Table 20.1.1 Recommended onion varieties
Transplants		Seeded, main season
	Mars (red) (F₁)¹	Listed in approximate order of
	Renegade (F₁)	maturity
	Redwing (F₁)		Early Yellow Globe strains
	Corona (F₁)		Benny’s Red (red)
	Viceroy (F₁)		Precedent (F₁)
	Sweet Spanish types		Gazette (F₁)
			Mars (red) (F₁)
Sets			Ricochet (F₁)
	Ebenezer		Mambo (red) (F₁)
	Stuttgart		Pennant (F₁)
			Festival (F₁)
Green or Bunching			Millennium (F₁)
	Beltsville Bunching		Duration (F₁)
	Southport White Globe		Bradley (F₁)
	White Portugal		Endurance (F₁)
	White Sweet Spanish		Crusader (F₁)
			Criterion (F₁)
Seeded, early
	Early Yellow Globe strains
	Norstar (F₁)
	Arsenal (F₁)
	Hustler (F₁)

1: F₁ = hybrid

Many additional varieties are suitable for production, and new ones are being developed each year. Growers should test new varieties routinely to determine which perform best on their farms, choosing those with tolerance or resistance to diseases such as pink root and Fusarium basal rot if their fields have a history of these diseases.

20.2 Planting Methods

Onions are a cool-season crop that requires a long growing season to produce good bulb yields of high quality. In New York, most dry bulb onions for commercial production are grown on muck soils because of the advantages offered by these soils. These include a uniform water supply, early crop establishment, and ease of mechanical harvesting. Most mineral soils require irrigation to produce a good yield.

Because onion plants begin to bulb in early summer when days reach sufficient length, it is important to establish the crop early to assure that plants are large enough to develop a large bulb. The crop also must mature early enough to allow curing of the bulb before cool, wet weather arrives in the fall. Onions may be grown from seeds, sets, or transplants, but

because of economics direct seeding is the usual practice in New York. Row spacing varies considerably depending upon weed and foliar disease management, seeder capability, and harvesting equipment. In-row spacing also varies depending upon variety, desired bulb size, row spacing, soil type, and other management considerations. To ensure good quality bulbs for storage and to produce acceptable yields, plants must not be spaced too far apart in the row.

Onion yields in New York vary considerably from year to year and from field to field. Average yields are around 300 to 350 cwt per acre but can reach 500 cwt or more per acre when growing conditions are good and proper management is practiced.

20.3 Fertility

Maintain a pH of 6.0 to 6.8 on mineral soils. Consider liming muck soils when pH is 5.2 or below. See Tables 20.3.1 and 20.3.2 for the recommended rates of nitrogen, phosphorus, and potassium.

20.4 Harvesting

A high percentage of onions grown in New York are stored for marketing throughout the fall, winter, and early spring. Late, hard, pungent varieties with good skin retention are preferable for storage after January 1. Undercutting several days before harvesting can improve keeping quality. To develop best skin color, onions should be cured for two weeks at temperatures between 75° and 80°F and relative humidity of 70 to 80 percent. After curing, temperatures should be lowered gradually to as near 32°F as possible for best keeping. Rapid cooling followed by a sudden warm period might break dormancy and cause onions to sprout early. A sprout inhibitor should be applied to all onions intended for sale after mid-November. See Table 20.4.1 for the recommended sprout inhibitor rate. Apply when about 50 percent of the tops are down, bulbs are mature, necks are soft, and five to seven leaves are still green. This treatment is not recommended if Botrytis leaf blight is moderate to severe because it destroys leaf tissue before the maleic hydrazide can be absorbed.

