Powdery mildew of roses
Class: Fungi
Common Name
Powdery mildew of roses
Scientific name
Podosphaera pannosa
Potential Hosts
Roses
Who am I?
Powdery mildew of roses is a fungal diseases with worldwide dispersion that 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 pannosa infects different varieties of roses.
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 losses due to nutrient extraction, reduced photosynthesis, increased respiration and transpiration, impaired aesthetics, and overall slower growth rates in roses.
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Control measures
Cultural
The sooner the better: It’s easier and more cost effective to overcome infection by controlling powdery mildew during its initial stage. Make it a routine to monitor the field regularly and search roses for the presence of powdery mildew on a weekly basis.
Sanitation: In most cases, it is beneficial that powdery mildew is generally host-specific; there is little worry regarding adjacent crops that are neglected. However, it is important to remove all the debris from outbreaks. Otherwise, it can affect roses the following season. There are many powerful commercial fungicides and the amount of inoculum originating from last year's outbreak matters significantly.
Simple sanitation measures are an important step and should be at the foundation of plant protection programs.
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 each belonging to a different group of chemicals.
Effective control requires spraying with high pressure and 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 used in one or more parts of the world and is sorted into groups according to 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 C. 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 14 days. Do not use products with 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 powdery mildew from developing resistance to specific chemicals. 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).
Some commercial fungicides have two active ingredients and two modes of action. After using such products, take into account that now you have two groups that you already used. So, make sure to exclude those two active ingredients in the next application.
Organic
Sulfur (dust, wettable, flowable, or micronized) and potassium bicarbonate can be applied.
Biological
Bacillus pumilis and bacillus subtilis
Note: Names marked in green are considered to be 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.