Onion Downy Mildew Info – Learn How To Control Downy Mildew On Onions
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By: Mary Ellen Ellis
The pathogen that causes onion downy mildew has the evocative name Peronospora destructor, and it truly can destroy your onion crop. But there are ways to prevent it and manage it if you see the early signs.
Downy Mildew of Onion Crops
Onions, garlic, chives, and shallots are all susceptible to being infected by the fungus that causes this type of downy mildew. The fungus overwinters in the soil in many locations, which means that it can become endemic in a garden or field, ruining crops year after year. The spores of the fungus spread and cause infection, especially in conditions that are cool, humid, and moist.
Onions with downy mildew have leaves with irregular spots ranging from pale green to yellow to brown in color. The seed stalks may also be affected. Both leaves and stalks may also host the spores of the fungus, which are initially gray and then become violet. Eventually, the leaf tips will die and the leaves will collapse entirely, with the spores taking over the dead tissue.
The impact on the edible bulb of the onion plant will be reduced size and it will develop a spongy texture. The bulb will not keep for as long as it normally would. Although the downy mildew doesn’t often kill the entire plant, it does reduce yield and results in onions of greatly lessened quality.
Preventing Downy Mildew on Onions
There are several ways you can prevent this disease in your onion and related plants:
Use onion varieties that are resistant to downy mildew. Use high-quality bulbs, seeds, and sets to start your garden. These are more likely to be disease-free. The infection often begins with infected plants and seeds.
Space plants adequately to allow for air flow. Avoid watering plants when they will not have a chance to dry quickly, such as in the evening or in very humid conditions.
Managing Onion Downy Mildew
The only real way to eliminate downy mildew in onion plants is to spray them with a fungicide. Dithiocarbamate fungicides are used for downy mildew on onions.
If you do get an infection that takes hold in your garden, try crop rotation. Plant something that resists onion downy mildew next year so that the fungus has nothing to grow on. Because this pathogen can survive most winters, it is also important to practice good garden hygiene, collecting and destroying dead onion matter at the end of the season.
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Several fungal species affect different crops to produce similar symptoms, all known as downy mildew.
Pseudoperonospora cubensis (affects curcubits), Plasmopara viticola (affects grapes), Bremia lactucae (affects lettuce), Peronospora destructor (affects onions).
In moist weather, a white downy fungal growth develops on the underside of the leaf.
Spots turn brown and dead areas develop.
It is mainly spread by wind from older crops, but spores can survive in the soil and on crop residue. Worse on lower, shaded leaves that stay moist.
Warm, moist weather favours the spread of the disease.
Curcubits, grape, lettuce, onion.
Plant resistant varieties. Plough in old crops as soon as harvesting is completed and ensure all crop residue is decomposed before planting.
Spray to fruit set with a protectant fungicide. In wet weather and after fruit set, use a systemic fungicide. If the disease is well advanced, it will be difficult to control.
Wider spaced plantings allow better air flow and spray penetration.
Chemical registrations and permits
Check the Australian Pesticides and Veterinary Medicines Authority chemical database and permit database for chemicals registered or approved under permit to treat this disease on the target crop in your state or location. Always read the label and observe withholding periods.
Symptoms and Signs:
IPM Strategies: Planting season Downy mildew can be prevented by planting onions when the weather is dry and temperatures are over 25° C.
Plot location It is better not to have multiple or old plantings in the same area because old crops will serve as inoculum for downy mildew in the new plantings. This is especially important when onions are planted for seed production. In the tropics, downy mildew is a problem when onion farms are located in the mountains or cool valleys. Irrigation Avoid the use of overhead irrigation. If overhead irrigation is used, apply it early in the day to allow time for the crop to dry. Downy mildew sporulates at night when the leaves are wet. Avoid damp growing conditions and maintain good soil drainage. Seed quality Use certified disease-free seed purchased from reputable seed merchants. Ensure the seed is in the original seed packet. When onion bulbs are used to establish seedlings they should be heat-treated to kill infection by exposing them to the sun for 12 days during which the temperature must exceed 40oC for about 4 hours. Infected onion sets should not be used to establish seedlings. Seedlings Seedbeds should be distant from old plantings. It is important to use new deep soil that has good drainage properties for the seedbeds. Sterilize the soil with hot water or ashes to eliminate the fungi from the soil. Inspect seedlings for any sign of disease and discard and destroy any that are suspected of being infected. Fertilization Nitrogenous fertilizer should be use sparingly. Sanitation Avoid dense planting of onions or planting near weeds and dense barriers because these help to maintain dew on the onion leaves which aids in disease infection and dissemination. Remove unharvested plant parts Destroy volunteer onion plants and crop debris as soon as the crop is harvested. Make a compost heap with the crop residues and cover it with a layer of soil. Do not use this compost on onions or any downy mildew-susceptible crops. Rotation Rotate crops by not planting onion or its relatives for at least two years and preferably not for four years.
Chemical control: Fungicide recommendations Contact the local authorities for the specific fungicides to use in your country. Remember, you must use only fungicides that are legal to use for this pest and crop in your country. When early symptoms of the downy mildew are detected in the field and environmental conditions exist for the disease sporulation and infection, apply protectant fungicides (dithiocarbamates, clorotalonil, copper, metalaxyl and fosetyl-A-1). Use of these fungicides should be rotated using seven day intervals when the weather is cool and damp and up to ten day intervals if the weather is dry. Overhead irrigation and rainfall will wash the fungicides off the plants. Fungicides should be applied after an irrigation cycle and may have to be re-applied after a heavy rainfall. Spray techniques Sprays should be applied using a knapsack applicator which is in good condition. It should be fitted with a hollow cone nozzle. The sprayer should walk slowly down the rows covering the whole plant with enough spray to coat the plant thoroughly but not so much that it runs off the plant.
Cerna O., S. Kline, W. Kline, D. Ramírez. M. Gaskell. 1994. Guía Sobre Producción de Cebolla para Exportación. Fundación Hondureña de Investigación Agriacutecola. San Pedro Sula. Honduras.
Hoffmann M. P., C. H. Petzoldt and A. C. Frodsham. 1996. Integrated Pest Management for Onions. New York State IPM Program Publication No. 119.
Lorbeer J and J. Andaloro. 1984. Diseases of Onions. Downy Mildew. NYSAES, Geneva, NY. Page 737.20.
Tropical Development and Research Institute. 1986. Pest Control in Tropical Onions. Tropical Development and Research Institute. College House. Wrights Lane. London. UK.
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