Table 20.3.1 Recommended nutrients based on soil tests for onions grown on mineral soils.
N pounds/acre	P₂O₅ pounds/acre			K₂O pounds/acre			Comments
	Soil Phosphorus Level			Soil Potassium Level
	low	med.	high	low	med.	high
*90-120*	*150*	*100*	50	*150*	*100*	50	*Total recommended*

30-40	100	50	0	100	50	0	Broadcast and disk-in.
30-40	50	50	50	50	50	50	Apply in band at seeding or transplanting
30-40	0	0	0	0	0	0	Apply at 4-5 week stage

Table 20.3.2 Recommended nutrients based on soil tests for onions grown on muck soils.
N pounds/acre	P₂O₅ pounds/acre			K₂O pounds/acre			Comments
	Soil Phosphorus Level			Soil Potassium Level
	low	med.	high	low	med.	high
*100-125¹*	*150*	*100*	50	*150*	*100*	50	*Total recommended*

100-125	150	100	50	150	100	50	Broadcast and disk-in.

1: On deep well-drained mucks, 50 pounds per acre may be sufficient for best yields, but higher rates may accelerate seedling growth and maturity of later hybrids. Muck onions have not responded to nitrogen sidedressings except in rainy seasons.

Table 20.4.1 Sprout inhibitor compound.

Common name		Rate/A	PHI	REI	Field
	Trade name	Product	(days)	(hours)	Use EIQ	Comments
maleic hydrazide
	Royal MH-30 1.5 lb/gal	1.33 gal	—	12	36	Temperature should be below 85˚F at application or below 80˚F if temperatures are expected to 85˚F later in the day. Compatible with most insecticides and fungicides, but separate application is recommended.

20.5 Disease Management

20.5.1 Bacterial soft rot, Erwinia carotovora, sub. sp. carotovora; Slippery skin, Pseudomonas gladioli pv. allicola; Bacterial canker and Sour skin, Burkholderia cepacia

Time for concern: Any time after bulbing, in storage and transit

Key characteristics: Bacteria can spread by splashing rain and enter through natural openings and mechanical injuries. The affected tissue becomes soft, spongy, and watery, and may cause interior breakdown in the bulb. B. cepacia, a versatile bacterium found in soil and water, is the most important disease of onions. B. cepacia strains comprise a heterogeneous group. Thus, copper-based bactericides can vary in their ability to reduce bacterial populations as shown in recent tests conducted in plate cultures. See Reference 1.

*Management Option*	*Recommendation*
Resistant varieties	Avoid planting susceptible varieties such as Spanish onions.
Harvest	Minimize injuries to maturing and harvested bulbs. Undercutting. lifting, and wind rowing of bulbs is an important practice.
Postharvest	Dry onions thoroughly before storing at a temperature of 32° to 33°F and less than 70 percent relative humidity with good ventilation.

*Management Option*			*Recommendation*
Crop rotation			B. cepacia populations can be reduced with rotations, although the specific crop(s) to be used remains under investigation.
Scouting/thresholds, Site sel-ection, Seed selection/ treatment, Sanitation			These are not currently viable management options.
Compound(s)
Common name		Rate/A		PHI	REI	Field
	Trade name	Product		(days)	(hours)	Use EIQ	Comments
copper compounds
	Champ F or OLF	0.66-1 pt		0	12/24 (see label)	9-14	Label varies with manufacturer and formulation. Use of copper may reduce bacterial decay.

20.5.2 Fusarium basal rot, Fusarium oxysporum f. sp. cepae

Time for concern: Later stages of the growing season and in storage

Key characteristics: Roots of plants infected early in the growing season can decay and die; plants turn yellow and wilt. The bulbs of plants infected later in the season may appear normal, but inner neck tissue may feel soft. Portions or all of the basal plate develop a firm dry rot, turn brown, and die. Under moist conditions, the diseased scales can develop white mold. See Reference 1.

*Management Option*	*Recommendation*
Resistant varieties	Plant resistant varieties such as Eskimo, Frontier, Benchmark, Millennium, Condor, Prince, Copra, Duration, and Celtic.
Crop rotation	The disease persists as spores in the soil. Minimum three year rotation with crops other than onions.
Harvest	Avoid mechanical damage to bulbs during spray applications and harvest.
Postharvest	Keep onion storage areas dry and well ventilated.
Scouting/thresholds, Site selection, Seed selection/ treatment, and Sanitation	These are not currently viable management options.
Compound(s)	No pesticides are available to manage Fusarium basal rot.

