Powdery mildew of tomato
Who am I?
Unlike other pathogens responsible for powdery mildews (e.g., Leveillula taurica), Oidium neolycopersici is an external powdery mildew. It immediately produces white powdery spots that cover the upper side of leaflets rather than the underside. In time, dense, white mycelial network colonize the upper surface of the leaves. Spores can be carried by the wind.
Powdery mildews favors of warm, dry climates. The fungus does not need the presence of water on the leaf surface for infection to occur. However, spores need dew to germinate.
Powdery mildew will not kill its host quickly, and fruits are not affected.
The sooner the better: It’s easier (and more cost effective) to overcome powdery mildew by controlling it in the initial stages of infestation. Make it a routine to monitor the field regularly and search plants for the presence of powdery mildew on a weekly basis.
Sanitation: In most cases, it is beneficial that powdery mildew is generally host-specific, which means adjacent neglected crops are not of worry. However, it is important to remove all the debris of infestation because otherwise it can affect host-specific crops the following season. There are many powerful commercial fungicides, and the amount of inoculum originating from last year's infections significantly matters.
Simple sanitation measures are an important step and should be at the foundation of plant protection.
Products based on tea tree oil, Potassium Hydrogen Carbonate+copper sulfate, and Sulfur (dust, wettable, flowable, or micronized)
Bacillus pumilis and bacillus subtilis
There are differences in the dynamical nature and behavior of powdery mildew on different hosts. Growers and consultants tend to have different treatment methods, approaches, fungicides preferences, and secret tricks. Still, some consensus exists: prevention, rotation, and the use of several fungicides that each belong to different groups (with respect to mode-of-action).
Effective control requires spraying with high pressure, high volume of water; good coverage is of the essence. Having a fixed or dynamic schedule for spraying application is a common strategy.
The following is a list of generic names for fungicides known to help manage powdery mildew and is sorted into groups according to their mode of action:
Group 1: Penconazole, Triadimenol, Tebuconazole, Myclobutanil, Tetraconazole, Propiconazole, Prochloraz, Cyproconazole , Difenoconazole, Fenbuconazole, Triflumizole, and hexaconazole
Group 2: Azoxystrobin, Pyraclostrobin, Trifloxystrobin, and Kresoxim-Methyl
Group 3: Sulfur, copper sulfate, bicarbonates, mineral oils, neem oil, and detergents\soap-based products
*Sulfur can cause injury to foliage and fruit when applied on days with a temperature above 32 degrees Celsius. Do not apply within 2 weeks of an oil application.
*When powdery mildew is present, yet the symptoms have not appeared, consider spraying applications of fungicides once every 7-14 days. Don’t use products based on the same active ingredient in consecutive treatments except in group 3, as there are no restrictions there.
*Use fungicides belonging to different groups to prevent pathogens from developing resistance to a specific chemical.
*Some commercial fungicides have two active ingredients and two modes of action. After using such products, take into account that you now have two groups that you already used. Make sure to exclude those two active ingredients in the next application.
It is important to remember that if powdery mildew develops resistance to fungicides within a group, the pathogens are likely to be resistant to all members of that group (except group 3).
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 after the legal uses and permissions with respect to the laws in their country and destination markets. Always read the Instructions written on the label and in any case of contradiction work with accordance to the product label. Keep in mind that information written on the label usually apply only to local markets. Pests control products intended for organic farming are generally considered to be less effective in comparison to conventional product. And so one must keep in mind that when dealing with organic, biologic and, to some extent, small number of conventional chemical products, a complete eradication of a pest or a disease will often require several iterations of a specific treatment or combination of treatments.