Powdery mildew

Powdery mildew is probably the most common vegetable disease urban gardeners encounter. Those powdery little spots appear mid summer, and by the end of the summer they’ve covered entire leaves with their fuzzy white powder. While you might not be so sad to see those overly prolific zucchini decimated, the cucumbers and pumpkins won’t be far behind. So what causes this disease? Why do we get it every single year? And what can we do about it?

What causes it?

Powdery mildew is caused by a fungal, “obligate” pathogen. So what does that actually mean? An obligate pathogen means that it requires a living host, in this case a plant, to reproduce and survive.  Think of it like this: the pathogen wants to hijack the plant's resources for its own benefit, instead of producing those delicious vegetables. This lifestyle strategy is going to affect how we manage the disease and how it infects your plants. While powdery mildew is most common on cucurbits like cucumbers, squashes and melons other plants such as tomatoes, onions, peppers, chards and even sugar peas. However, many of these powdery mildew diseases are caused by different fungal species. Because powdery mildews are obligate pathogens this often means they have a very specialized relationship with their host plant. The powdery mildew on your squash is going to be different than the powdery mildew on your tomatoes, and it can NOT go from one to the other. I'll be providing information about powdery mildew management in general, but some specifics might not be appropriate for all species.

Where does it come from?

So I’ve just told you that powdery mildew is an obligate pathogen and it requires a living host. You’re first thought is probably, “If it requires living plant tissue, and isn’t living in the soil over the winter, how does it show up every year?”  Powdery mildew is an airborne pathogen. It’s spread as air currents flow across the U.S. Just because places like Michigan have winter, not all places do. Powdery mildew can survive in those places, and then as the weather warms it can slowly creep its way farther north. Even if we could somehow prevent it from moving up from the south, greenhouse grown plants could be another source of the pathogen. 

Early symptoms of powdery mildew. The small white spots can be rubbed off (but don't rub them otherwise you can spread the pathogen).

What does it look like?

The symptoms of powdery mildew are pretty much what you would expect. They start as small white powdery spots, sometimes they’re also a bit chlorotic (yellow), on the upper sides of the leaf and as time progresses those spots will expand and spread. Often lower leaves show symptoms first.

Conditions that favor the disease

Powdery mildews like high relative humidity, without a lot of a leaf wetness (Granke et al 2012). This is one of the reasons the first leaves to show symptoms are generally the lower leaves. The lower leaves are more protected from rainfall and are buffered by the surrounding leaves so they often have a higher relative humidity than upper leaves in the canopy. This is one reason why adequate plant spacing is so important for minimizing diseases. Low humidity promotes spore dispersal (like tiny airborne seeds), while the high humidity promotes infection (Clemson). Temperature can also be a factor - powdery mildew does not like it too hot, so those hot hot days are not going to be good for the pathogen.

Later symptoms of powdery mildew. This shot is of a leaf that was lower in the leaf canopy than the top picture. I moved the top leaves aside and hiding underneath was this plethora of powdery mildew spores.

Management

While prevention is the best way to manage a disease, at this point in the growing season minimizing the effect of disease is the next best thing. Whether you want to use chemical sprays or organic methods, early detection of the disease (through scouting) is critical for successful management. Studies have shown that sodium bicarbonate (baking soda) and whole milk can help minimize the growth and spread of powdery mildew. Baking soda, when used at a rate of 0.5% (weight per volume) combined with an oil, like SunSpray Ultra-Fine Spray Oil (also at a rate of 0.5%), reduced disease severity when sprayed on the plant's leaves (Ziv and Zitter 1992). Commercially available products like Kaligreen (82% potassium bicarbonate) can also help suppress the disease (Mcgrath and Shishkoff 1999). Whole milk, not skim, reduced disease severity at a higher rate than just baking soda in a more recent study by Ferrandino and Smith (2007).  The authors used a 50% milk to water spray, and did state that good coverage was needed, however that the cost may be prohibitive for large areas.

While these are options that should be readily available in most households, these are not as effective as chemical fungicides when the disease severity is high. So if you plan to use these products then you should definitely be careful to scout frequently and do your best to catch it early. If the infected leaves are few and far between you could easily remove a few leaves here and there. But likely you will have latent infections (when the plant is infected but you can’t see the disease symptoms), and you’ll have more diseased leaves soon.

Don't confuse variegation with powdery mildew. This squash has variegated (patterns of light and dark) leaves, but no powdery mildew.

Remember, powdery mildews need a living host. This means that the pathogen’s goal is to make your plants make food for them instead of you. Your goal is to keep ahead of the pathogen so that your plants can spend all their photosynthetic energy on making those delicious veggies you want to eat.

References contributing information in this post:

Clemson University Cooperative Extension bulletin

Ferrandino F.J., and Smith V.L. 2007. The effect of milk-based foliar sprays on yield components of field pumpkins with powdery mildew. Crop Protection 26:657 -663.

Granke L.L., Crawford L.E., and Hausbeck M.K. 2012. Factors affecting airborne concentrations of Podosphaera xanthii conidia and severity of gerbera powdery mildew. Hortscience 47:1068-1072

Mcgrath M.T, and Shishkoff N. 1999. Evaluation of biocompatible products for managing cucurbit powdery mildew. Crop Protection 18:471-478.

Ziv O., and Zitter T.A. 1992. Effects of bicarbonates and film-forming polymers on cucurbit foliar diseases. Plant Disease 76:513-517