20.5.3 Botrytis leaf blight (BLB), Botrytis squamosa

Time for concern: Mid- to late season

Key characteristics: Symptoms include browning (blighting) and early death of leaves, undersized mature bulbs, and reduced yield. The fungus initially causes white spots, 1/32 to 3/16 inch in diameter, on infected leaves. The spots are surrounded by a greenish white halo that initially appears water-soaked. The centers of the lesions usually become sunken, straw yellow, and develop a characteristic slit that is oriented lengthwise in the lesion. See References 1 and 2.

*Management Option*			*Recommendation*
Scouting/thresholds			Sample for BLB while sampling for onion thrips. It is important to sample areas bounded by trees or barley and low spots where air circulation is limited. Sample 15 plants randomly to represent most of the field, including trouble spots. For each plant, record the number of lesions on the three oldest (outer) green leaves. Use Appendix 21.1 Onion grower report form. Total the number of lesions. Do not treat if the number of lesions is less than the lower limit in Table 21.5.1. Treat if the number of lesions is greater than the upper limit. If the number falls between the two values, sample five more plants. Continue this process until 50 plants have been sampled.
Pesticide use			Follow sequential sampling plans, action thresholds, and forecasting schemes to optimize timing and reduce the number of sprays. Reducing the number of fungicide applications may postpone BLB resistance to a fungicide.
Blight Alert			In order to use the Blight Alert forecasting system, it is necessary to have a hygrothermograph near the field, a calculator programmed with the Inoculum Production Index (IPI) forecasting method, access to local National Weather Service forecasts of the chance of rainfall, and the planting date of the field. After a fungicide application is made, growers may assume one week of protection from infection under most circumstances. After the one week period has expired, it is most effective to reapply a fungicide when both of the following conditions are met: the IPI is greater than seven and the chance of rain is 30 percent or greater. If the forecast calls for no fungicide eight days after the first application, make another forecast on the ninth day. Continue this process until a fungicide application is required.
Weather forecast system			If unable to use Blight Alert, listen to the National Weather Service forecasts one week after applying a fungicide. If the prediction on Day 8 is for at least a 30 percent chance of rain anytime during the following 36 hours, the recommendation would be to spray. If not, the recommendation would be to wait until Day 9 and to check the forecast again. Continue this process until a fungicide application is required.
Resistant varieties			Sweet Spanish type onions are more likely to be affected.
Site selection			Seed fields should be located well apart from commercial onion production fields.
Postharvest			Severed onion tops should be removed from the field and destroyed.
Sanitation			To reduce incidence and severity of BLB, cull piles should be destroyed and volunteer onions should be rogued.
Crop rotation and Seed selection/treatment			These are not currently viable management options.
Compound(s)
Common name		Rate/A		PHI	REI	Field
	Trade name	Product		(days)	(hours)	Use EIQ	Comments
boscalid
	Endura DF	6.8 oz		7	12	13	For use on all varieties of onions. Do not make more than 2 applications of Endura (Group 7) before alternating to a labeled fungicide with a different mode of action.
chlorothalonil (dry bulb)
	Bravo, Equus or OLF 6 lb/gal	0.75 pt, 1-2 pt		7	12	16 22-43	See note under iprodione for use of low rate. Note Bravo eye wash requirement and reduced seasonal amounts for shortened reentry interval on label. Adjust fungicide rates and frequency of application according to disease hazard.
chlorothalonil + zinc (dry bulb)
	Bravo Zn 4.17 lb/gal	1.5-3 pt		7	12	23-46

