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Over the Garden Fence

Weather influences foliar disease

The weather can, does, and will influence foliage disease each year, starting in the early weeks of spring. While early spring was a long time ago, many diseases are now quite visible in the home landscape.

Ornamental flowering crabapples and apple trees have had two foliage diseases in force this summer. Both are bad, one more so than the other. Apple scab is the one that will cause the leaves to fall to the ground by mid-summer, robbing the tree of producing energy to create next year’s flower and foliage buds. The fungus overwinters on those fallen leaves, so a big thing would be to rake up those leaves this fall. Cedar apple rust (CAR) is the other disease and looks quite colorful on the tree leaves with red and yellow surrounding the infection points. CAR overwinters on needled evergreens – certain junipers and cedars. CAR will float back through the air early next spring to start all over again.

The maple trees have had several fungal diseases. One very noticeable is tar spot, where, as it matures, it appears as a ½-inch circle of “tar” on the leaf, and there can be many such spots. Earlier in the season, the spot is yellowed and filled with fine black dots.

I covered powdery mildew last week in detail, yet it deserves mention again. This one is a disease that waits for hot, humid weather, and then can cover entire plants in a just a few days.

Lawn disease has really been common this season. Lawn diseases can happen from spring through fall. It could be spots just a couple inches in size or regrettably it can take out large patches or the entire lawn. One disease called Helminthosporium attacks in the spring as “leaf spot” and comes back later in the summer as “melting out,” where large patches of grass just seem to melt away and disappear. There are a number of “patch disease” fungi as well. Older lawns are more susceptible to any lawn disease, as are lawns that are stressed from exposure, lack of moisture, etc.

The key to controlling any fungal disease is timing. Any fungicides will need to be applied starting 10 to 14 days ahead of any expected outbreak and repeated until the weather changes and is no longer favorable for disease development. For example, with cedar apple rust and apple scab, both get started while the weather is cool and moist in early spring (i.e. sweatshirt weather and earlier). For optimal production, preventive sprays will continue through about three-quarter flower petal fall, so that is quite a while and a lot of sprays. Lawn disease is not different, though timing depend on whether you get a spring or summer disease. For homeowners, the threshold to justify treatment is often that of aesthetics and not economics. If you choose to treat, make a note now in your 2021 calendars and always read and follow label instructions.

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.