Pest and Disease Problems in Onions Home Grown Onions  luv2garden.com  - Abigal Gordon's Gardens ®     How to Grow Onions Onion Varieties   Insect Pests of Onion Plants Maggots Worms Aphids Slugs and snails   Diseases of Onion Plants Botrytis Neck Rot Botrytis Leaf Blight Downey Mildew Pink Root  Purple Blotch Black Mold

	Common pests of most Onion crops are Root Maggots and Onion Maggots which attack germinating seedlings, they feed on the developing roots and epicotyl. as well as the expanding bulb during later stages of plant development.  They lay their eggs near the plants base during the spring when they emerge from the pupae stage. Eggs hatch and the larva will feed on the onions for up to 3 weeks . They are more prevalent in coastal and cool damp areas. Maggots will hollow out an entire onion if given the opportunity. Larger bulbs will turn yellowish and rot. Destroy all affected plants.  Destroy means just that, destroy em, Nuke em get rid of them but - DO NOT PUT THEM IN THE COMPOST HEAP as this will perpetuate your problem. Tanglefoot is an organic substance that is sprayed or spread on plant stems and catches anything that attempts to climbs up. Yellow Sticky Traps will capture the adult flies when they emerge in the spring. Spreading wood ashes and spraying Hot Pepper Wax around the plants to discourage egg laying is helpful also.
	Beet Army Worm and Cut worms Host plants of the beet armyworm include asparagus, beans , peas, beets, celery, lettuce, potato, tomato, many flowers, and weeds as well. It does not tolerate cold and dies off during the winter in Northern regions.  The larvae feed on the foliage and at times can completely defoliate some plants. As they travel about they will sometimes leave strands of silk behind, netting the leaves with a silvery film. Most damage can be avoided by using hardy transplants. Onion plants can  also be protected with Bacillus thuringiensis or spinosad sprays.
	Slugs and snails like to feed on young seedlings and succulent parts of plants. They leave a trail of mucus on the surfaces on which they crawl. Moist, humid environments favor slug development, and slugs usually overwinter in sheltered locations outdoors, They are a bothersome pest in many garden and landscape situations. Large ragged holes in the onion leaves are generally a sign of slugs. Look for slimy trails as further evidence of their presence. You can control the pests by handpicking if caught early enough. Diatomaceous Earth may be used as a barrier to control snails and slugs as well as flea beetles, sawfly,  thrips, mites, and many other insect pests.
Aphids are Yellowish-pink to pale green plant lice that suck ......plant juices. They are a soft-bodied, oval/pear shaped insect and are commonly found on nearly all varieties of plants, vegetables, field crops, and fruit trees.   Hot Pepper Wax repels bugs from your plants, fruits and flowers. 100% natural spray uses pepper extract and wax to effectively discourage pests. Helps to reduce stress on cuttings. Leafhoppers and leaf miners are also minor pests of Onion Plantings.

Onion Diseases

Most onions are susceptible to fungal infestations, at times it can be a severe problem , particularly under moist conditions.

 

