Powdery Mildew of Cucurbits Alert


Symptoms include the formation of white powder-like spots on the upper side of leaflets. When left untreated, white powdery-like texture can cover the entire leaf, leading to defoliation eventually. Powdery mildew can cause considerable loss due to nutrient removal, reduced photosynthesis, increased respiration and transpiration, and impaired growth. Fruits are not affected.

Who Am I?

Powdery mildew of cucurbits is a fungal disease that targets only members of cucurbits. The fungi does not need the presence of water on leaf surfaces for infection an to occur. However, spores need dew to germinate and can be carried by the wind.

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 the infestation and overcome it during the initial stage of infestation.

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

*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.

*Never mix products based on sulfur with any kind of oil. Have a period of at least 14 days separating an oil application and a previous sulfur application—and vice versa.

*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 you have two groups that are 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).

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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