Compound(s)
Common name		Rate/A	PHI	REI	Field
	Trade name	Product	(days)	(hours)	Use EIQ	Comments
chlorothalonil (bunching)
	Bravo, Equus or OLF 6 lb/gal	1.5-3 pt	14	12	33-65	Note eye wash requirement and reduced seasonal amounts for shortened reintry interval on label.
cyprodinil + fludioxonil
	Switch 62.5 WG	11-14 oz	7	12	11-13	Begin applications of Switch (Groups 9 +12) when conditions become favorable for disease but before infection occurs.
iprodione (dry bulb)
	Rovral 4F or OLF 4 lb/gal	0.5-1 pt	7	24	<1-5	Use at the lower rate of Rovral (Group 2) if tank-mixed. Research has shown that the effectiveness is improved when mixed with full rates of maneb or mancozeb, or chlorothalonil at half the normal rate.
maneb (dry bulb and bunching)
	Maneb 75DF 0.75 lb/lb	1.5 lb, 2-3 lb	7	24	24 32-48	See note under Rovral for use of low rate.
mancozeb (dry bulb)
	Dithane 75DF or OLF	3 lb	7	24	33	See note under Rovral for use of low rate.
mancozeb + copper hydroxide (dry bulb)
	ManKocide	2.5 lb	7	24	73
pyraclostrobin
	Cabrio EG	12 oz	7	12		Not for aerial application in NYS. For use on all varieties of onion. For suppression only. Do not make more than 2 sequential applications before alternating to a labeled fungicide with a different mode of action.
pyraclostrobin + boscalid
	Pristine 38 WDG	14.5- 18.5 oz	7	12		Not for aerial application in NYS. For use on all varieties of onion. Do not make more than 2 sequential applications of Pristine (Groups 11 + 7) before alternating to a fungicide with a different mode of action. If downy mildew occurs during the Pristine application, immediately follow this application with a downy mildew fungicide with a different mode of action.
pyrimethanil
	Scala SC or	18 fl oz	7	12	9	For both green and dry onions. Do not apply more than 54 fl oz of Scala (Group 9) per crop.
	Scala + protectant	9 fl oz	7	12

Table 25.1.1 Thresholds for Botrytis leaf blight

Cumulative number of lesions on 3 oldest leaves of each plant

Number of plants Lower limit Upper limit

15 17 73

20 28 92

25 39 111

30 50 130

35 62 148

40 74 166

45 86 184

50 99 201

20.5.4 Botrytis neck rot, Botrytis allii

Time for concern: Harvest and storage

Key characteristics: Infection is favored by cool, wet conditions and poor drying and curing of harvested onions. The neck tissue of infected bulbs may shrivel; bulb scales may become soft and brown, and a superficial gray mold may develop. This disease often affects injured onions in storage. See Reference 1.

*Management Option*			*Recommendation*
Resistant varieties			No resistant varieties are available.
Seed selection/treatment			Plant disease-free seed and transplants.
Fertilizer use			Avoid excessive and late-season applications of nitrogen fertilizer, which will delay onion senescence, promote larger necks, and encourage diseases.
Undercutting			Before topping and storing, undercut and windrow onions until inside neck tissues are dry. Leave several inches of neck on the bulb. Dry and cure well before storage to allow injuries to heal quickly. Artificial curing (forced, heated air at 93° to 95°F) for five or more days is beneficial.
Harvest			Harvest only fully mature bulbs during dry weather.
Postharvest			Sort carefully before storage to minimize losses. Do not store bruised or damaged onions or those harvested from wet areas. Store onions at 32° to 33°F and less than 70 percent relative humidity.
Sanitation			Destroy cull onions and onion debris.
Scouting/thresholds, Crop rotation, and Site selection			These are not currently viable management options.
Compound(s)
Common name		Rate/A		PHI	REI	Field
	Trade name	Product		(days)	(hours)	Use EIQ	Comments
chlorothalonil (dry bulb)
	Bravo, Equus or OLF 6 lb/gal	1-2 pt		7	12	22-43	Note eye wash requirement and reduced seasonal amounts for shortened reintry interval on label. Adjust fungicide rates and frequency of application according to disease hazard.
chlorothalonil + zinc (dry bulb)
	Bravo Zn 4.17 lb/gal	1.5-3 pt		7	12	23-46
iprodione (dry bulb)
	Rovral 4F 4 lb/gal	1-1.5 pt		7	24	7	Use at the lower rate if tank mixed. Research has shown that the effectiveness of Rovral (Group 2) is improved when mixed with full rates of maneb or mancozeb, or chlorothalonil at half the normal rate.
maneb (dry bulb and bunching)
	Maneb 75DF 0.75 lb/lb	2-3 lb		7	24	32-48
mancozeb (dry bulb)
	Dithane 75DF or OLF	3 lb		7	24	33

