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Columns

Down the Garden Path: Weather influences foliar disease in the landscape

Richard Hentschel
Richard Hentschel

The weather can 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 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 its important to rake up those leaves this fall. Cedar apple rust 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 half-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, but it deserves mention again. This one is a disease that waits for hot, humid weather, and then can cover entire plants in a only a few days.

Lawn disease has been common this season. Lawn diseases can happen from spring through fall. It could be spots only a couple of inches in size or, regrettably, it can take out large patches or the entire lawn. A 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 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 fungal disease is timing. Fungicides need to be applied 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 when 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, although timing depends on whether you get a spring or summer disease. For homeowners, the threshold to justify treatment often is that of aesthetics, not economics. If you choose to treat, make a note now in your 2021 calendars and always read and follow label instructions.

University of Illinois Extension Master Gardeners have been on the job working the entire season by email, so if you have gardening questions contact volunteers near you: Kendall County atáuiemg-kendall@illinois.edu, DuPage County at uiemg-dupage@illinois.edu, and Kane County atáuiemg-kane@illinois.edu.

• Richard Hentschel is a Horticulture Extension educator with University of Illinois Extension, serving DuPage, Kane and Kendall counties. This column originates on his blog at http://go.illinois.edu/overthegardenfence. To get more tips from Hentschel, watch his “This Week in the Garden” videos on Facebook and YouTube.

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