Onion Diseases and Pests

Diseases can be a major problem for onion growers. Some fungal diseases remain in the soil for decades; some reveal themselves only when onions are planted. Though research and selective breeding has resulted in many resistant and partially resistant cultivars, many diseases still have the ability to seriously damage onion crops. Some of the more serious diseases include pink root, botrytis blast, downy mildew, purple blotch, white rot, and neck rot. Pests are also a problem, onion thrips being the most wide-spread. Onion maggots can also be damaging.

Diseases

Downy mildew ( Peronospora destructor ) is a fungus that overwinters on infected plants or old onion plant material in the field. The plants may be dwarfed, distorted and pale green. Bluish, downy spots appear on the leaves. The spots will enlarge, often girdling the leaves, later turning black. Secondary leaves may develop on the plants retarding normal bulb growth. Bulb quality is poor and often spongy. Controls for this fungus include using a 3-year crop rotation, destroying onion plant refuse, eradicating wild onions, and avoiding poorly-drained soils. Chemicals can be used to help control this fungus. There are no resistant onion cultivars.

Neck Rot ( Botrytis allii ) is a fungus that overwinters on plant debris, in the soil, in infected bulbs, and as sclerotia. Most common causes are excessive nitrogen in the soil, which delays crop maturity; late-season rain and late irrigation; inadequate or improper curing; improper storage or rotation; and tight storage. The disease often appears just before and during harvest, entering the bulb through punctures or the neck. Symptoms are first watery-looking neck tissue that becomes yellow, then grey mold develops between the scales, and finally, small, black sclerotia appear around the neck. Bulbs become soft and secondary infections follow. In the field, neck rot will winter over in small plants and weak plants may die. Spores will continue to infect the crop. Seed lots may be infected. Controls include allowing the green tops to mature well and then dry well before topping, proper field curing, minimal and careful handling to minimize bruising, and proper storage temperatures. Before the following crop year, onion refuse and culls should be removed, and treated seed should be used. No chemical controls will adequately control neck rot.

Smut ( Urocystis magica and U. colchici ) is a fungus that survives multiple years. It attacks only onions. Symptoms appear first in the emerging plant stem as a raised, blister-like lesion near ground level. The blister erupts to expose black, powdery spore masses. Many seedlings will die; the survivors produce distorted bulbs with dark streaks and smutty lesions. Oddly enough, small onion sets and young plants that are set out don't appear to run the same risk of infection. Chemical controls include treating the seed and soil drenches at seed planting time.

Purple blotch ( Alternaria porri ) is a fungus that overwinters on infected bulbs and on dead, diseased onion culls and field refuse. The fungus often takes hold after injury to the onion. The spores require both temperatures, between 77 and 81 degrees, and persistent moisture to cause infection. 'Sweet Spanish' onions are very susceptible. The symptoms are purplish leaf spots with white margins. The spots will become covered with a brownish-black powdery growth and, within hours, the spotted leaves will turn yellow and droop. The fungus acts quickly, usually within two days. Affected bulbs may decay during and after harvest. Controls include cleaning up cull piles and onion refuse, and properly curing bulbs in the field. Chemical sprays can be used. Resistant varieties include the 'Fiesta' cultivar.

Pink root ( Phoma terrestris ) is a fungus that may live in the soil for several years. It may appear anytime in the onion plant's growth. Infected roots turn first yellow, then pink, bright pink, red, and finally dark purple and they dry up and disintegrate. New roots will also become diseased and die. Because the bulbs develop so poorly, they are normally small and have stunted tops. Controls include using a 5-year crop rotation, and closely monitoring fertilizer and water needs. Fall soil fumigation before spring planting is also effective. Partially resistant and resistant cultivars are available ('Yellow Globe Danvers' types, 'Fiesta', 'El Capitan', and others, also some partially resistant 'Sweet Spanish' cultivars).

White rot ( Sclerotium cepivorum ) is a fungus that can survive in the soil as long as 30 years or more. It only attacks allium species. It can infect plants from 12 inches below the surface and spread rapidly to adjacent plants. It proliferates in cool soils below 75 degrees F. Once white rot is in a field, it is very difficult to cultivate onions successfully. Symptoms initially include decaying leaves, then rotted roots. The plants can be easily uprooted. Fluffy spores can be seen on the roots and bulb. The bulbs will be watery and the outer scales will crack. Laws in Idaho, Washington, and Oregon forbid importing infected bulbs in many parts of each state. Controls include only planting disease-free bulbs in disease-free soil, avoiding movement of soils from contaminated fields to disease-free fields, washing equipment, and removing diseased plants and surrounding soils. Chemical controls can be used to only partially control this fungus.

Botrytis blight ( Botrytis squamosa ) is a fungus that is spread by moving air. Optimum temperatures for disease are 55 degrees F. to 75 degrees F. Dampness will encourage infection. Older leaves are most susceptible. Symptoms are small white spots on the leaf that eventually become tan and cover the leaf surface. Lesions will also occur on the scapes. Controls include removal of onion debris and old culls, 2-3 year crop rotation, avoidance of extended overhead irrigation, and maintaining a dry atmosphere while curing and during storage. Chemical controls may also be used. There are no resistant cultivars.

Pests

Onion thrips and onion maggots are important widespread pests. Onion thrips blanch and deform the leaves; the injuries being worse in hot, dry weather. Chemical controls are fairly successful. Onion maggots are larvae of a small fly that lays its eggs in cracks in the soil or on the base of the plant. The small maggots feed on the stem. Young plants will usually die quickly, but in older plants the maggots eat into the bulb. Controls include crop-rotation, removal of onion debris and old culls. Both insects can be somewhat controlled with chemicals.

Background Information

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