- English: Downy Mildew
- Spanish: Mildiu Algodonoso, Mildiu Lanoso
- Peronospora destructor (Berk.) Casp. In Berk.
- Americas, North, East and South Africa, West, South, and East Asia, Europe, Australia, and New Zealand. This fungus is present where temperatures are cool. In the tropics it attacks mainly onions planted in the mountain regions.
Symptoms and Signs:
- Oval or cylindrical areas of varying sizes develop on infected leaves and seed stalks. These areas are pale greenish-yellow to brown in color. Symptoms often appear first on older leaves. If weather conditions are moist and temperatures are low, masses of gray to violet fungal spores envelop infected leaves, which become girdled, collapse, and die. The dead leaf tissue is rapidly colonized by purple blotch, which is dark in color and obscures downy mildew.
- Downy mildew seldom kills onion plants, but bulb growth may be reduced. Bulb tissue, especially the neck, may become spongy and the bulb may lack keeping quality. This disease is one of the most destructive of onion seed production world-wide.
- The downy mildew fungus can rest in perennial varieties and in infected onion bulbs left in the field and in cull piles. Resting sexual spores (oospores) may persist in the soil to infect seedling onions planted the following season. During the onion growing season the fungus produces spores (conidia) that are carried by wind to infect new onion plants. Spores are produced on nights with high humidity and moderate temperatures (4-25° C) with an optimum temperature for sporulation of 13° C. The spores mature early in the morning and are dispersed during the day. They remain viable for about 4 days. For germination the spores require free water and the optimal temperatures 7-16° C. Rain is not needed for infection when dew occurs continuously during the night and morning.
- After the fungus is established, it completes its life cycle in 11 to 15 days. New spores infect new plants or leaves. As the upper portion of the onion leaf is killed, the fungus can infect the next lower part of the leaf. The entire leaf may thus become infected and die. During favorable environmental conditions the infection may result in a severe epidemic. During dry weather, the spores usually disappear and the number of lesions declines. However, the disease cycle recommences when wet, cool weather recurs.
- Onion, garlic and other plants of the allium family.
- The best way to manage the disease is on a preventive basis. Inspect the tips of old onion leaves twice a week for plants with disease symptoms before initiating a fungicide application. Downy mildew produces spores in periods with no rain and low to moderate temperatures at night (<24°C) and with relative humidity 95% between 2:00 AM to 6:00 AM. Infection may occur the night following sporulation if the temperature is 6-22°C and dew is present on the leaves within the first five hours of darkness for a span of at least 3 hours.
- 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.
- 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.
- 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.
- Nitrogenous fertilizer should be use sparingly.
- 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.
- 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 Agriacute;cola. 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.
- Technical content
- Alfredo Rueda and Anthony M. Shelton
- Cathy Weeden, Ben Shelton and Linda McCandless
- From the Archives of the New York State Agricultural Experiment Station in Geneva
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