Powdery Mildew of Apples Alert


This disease usually appears on the upper surfaces of leaves as white-grayish spots with a powdery-like texture. When left untreated, the white powdery-like texture can cover the entire leaf leading eventually to defoliation. Powdery mildew can cause considerable loss due to nutrient removal, reduced photosynthesis, increased respiration and transpiration, and impaired growth.

Who Am I?

Podosphaera leucotricha is a fungal disease known by its common name powdery mildew of apples and pears, which are its only hosts.

Control Measures

Monitoring: Make it a routine to monitor the field regularly and search plants for the presence of powdery mildew on a weekly basis. It is easier and more cost effective to control and overcome powdery mildew during the initial stage of infestation.

It is a common practice to prevent powdery mildew by spraying early in the season.

*Don’t use products with the same active ingredient in consecutive treatments. Use fungicides belonging to different groups to prevent pathogens from developing resistances to a specific chemical.

The following is a list of generic names for fungicides known to 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, and Triflumizole; 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.

*Some commercial fungicides have two active ingredients and thus sometimes two modes of action. After using such products, take into account that now two groups are already used; make sure to exclude those two active ingredients in the next iteration.

It is important to remember that if powdery mildew develops a resistance to fungicides within a group, the pathogens are likely to be resistant to all members of that group (except group 3).

Sulfur (dust, wettable, flowable, or micronized) and potassium bicarbonate

Bacillus pumilis and bacillus subtilis.

*Names marked in red are considered to be highly poisonous to beneficial insects.

*Names marked in green are considered to be organic and IPM (integrated pest management) compatible.

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.

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