Compound(s)
Common name		Rate/A	PHI	REI	Field
	Trade name	Product	(days)	(hours)	Use EIQ	Comments
pyraclostrobin + boscalid
	##Pristine 38WG	14.5-18.5 oz	7	12		## 2(ee) Begin application of Pristine (Groups 11 + 7) prior to onset of disease development and continue on a 14-day interval. If applications intervals shorter than 14 days are needed, rotate to another fungicide with a different mode of action. Use the higher rate when disease pressure is high. No aerial application in New York.
pyrimethanil
	Scala SC or	18 fl oz	7	12	9	For green and dry bulb onions. Do not apply more than 54 fl oz of Scala (Group 9) per crop.
	Scala + protectant	9 fl oz	7	12

20.5.5 Damping-off, Pythium spp.

Time for concern: Seedling stage

Key characteristics: Seeds may rot and fail to germinate or newly emerged seedlings may rot at the soil line and wilt or collapse. The roots and shoots of surviving seedlings may have a brown rot and fail to thrive. See Reference 1

*Management Option*			*Recommendation*
Scouting/thresholds			Record the occurrence and severity of damping-off. No thresholds have been established.
Resistant varieties			Avoid susceptible varieties such as Spanish onions.
Seed selection/treatment			Plant disease-free seed treated with an appropriate fungicide.
Transplants			Grow transplants in steam-treated soil or soilless mix to ensure freedom from disease.
Crop rotation, Site selection, Postharvest, and Sanitation			These are not currently viable management options.
Compound(s)
Common name		Rate/A		PHI	REI	Field
	Trade name	Product		(days)	(hours)	Use EIQ	Comments
mefenoxam
	Ridomil Gold EC or OLF	0.5-1 pt		—	48	7-14	Application may be made PPI or as a soil surface spray after planting.

20.5.6 Downy mildew, Peronospora destructor

Time for concern: Mid- to late July through the end of the season

Key characteristics: Effects include heavily blighted onion leaves, reduced bulb growth, destroyed seed stalks, and spongy neck. Pale greenish yellow to brown elongated areas of varying sizes develop on infected leaves and seed stalks. Symptoms first appear on older leaves. If weather conditions are moist, masses of gray to violet fungal spores will envelop the infected leaves. Leaves become girdled and collapse. Cool temperatures of less than 72^oF and the presence of rain or dew providing relative humidities greater than 95% are required to initiate infection. Sporulation occurs at night and spores are easily windborne considerable distances during the early morning hours. Spores are very susceptible to drying, but can survive on host tissue 1-3 days. See References 1 and 3.

