Downy Mildew
Class: Fungi
Common Name
Downy mildew
Scientific name
A collective of different pathogens: Peronospora, Plasmopara, Bremia, Pseudoperonospora, Hyaloperonospora
Potential Hosts
Members of the cucurbits (cucumber, squash, melon, pumpkin, zucchini, watermelon, etc), sage, parsley, coriander, dill, basil, sunflower, grapevines, onion, brassicas (cabbage and cauliflower), pea, roses, daisies, and lettuce
Who am I?
A general name for a large group of pathogens that can cause major damage to cultural crops, ornamentals, and landscapes. The disease is usually host-specific, which means that certain pathogens can only infect plants from the same genus or family.
Traditionally downy-mildews were considered to be fungi until advancements in microbiology confirmed that they aren’t true fungi. They are Oomycetes, a fungus-like eukaryotic microorganisms closely related to algae.
Downy mildews produce a gray to whitish, thin mycelium layer upon lower leaf surfaces. The first sign of downy mildew on the upper side of leaves is yellowish angular spots, bounded by vines. It favors relatively high moisture conditions, low light, and low temperatures.
Control measures
Cultural
The sooner the better: It’s easier and more cost effective to overcome infestation and successfully control downy mildew at its initial stage. Make it a routine to monitor the field regularly and search plants for the presence of downy mildew on a weekly basis.
As with any moisture-favorable disease, various agro technical measures taken during crop growth can reduce chances of infection and spread:
Improve air circulation: promote drying foliage and shorten the duration of wetting periods by introducing net curtain vented areas.
Proper soil drainage: the presence of standing water will promote the spread of the fungus.
Other moisture reduction techniques include covering the ground with polyethylene sheets to reduce evaporation from the soil.
Conventional (chemical)
When conditions for downy mildew are expected, consider spraying once every 7-14 days with applications of fungicides. Don’t use products based upon the same active ingredient in consecutive treatments. Use of fungicides belonging to different groups to prevent pathogens from developing resistance to a specific chemical.
The following is a list of generic names of fungicides (sorted by groups) and used in one or more parts of the world:
Group 1: Cellulose synthesis inhibitors such as Dimethomorph and Mandipropamid, Iprovalicarb, and Benthiavalicarb – isopropyl
Group 2: RNA inhibitors such as Metalaxyl – M
Group 3: Respiration inhibitors such as Fenamidone and Propamocarb HCL+Fluopicolide
Group 4: Resistant inductors such as potassium salts of phosphorous (phosphoric) acid
Other: Cymoxanil
Organic
Products based on tea tree oil and Potassium Hydrogen Carbonate+copper sulfate.
Biological
Bacillus subtilis.
Caution and careful notice should be taken when using any plant protection products (insecticides, fungicides, and herbicides). It is the grower’s sole responsibility to keep track of the legal uses and permissions with respect to the laws in their country and destination markets. Always read the instructions written on labels, and in a case of contradiction, work in accordance to the product label. Keep in mind that information written on the label usually applies to local markets. Pest control products intended for organic farming are generally considered to be less effective in comparison to conventional products. When dealing with organic, biologic, and to some extent a small number of conventional chemical products, a complete eradication of a pest or disease will often require several iterations of a specific treatment or combination of treatments.