray: Mancozeb (dry bulb onions only) (Dithane M-45, Manzate 200)--1.6 to 2.4
lb ai/100 gal/A.
Bravo 720 at 1.5 to 3 pt/a (2 pt maximum) on dry bulb onions in sufficient
water to cover.
Maneb at 1.6 to 2.4 lb ai/A. Do not apply more than 16.8 lb ai/A/season. Do
not apply within 7 days of harvest.
Ridomil/Bravo 8l W l.5 to 2 lb/A.
Rovral WP--1.5 lb/A in 50 to 100 gal of water.
Spray fixed copper. Several formulations and products are available. See
product labels for rates.
Tolerance: Mancozeb (dry bulb onions only)--Tolerance 0.5 ppm. Do not apply
within seven days of harvest. Do not apply to exposed bulbs.
Maneb--Tolerance 7 ppm. No time limitation.
Bravo 720--Tolerance 5 ppm on green onions, 0.5 ppm on dry bulb onions. Do not
apply within 14 days of harvest (green onions) or 7 days of harvest (dry
onions). Do not apply more than three times per season on green onions. Do not
apply to sweet spanish onions except in Idaho and Oregon.
Ridomil/Bravo - See cautions under downey mildew.
Rovral WP--Tolerance 0.5 ppm. Do not apply within 4 days of harvest.
Copper: Exempt.
---------------------------------------------------------------------------
ONIONS -- RUST (Seed Onions Only)
Cause: Puccinia asparagi, a fungus. The fungus actually survives on asparagus
which commonly grows wild along ditch banks of irrigated areas in southern
Idaho and eastern Oregon.
Symptoms: The uredial stage attacks the onion leaves and seed stalks. Light
yellow to orange or reddish, powdery pustules develop. With pustules enlarge-
ment, the leaves or seed stalks become girdled. The affected tissues at and
above the girdling turn yellow and die prematurely, thus reducing seed yield.
~Control: Destroy wild asparagus or spray the asparagus and adjoining onions
with mancozeb.
Residues: See under Onion--Purple Blotch.
---------------------------------------------------------------------------
ONION -- SEED TREATMENT
Aids in control of smut, seed decay, and damping-off.
Thiram-- or 2 t/lb or 1 lb product for 10 lb seed for smut.
---------------------------------------------------------------------------
ONION -- SMUT
Cause: Urocystis cepulae, a fungus. This fungus survives saprophytically in
the infested soil for several years. It attacks only members of the onion
family including onion, leek, and Welsh onion.
Symptoms: The disease appears first in the stems of the young plant as it
emerges. Infected areas have a raised blisterlike appearance near the ground
level. These rupture to expose black powder masses of spores. Numerous
seedlings are killed during the emergence period, and those which live produce
distorted bulbs having darkish streaks and numerous smutty lesions.
~Control:
1. Pelleted onion seed using Thiram with a methocel sticker has been used in
some areas. Such pelleted seed is applied at the rate of 5 lb/A.
2. Small onion sets and young onion plants may be set out in infested soil
without apparent danger of infection.
3. Control smut by an in-furrow drench at seeding of mancozeb or Maneb at 2.4
lb actual/A (29,000 linear feet of furrow).
---------------------------------------------------------------------------
ONION -- STINKING ROT (soft rot)
Cause: Unknown species of Pseudomonas and Erwinia.
Symptoms: First symptoms appear in late June. Either the inner or outer whorl
of leaves, but usually not both, become flaccid and rapidly change from green
to white to brown and collapse to the ground. Soon the disease progresses to
the apparently unaffected parts, with water-soaking and collapse following.
After collapse of the foliage a watery, foul-smelling liquid exudes from the
bulbs.
Further spread of the disease is reduced by warm, dry summer weather. Slightly
affected plants may recover, but usually are distorted and chlorotic. Bulbs
infected late in the season may appear, but decay during storage. The disease
does not spread from bulb to bulb in storage.
This disease has been widely distributed in western Oregon onion-growing
areas, and occurs in Oregon and Southwestern Idaho onion-growing areas. There
is a lower incidence of this disease in furrow or rill-irrigated fields.
~Control:
1. Some control can be obtained by using water from deep wells, rather than
from ditches.
2. Reduce mechanical injuries during cultivation and weeding.
3. Thoroughly cure onions before storing.
---------------------------------------------------------------------------
ONION -- WHITE ROT
Cause: Sclerotium cepivorum, a fungus. The fungus can remain viable in the
soil for many years without the presence of the host--probably as long as 8 to
10 years. Other hosts include garlic, leek, shallot, and other allium sp.
Symptoms: The leaves of a diseased plant decay at the base, turn yellow, wilt,
and topple over. The older leaves collapse first. Roots are rotted and the
plant can be easily pulled up. Fluffy mycelium may be present on the remaining
roots and bulb. Affected bulbs may become watery and the outer scales cracked
as they dry and shrink. Small (1/50 inch) sclerotia are formed in affected
parts of the bulb and on the surface, often around the neck. The sclerotia are
smaller and rounder than those of Botrytis (the cause of the neck rot dis-
ease).
~Control: Idaho law forbids the import of sets, bulbs, or other material
infected with white rot. Avoid the movement of contaminated soil or machinery
into clean fields.
Treatment of heavily infected areas of organic soils with 400 lb of methyl
bromide/A tarped has been useful in reducing inoculum in infested fields.
Spray 2 lb of Ronilan 50 WP in a 4- to 6- inch band in 400 gal. of water/A or
in a 2- to 4- inch band in 100 gal of water/A over the seed row. Apply at
least 2 more applications of 2 lb. of Ronilan 50 WP in 100 gal of water/A at
4-6 week intervals.
Residues:
Ronilan--Do not apply more than 10 lbs. Ronilan/A in one crop season. Do not
apply within 18 days of harvest. Do not apply through any type of irrigation
system.
______________________________________________________________________________
______________________________________________________________________________
---------------------------------------------------
---------------------------------------------------
This information was adapted by the Oregon State
University Extension Service from Oregon and
Pacific NW publications as well as other vegetable
production guidelines from public institutions
in the U.S. and Canada. These guidelines are
intended as a general guide for Oregon commercial
vegetable producers. Address comments and
questions to your County Extension Agent or Dr. N.
S. Mansour, Department of Horticulture, Oregon
State University, Corvallis, OR 9733l.
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