*Management Option*			*Recommendation*
Scouting/thresholds			Look for downy mildew while scouting for other pests. If you suspect downy mildew, remove the infected leaves, place them in a plastic bag, and mark the location. Send the sample to an IPM specialist to confirm your diagnosis. Once diagnosed, a fungicide treatment is required. A forecasting system called Downcast, developed by Canadian researchers, can be used to determine fungicide application timing. See Reference 1 for a more detailed description of Downcast.
Resistant varieties			No resistant varieties are available.
Crop rotation			Minimum two year rotation away from onions to break the disease cycle.
Site selection			Do not grow perennial onion species in the vicinity of commercial onion fields.
Transplants			Transplants should be mildew-free.
Sanitation			Practice good sanitation. The fungus overwinters in volunteer onions as oospores and as mycelium on infected bulbs in cull piles. Remove and destroy cull and volunteer onions.
Seed selection/treatment and Postharvest			These are not currently viable management options.
Compound(s)
Common name		Rate/A		PHI	REI	Field
	Trade name	Product		(days)	(hours)	Use EIQ	Comments
azoxystrobin
	Quadris F	9.2-15.4 fl oz		0	4	2-4	Do not apply more than 1 application of Quadris or Amistar (Group 11) before alternating with a fungicide that has a different mode of action. See comments below for maneb/mancozeb program. Do not spray these products where spray drift may reach apple trees or use same sprayer to apply sprays to apple trees.
	Amistar 80 WDG	3-5 oz		0	4	3-4
chlorothalonil
	Bravo, Equus or OLF 6 lb/gal	1.5-3 pt		14, green 7, dry	12	33-65	Note eye wash requirement and reduced seasonal amounts for shortened reintry interval on label. These products are labeled only for disease suppression. Consider using Ridomil Gold Copper as an alternative.
copper compounds
	Champ F or OLF	1.3 pt		0	12/24 (see label)	19	Apply when plants are 4-6 inches high and repeat at 7-10 day intervals
dimethomorph
	Acrobat 50WP, Forum SC	6.4 oz, 6.0 oz		0	12	5 4	For both green and dry bulb onion. Acrobat and Forum (Group 15) must be applied as a tank-mix with another fungicide labeled for control of downy mildew.
fenamidone
	*†Reason 500 SC	5.5 fl oz		7	12	3	Not for use or sale in Nassau or Suffolk Cos. For optimum results, begin applications of Reason (Group 11) as soon as environmental conditions are favorable for disease development. Do not make more than 1 application before alternating with another effective fungicide with a different mode of action.

Compound(s)
Common name		Rate/A	PHI	REI	Field
	Trade name	Product	(days)	(hours)	Use EIQ	Comments
fosetyl-Al (dry bulb)
	Aliette WDG 0.8 lb/lb	2-3 lb	7	12	18-27

maneb
	Maneb 75DF or OLF 0.75 lb/lb	2-3 lb	7	24	32-48	Maneb and mancozeb are the preferred materials for downy mildew control, and should be used at the full labeled rates. Begin sprays when conditions are favorable for disease and repeat every 7 days if the weather is cool and wet. Do not apply to exposed bulbs. Add a surfactant to the solution to improve performance. NOTE - if Botrytis leaf blight and purple blotch are also occurring, overall best control is achieved with full rates of the EBDC's listed plus the 2 pt rate of chlorothalonil for Botrytis and Alternaria. If Botrytis is particularly severe, consider the full rate of an EBDC + the 2 pt rate of chlorothalonil + the lowest rate of Rovral.
mancozeb (dry bulb)
	Dithane 75DF or OLF	3 lb	7	24	33
mancozeb + copper hydroxide (dry bulb)
	ManKocide	2.5 lb	7	24	73
mefenoxam + copper hydroxide
	Ridomil Gold Copper	2 lb	7, green 10, dry	48	43	Apply when disease is first reported and continue at 14 day intervals until the threat of downy mildew is over.
mefenoxam + mancozeb (dry bulb)
	Ridomil Gold MZ	2.5 lb	7, dry only	48	75
mefenoxam + chlorothalonil
	Ridomil Gold Bravo	2 lb	21, green 7, dry	48	106
pyraclostrobin
	Cabrio EG	12 oz	7	12		Not for aerial application in NYS. For use on all varieties of onion. Do not make more than one application of Cabrio (Group 11) before alternating to a labeled fungicide with a different mode of action.
pyraclostrobin + boscalid
	Pristine 38 WDG	18.5 oz	7	12		Not for aerial application in NYS. For use on all varieties of onion. Do not make more than one application of Pristine (Groups 11 +7) before alternating to a labeled fungicide with a different mode of action. For disease suppression only.