	Botrytis Neck Rot  a major disease of onions. Susceptibility varies by variety. In general, white cultivars are more susceptible, red and yellow cultivars may also sustain heavy damage. Symptoms generally appear after harvest, although infections occur in the field. The fungus enters bulbs through the neck when the tops are cut  .A gray mold develops between the onion scales and black sclerotia may develop around the neck of the bulb. Secondary invasion by soft rot bacteria may cause a watery rot There is no treatment for Botrytis Neck Rot, only prevention. Inspect stored Onions regularly and discard any infected ones.
Avoid late season applications of nitrogen. Don't irrigate within 2 weeks of lifting onions. Harvest only when the crop is mature, and during dry weather. Provide good ventilation for curing onions before storing. If you are planning to store Onions grow varieties that are known to store well, such as Candy Hybrid, Yellow Ebenezer,. Good storage onions are low in water and high in sulfur, and should have 3 or more wrapper scales, and the neck will tighten when dried. Stronger flavored onions last longer in storage than the sweet onion varieties. Try storing all your sweet onions no more than two months . If this disease has been a problem in the past you should use resistant varieties such as Red Burgundy .
	Botrytis Leaf Blight occurs in poorly drained damp areas. The bacteria is spread by garden tools and surface water .Bacterial leaf blight usually occurs only on the upper leaves. Small water-soaked lesions form and eventually coalesce into larger lesions that eventually cover the entire leaf.  There are a number of fungicides registered for treatment of Botrytis Leaf Blight , many are only available to commercial growers.  Remedy Fungicide has proven effective against this blight.
Downy Mildew Downy mildew attacks onions, shallot, leek, garlic, and chive Its more prevalent in cool, moist weather and rarely occurs in warmer climates. Symptoms are Light green to yellow and light brownish oval to irregular cylindrical areas on infected leaves or seed stalks These areas may consist of alternating yellow and green layers of tissue. Downy mildew also produces minute fruiting bodies and spores called sporangia on the surface of the leaves and seed stalks . The masses of spores are at first transparent to grayish, and then become a light violet color. Leaves become girdled in the region where mildew develops and then collapse. This results in dead leaf tips The dead leaf tissue is rapidly colonized by purple blotch . Downy mildew seldom kills onion plants, but bulb size is reduced. Bulb tissue, may become spongy and the bulb may lack storage quality  Manzate or Dithane are both effective against Downy Mildew.
	Pink Root  Infected roots first turn light pink, then darken through red and purple, shrivel, turn black, and die. The pinkish red discoloration may extend up into the scales of the bulb. New roots also may become infected. If infection continues, plants become stunted. Yellow /Light Brown to Light pink of roots that later becomes dark pink/red and sometimes purple. Severely infected plants exhibit tip dieback, and stunted growth of both plant and bulb. Weakened foliage often becomes infected with other opportunistic diseases such as purple blotch. Pink root rot symptoms above the ground is often misdiagnosed as nutrient deficiency or drought stress .   Pink Root seldom results in plant death. Infection is confined to roots and outer scales of the bulb. Severe infection will reduce bulb size. Crop rotation and proper cultural practices are the best defense against onion pink root.  Fungicides are only moderately effective, and generally not worth the expense for the success rate they have displayed against this disease.   1.] Crop Rotation in your garden planting area 2.] Remove and destroy all plant refuse in the fall and use deep cultivation to bury any remaining refuse. 3.] Do not place diseased plants in the compost heap, as this will only serve to carry bacterial or fungal infestation into the next growing season.
	Purple Blotch causes small water-soaked lesions on leaves and stalks that rapidly develop white bleached out centers. As the lesions grow they become brown to purple in color occasionally with a red or red/purple border. The lesion surface may be covered with brown to dark gray masses of spores. Purple blotch lesions often occur along with other diseases in weakened plant tissue and also may be invaded by other fungi. Purple Blotch progresses in plants weakened by poor care or other pathogens as well as in overly moist conditions from rain or dew. Susceptibility to infection is higher on older leaves when compared to younger leaves and is higher on leaves infested with aphids and onion thrips . Cultural control strategies include reducing plant density and promoting good field drainage to reduce the duration of leaf wetness. Once disease occurs in a field, long rotations are recommended. Fungicides such as mancozeb can reduce the severity of purple blotch. Alternating applications of varying fungicides is advisable.  Fungicides containing chlorothalonil are also effective.
	Black Mold occurs sporadically but is potentially serious. It causes damage primarily to stored onions, rendering them unpalatable and unmarketable. The scientific name is “allium aspergillus” or “aspergillus niger” but who cares about scientific names - so long as it ain't growing on YOUR onions. Should your hands come in contact with black mold, a thorough scrubbing with soap and water is advisable, to prevent spreading the fungi. Onions carrying black mold are generally damaged , to be safe, you should discard - nuke- obliterate and destroy any potentially contaminated onions. If you enjoy fungus in your Garden - put them in your compost heap - you'll be sure to have more fun with the same black mold next season. No particular fungicide has proven to be highly effective. And there's no absolute way to ascertain whether your onions are actually contaminated while still in the ground. "Research indicates that a good fungicide control program for foliage diseases will reduce the incidence of black mold. Storage and transit temperatures below 55°F (12.8°C) and as low as 33°F (0.6°C) are recommended to suppress black mold development. Handling of bulbs to avoid bruising also reduces injury and invasion sites for the fungus."   UC Pest Management Guidelines

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References -

Identification of Diseases and Disorders of Onions  

Colorado State Univ. Botrytis, Downy Mildew and Purple Blotch of Onion