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Over the Garden Fence

Why are some plants’ leaves white now?

What do lilacs, phlox, vine crops, peonies, and lawns all have in common this time of year? First clue – it is weather related. Second clue – if you touch it, it will rub off. Final clue – it looks like it came out of the kitchen pantry and you would sprinkle on your pastries, pancakes, and waffles.

There may be a bunch of guesses, yet the right answer is a foliar fungal disease called powdery mildew (PM). There are many more plants in the home landscape that can be infected by PM, the ones listed above are a few of the more common plants we are all familiar with. Hosts for this foliar disease can be herbaceous like a phlox or woody like the lilac. The giveaway clue was what came out of the kitchen pantry, powdered sugar, as the biggest visual clue of PM will be the foliage covered in a white powdery looking material (the fungus itself). Plant pathologists consider this a sign that the pathogen is present, something we can see. Other symptoms include stunting, dwarfing, curling of leaves, chlorosis (yellowing), and disfigured shoots and flowers. These symptoms are how the plant is reacting to the pathogen.

As PM has a large range of host plants it can impact, the fact that there are more than 1,000 different fungi in six genera makes sense. Too much science? Just know that for each host plant, there is a specific PM fungus, so while all the plants look like they have the same thing (and they do in a sense) each plant is host to its own version of PM.

The weather-related part has to do with our weather pattern of hot days with humidity. This time of year we see a lot of PM, starting around August and continuing into fall. It can get started as early as June though. Maybe you saw it on the phlox much earlier this summer?

There are some best practices for slowing, preventing, and limiting PM. Planting disease resistant varieties is a great start, especially with vegetables and herbaceous flowering plants. Stress can really impact how plants resist disease, so keep the vigor up, water before a drought shows up, and during as well. When we are out there watering, keep the foliage dry or if you have to wet the foliage to do the watering, early in the morning is best, as having the foliage dry by nightfall is critical. Stay out of the garden and flower beds when the plants are wet, you just help spread PM around when you do.

If you have that prized plant or one that has come down through the family, chances are there is limited or no disease resistance there. Preventive fungicide sprays may be your best bet if all other cultural efforts do not work. Sprays need to be applied at least two weeks before you ever notice any symptoms and then repeated per the label instructions. Always read and follow all safety and label instructions. Learn more about PM with the Home, Yard and Garden Pest Newsletter

About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.