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.
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.
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.
Sulfur (dust, wettable, flowable, or micronized) and potassium bicarbonate can be applied.
Bacillus pumilis and bacillus subtilis
Note: Names marked in green are considered to be IPM (integrated pest management) compatible.