This story was originally published in September 2020.
It appears as white spots on leaves at first. Soon, it expands, growing a powdery appearance that looks like powdered sugar covering leaves. Have you seen this foreign substance on your plants? If so, they may be afflicted with powdery mildew.
“It looked like somebody walked into your garden with a dough boy and dropped it on your plant,” said Alicyn Smart, assistant extension professor and extension plant pathologist at the University of Maine Cooperative Extension. “Typically we see it on the topside of the leaf but we can also see it on the stem of plants as well as in some cases fruit.”
Powdery mildew is a fungal disease that commonly affects gourds like squash and pumpkins. It can also afflict a number of plants in Maine gardens, including tomatoes (particularly when grown in a greenhouse) and ornamental plants like lilacs, bee balm and phlox.
“There are many species of powdery mildew, and many plants that can be infected, usually only by one of the powdery mildew species,” said Caleb Goossen, organic crop and conservation specialist at the Maine Organic Farmers and Gardeners Association.
Smart said that under a microscope, powdery mildew looks almost like white Tic Tacs stacked on top of each other. When hit with rain or wind, the spores will move across the leaf on that plant or to another plant. Though environmental conditions can cause powdery mildew outbreaks on multiple crops, the fungus present on one type of crop is unlikely to spread to another.
“If you have powdery mildew on your lilacs, and then you notice it on your winter squash, that is purely a coincidence,” Goossen said. “You have two species of powdery mildew, it did not spread from your lilacs to your winter squash.”
Plants tend to be most susceptible to powdery mildew infection when flowering or fruiting because they are devoting so many of their resources developing flowers or fruits instead of defending against fungi. Powdery mildew spreads best in humid conditions, but unlike most fungal diseases, it doesn’t require the leaf to be wet when it infects it.
“The interesting thing about powdery mildew is that most fungal diseases prefer a moist leaf surface whereas powdery mildew requires high relative humidity and dry leaf surfaces,” Smart said. “We were lucky during the summer and now we’re starting to see it.”
Powdery mildew interferes with photosynthesis, causing leaves to yellow or die back. Powdery mildew won’t make your garden crops inedible, but it may reduce the sugars available to the fruits and impact the taste.
“When winter squash are heavily infected, they lose their leaves, which can mean there’s not enough sugar being made to have really tasty winter squash,” Goossen said. “Also the squash may not store as well into the winter, and the lack of leaves can also lead to sunscald of the fruit, because leaves usually shade the squash as it develops.”
Smart said that a little bit of powdery mildew is “not a big deal” and may not impact that plant too severely. If you catch powdery mildew early, when the leaves are exhibiting only a few white, dusty spots, you can try to address it in a number of ways. You can remove the leaves, or Smart said you can splash them with a little water, though this comes with its own risks.
“The spores will explode with the contact of water, but then you’re opening the door to other fungal diseases that do like moist leaf surfaces,” Smart said. “It will do more harm than good if the plant is already stressed due to powdery mildew and other diseases likely to take advantage of that situation.”
You can also apply some fungicides that will address powdery mildew.
“Effective fungicide treatments for powdery mildew usually contain sulfur, or potassium bicarbonate and horticultural oil can be applied together,” Goossen said. “Care needs to be taken care with any of these treatments, even if they are an OMRI-listed product, which signifies that it’s allowable for use in organic growing, as sulfur or horticultural oil can burn plant leaves, or possibly be dangerous if not used properly.”
To prevent powdery mildew in future years, make sure you remove plant material after the growing season is done.
“Powdery mildew is called a facultative fungi, meaning it requires a host,” Smart said. “It hangs out on plant material in the winter. That’s why one of the management [techniques] is to simply remove the plant material from wherever you’re growing your plants.”
Also, be careful about spacing and pruning in order to increase airflow.
“The spacing would simply cause more air movement so that there wouldn’t be high humidity near the plants which is usually the case if plants are overcrowded,” Smart said. “Within the canopy has high humidity which is favorable to the disease. Also, you don’t want to over-fertilize.”
Goossen and Smart agreed, though, that one of the best ways to avoid powdery mildew is to choose varieties of plants that are resistant to it. When shopping for seeds, check the descriptions on seed packets for powdery mildew resistance, or filter your search on seed company websites for powdery mildew resistance.
“In especially vegetable crops, like squashes and tomatoes, there are resistant varieties out there,” Smart said. “That’s the kind of lowest hanging fruit people can use to mitigate this disease.”
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