In this crazy-weather year, we are seeing spring and summer diseases at the same time, which means that powdery mildew is already here. Ordinarily it comes in after the spring rainy season.
Powdery mildew (PM) is a fungus disease that looks just like it sounds — it’s a white mildew rather than the gray coating you see on the boots left at the back of the closet. PM fungi grow right on the surface of a living plant. They insert absorbing outgrowths called haustoria into the epidermal cells to extract nutrients and moisture from their host plants. Thus PM-parasitized plants grow a little less vigorously — but they also look much less ornamental, because they are spotted or coated with white mildew. One good thing about powdery mildew is that it almost never kills the plant that feeds it.
Many crops have a PM disease, and some unlucky species have more than one. Some of the PMs are rather host-specific, while others can choose from a menu that includes multiple plant families. A number of our favorite perennials are favorites of PM fungi: phlox, monarda, dahlia, and peony come quickly to mind as plants that have PM as their nemesis. Chains of spores called conidia are produced on the strands of mildew on the plant surface; once mature, the conidia are easily wafted about the greenhouse or garden on air currents. If they land on a compatible host plant, only low levels of moisture are needed for PM spores to germinate and infect. (This is in sharp contrast to the spores of Botrytis cinerea, which need a very wet leaf surface for a minimum of four hours to germinate and infect). It’s hard to stop a PM disease by improving cultural conditions because mildews thrive in circumstances that are ideal conditions for the plant.
There are some tricks to monitoring for powdery mildew. It’s such an obvious disease, you would think it would be easy. But powdery mildew can hide on the undersurface of leaves, so that you won’t realize there is inoculum unless you scout very thoroughly. On some plants, PM forms classic colonies, circular patches of white that each developed from a single spore. In other cases, PM forms thin coatings of hyphae across the leaf surface, and you may not notice the fungus until the leaves begin to turn yellow or purple. And the biggest challenge is to identify powdery mildew on sedum: Quite often, the symptoms are brown scabby spots that don’t look a thing like normal PM. It will take a hand lens or microscope and some sleuthing to figure out that a PM fungus is growing on the leaf surface and causing those dead areas. Ordinarily PM grows without generating chlorosis (yellowing) or necrosis (browning) in its hosts, but there are some exceptions to this rule, including the brown scabby spots formed on sedum. We think of PM as a leaf disease, but at times unusual plant parts may be infected: with some PMs you will see PM on stems, or on the calyx of the flower, or only on the petals.
Preventing PM takes some strategizing. Avoiding excessive relative humidity is always helpful — keep the greenhouse below 85 percent RH. Circulate the air, and don’t crowd plants. If you get a biocontrol there first, say a Bacillus or Streptomyces species, and continue applications at frequent intervals, you may be able to effectively block the accumulation of mildew on a susceptible plant. But it’s best to choose genetics that do most of the work for you. Usually there are alternative cultivars in the same color that are not so susceptible, so shop for the least susceptible representatives of the crops you want to grow. Another approach is to scout carefully and respond to the first colony with a well-thought-out fungicide control program, quickly implemented.
Rotations of fungicides with different modes of action will allow very good control of PMs. Try switching between contact and systemic materials, or a three-way rotation with contact, systemic and biological. Potassium bicarbonate is one of the effective contact materials, and there also are a good number of partly or fully systemic fungicides that are very effective against PM. Some of these come as strategic mixtures to slow the development of fungicide resistance. Thoughtful PM management programs, started before the disease appears, will allow you to grow healthy crops of even phlox and monarda. Practice IPMM — integrated powdery mildew management — for best results.