20.5.7 Flower blight, Botrytis squamosa, B. cinerea, and B. allii

Time for concern: During seed stalk development and flowering

Key characteristics: Effects include heavily blighted onion leaves, reduced bulb growth, destroyed seed stalks, and spongy necks. Pale greenish yellow to brown elongated areas of varying sizes develop on infected leaves and seed stalks. Symptoms first appear on older leaves. If weather conditions are moist, masses of gray to violet fungal spores will envelop the infected leaves. Leaves will become girdled and collapse. See References 1 and 3.

*Management Option*			*Recommendation*
Scouting/thresholds			Record the occurrence and severity of flower blight. No thresholds have been established.
Resistant varieties			No resistant varieties are available.
Site selection			Plant seed fields several miles from bulb fields.
Sanitation			Remove all potential sources of fungal spores such as cull piles, volunteer onions, and field debris.
Crop rotation, Seed selection/treatment, and Postharvest			These are not currently viable management options.
Compound(s)
Common name		Rate/A		PHI	REI	Field
	Trade name	Product		(days)	(hours)	Use EIQ	Comments
chlorothalonil
	Bravo, Equus or OLF 6 lb/gal	1-2 pt		7	12	22-43	Note eye wash requirement and reduced seasonal amounts for shortened reentry interval on label.

20.5.8 Nematodes, primarily northern root-knot (Meloidogyne hapla) and the lesion (Pratylenchus spp.)

Time for concern: Before and during planting

Key characteristics: Root-knot nematode is the most prevalent and damaging to onions at present, especially in the western production areas. Severely infected onions initially exhibit stunting, uneven growth, thicker necks, smaller bulbs, and delayed maturity. The diagnostic symptoms are found on roots as galls or root thickenings of various sizes. Similarly, above-ground symptoms of onions infected with lesion nematodes are of general stunting and infected roots may or may not show brown to black lesions. The presence of nematodes in roots or in soil around roots is the only definitive evidence of their involvement. See reference 14.

*Management Option*		*Recommendation*
Scouting/thresholds		Record symptoms of damage and assay roots and soil for the presence and density of nematodes. Threshold level of root-knot nematode on onions in organic soil is between 4-8 eggs/cc soil. A density as low as 1 lesion nematode/cc soil has caused damage to onions.
Resistant varieties		No resistant varieties are available.
Crop rotation		Both nematodes have a wide host range, thus it is difficult to design a practical, economic, and effective crop rotation. Grain crops such as wheat, rye, oats, barley, corn, and sudangrass are not hosts for the root-knot nematode. If possible, avoid potato, carrot, lettuce, celery, soybeans and beans in the rotation as these crops are good hosts to the root knot nematode.
Cover Crops		All the grain crops listed above are suitable for use as cover crops against the root-knot nematode. Sudangrass is a non-host to root-knot nematode, but is a host to lesion nematode. However, the soil incorporation of green manure of sudangrass before the first frost will reduce the population of both nematodes and their damage to onions. Certain white clover and flax lines have given similar results.
Seed selection/treatment		Select vigorous, disease free and properly treated seeds.
Soil Treatment		Where economical and possible, the use of pre-plant soil fumigants (Telone, Vapam) is highly effective in controlling root-knot and lesion nematodes. Of the nonfumigant-type nematicides, only *†Vydate has been registered for use on onions under a special Local Need (24C) labeling in organic soils. See below for details.
Site selection, and Postharvest		These are not currently viable management options.
Compound(s)
Common name	Rate/A

Pest Management Guidelines A Cornell Cooperative Extension Publication	Cornell Guide for Pest Management of Vegetables
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20.1 Recommended Varieties

Temperature should be below 85˚F at application or below 80˚F if temperatures are expected to 85˚F later in the day. Compatible with most insecticides and fungicides, but separate application is recommended.