Powdery Mildew Being a Pain? Effective Products for PM Relief.

Powdery mildew on cut roses in Ontario.

With May’s weather continuing to be up and down, humidity control is a problem in the greenhouse, meaning Powdery Mildew (PM) is too.  This pest is rearing it’s ugly head in crops like Kalanchoe and Dahlia.

We won’t cover the basics of PM here:  biology, spread and prevention were covered in a previous post.  Instead, we’ll focus on monitoring, and which control products to use once you’ve found an infection.

This post was written with help from Plant Pathologist Ann Zemke of the Chase Research Group.


Assuming you are doing all the preventative measures possible to avoid powdery mildew (see the cultural controls in our previous post), it’s still necessary to monitor crops on a regular basis for infection.  Scouting should occur on at least a weekly basis, or MORE if favorable conditions for this disease have occurred. 

Make sure the person doing the scouting knows that to look for, as initial signs can be subtle. Epidemics that seem to develop overnight are often the result of undetected low level infections that have spread spores throughout the greenhouse. 

The very beginning of a mildew issue often resembles spray residue when it first colonizes the plant. And, these first colonies are usually located on lower leaves; an area that often gets missed in a cursory walk-through.

Note that PM can look different on different plants.  Not every species will develop the characteristic talcum-powder-like spots.  Some plants simply develop a purplish discoloration, which can mimic nutrient issues.   Know what PM infection looks like in YOUR crop.

Surprisingly, some plants like Kalanchoe often develop brown scabs where Powdery Mildew infection occurs, and never develop the characteristic talcum-like white spots. Photo by OMAFRA.

Once you find an initial infection, immediately rogue infected plants or, if necessary, prune out diseased tissue. Always perform this operation when plants are wet or immediately place diseased material into a plastic bag to prevent spores from spreading.

Control Options:

Unlike most fungi, powdery mildews only colonize the surface of plants, making chemical eradication possible. To be specific, eradication means that these chemicals actually kill the internal infection and prevent further spread.

According to Plant Pathology consultant Ann Zemke, and supported by information from the IR4 program (an efficacy-testing program run by Rutgers University), the BEST products for PM eradication available in Canada are:

  • Nova (myclobutanil)
  • Compass (trifloxystrobin)
  • Heritage (azoxystrobin)
  • Palladium (fludioxonil + cyprodinil).  Currently, however, this combination fungicide is only registered for outdoor ornamentals, as well as greenhouse vegetables.

ALWAYS read the label for phytotoxicity warnings, and do a test batch on new varieties if necessary.  For example, Compass can cause injury to petunia, violets and impatiens, among others.

If you’re looking for products that are less toxic to workers and the environment (i.e. “biorational” pesticides), Milstop (potassium bicarbonate) is an excellent choice, both in terms of efficacy and for resistance management .  Note that it may leave residues on foliage if applied at high label rates, so it may not be appropriate for potted plants near sale.  Additionally, the microbial product Rhapsody containing Bacillus subtilis (var. QST 713) can provide good control of PM.   Though it’s unlikely to provide complete eradication by  itself, it can also be a useful component of a resistance management program for PM.

How Do You Know Your Products Are Working?

pesticide picThe mycelium (the white cottony matting) of the powdery mildew fungus should turn brown after it’s killed by a chemical or biorational product.  These dead colonies will remain on the leaf surface for a short time.  Plants in the treatment area should be monitored for incomplete kill or new growth to see if re-application of control products is necessary.

Multiple pesticide applications are likely to obtain control of a powdery mildew outbreak.   Products should be rotated to avoid development of resistance.

What To Do When all Else Fails:

  1. Still not getting control?  Your very next step should be to get your PM outbreak identified to species through the Guelph Pest Diagnostic Clinic. Many chemicals for PM only work on a few specific species of the species complex we call PM.  For example, Compass is a great product for Erysiphe azaleae and Spaerotheca pannosa species only.   But a product like Pristine can be effective against a third species,  Erysiphe polygoni, which may actually be what you’re dealing with.
  2. Consider your source of plants.  If you’ve received cuttings from the U.S. which may have already been heavily sprayed with fungicides (Dahlia’s, for example), you could be dealing with resistance issues.  If possible, attempt to get a spray history to see what chemistries should be avoided.
  3. Prevention is worth a pound of cure, especially when it comes to PM.  Consider revising your plan for next year.  PM prevention does NOT require the use chemicals.   Consider changing your ventilation strategy, increasing your plant spacing, or investing in resistant cultivars available for some plant species (e.g. African violet, begoniarose, pansy, zinnia, monarda, and phlox).

Unfortunately, it will take patience and persistence to eradicate an existing, well-established PM infestation.  This is why scouting, early detection, and a prevention plan for next year are the key to a better spring crop next year.

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