Powdery mildew of apples and pears

Class: Fungi

Common Name

Powdery mildew of apples and pears

Scientific name

Podosphaera leucotricha

Potential Hosts

Apples and pears

Who am I?

Powdery mildew is a multi-host fungal disease with worldwide dispersion. It usually appears on the surface of leaves as white or grayish spots with a powdery like texture, hence the name.

Generally, the disease is host-specific, which means that it can only infect plants from the same genus or family. In this case, the pathogen Podosphaera leucotricha infects apples and pears and to a much lesser extent peaches and quinces.

‏‏Powdery mildew is favored by warm climate, moderate to high humidity, low light conditions, and does not require the presence of free standing water.

Powdery mildew can cause considerable loss due to nutrient removal, reduced photosynthesis, increased respiration and transpiration, and impaired growth.

Control measures

Cultural

The sooner the better: It’s easier and more cost effective to overcome infestation by controlling powdery mildew during the initial stage 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, one could benefit from powdery mildew being generally host-specific and not worry about adjacent neglected crops. However, it is important to remove all the debris from infestations; 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 matters a lot.

Simple sanitation measures are an important step and should be at the foundation of plant protection programs.

Organic

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

Biological

Bacillus pumilis and bacillus subtilis

Conventional (chemical)

There are differences in the dynamical nature and behavior of powdery mildew on different hosts. Growers and consultants tend to have their own treatment methods, different approaches, fungicides preferences, and secret tricks. Still, some consensus does exist: prevention, rotation, and the use of several fungicides that each belong to a different group (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, 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 it’s 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 resistances to a specific chemical.

*Some commercial fungicides have two active ingredients and two modes of action. After using such products, take into account that now two groups were already used. Make sure to exclude those two active ingredients in the next application.

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).

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.

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