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August 2024
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You may have observed a major difference between this year and our last few growing seasons: plenty of precipitation.
According to the DNR’s Climatology Office, the amount of rain that’s fallen from April through August 2024 has been nearly the most on record over a good share of Minnesota. Overall, this is good news for our forests – but all that near- historic rainfall has had additional impacts to trees.
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Severe apple scab on crabapple trees resulting in
leaf loss from the lower canopy.
As shoots and leaves emerge in the spring, they’re particularly susceptible to infection from fungal pathogens. Extremely wet weather during this timeframe promotes sporulation (formation of spores) of these microscopic fungi, and rain spreads the spores around. The end result is a slew of common leaf diseases that we’ve seen in 2024: anthracnose on ash, maple, and oaks; apple scab on crabapples; oak leaf blister; Marssonina leaf blight on aspen; Septoria leaf spot on balsam poplar; and bur oak blight on bur oaks.
Continued rain through summer months exacerbates earlier spring fungal infections and promotes additional diseases, such as Tubakia leaf spot on oak.
Leaf disease pathogens frequently kill spots on leaves. Throughout the summer, these spots expand and merge, resulting in a larger brown blotch or a completely brown or almost black leaf. Heavily diseased leaves fall off the tree, although with some diseases, such as bur oak blight, trees shed many leaves that still are mostly green.
 Bur oak blight in mid-September. Note the rim of healthier green leaves.
An easy way to diagnose if a tree has a fungal leaf disease is to notice where the leaf browning or leaf loss is worse. If it is far worse in the lower and inner part of the canopy, and if there remains a rim of green leaves at the edge of the canopy in late summer, then you can almost be assured the culprit is a fungal leaf disease.
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Fortunately, fungal leaf diseases do not seem to have a major negative impact on trees. They certainly make the tree look unhealthy, but healthy trees will fully leaf-out the following spring.
Perhaps the most widely spread leaf disease this year in Minnesota is Marssonina leaf blight on aspen. Not only does this pathogen cause brown spots, blotches, and leaf drop, but it also stunts leaves.
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Severe leaf infection on a quaking aspen in mid-August.
This and other aspen leaf diseases frequently impact clumps of aspens, while healthy aspens growing immediately next to them are unaffected. The reason for this clumpiness is genetic tolerance to the disease or different leaf emergence timing in spring. |
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Marssonina leaf blight on quaking aspen. A susceptible aspen clone is on the left and a more resistant or tolerant clone is on the right.
Another common leaf disease this year is bur oak blight. Bur oak blight symptoms are particularly severe on scattered bur oaks in northwestern Minnesota, but they do extend down through the Metro area on scattered bur oaks.
Finally, let’s not forget the impact of the last several years of droughts. Droughts negatively impact tree growth and health, and the impact of drought can last for years. In fact, some of the early fall colors you might see in your tree could be due to the 2023 drought. Direct and indirect impacts from drought start at the edge of the canopy, killing the ends of branches.
So, ugly or missing leaves in the inner canopy? Not to worry. If the cause is a leaf disease, leave your tree alone and it’ll be fine next spring. It probably needed a lot of rain anyway to make up for the deficit it experienced in recent years.
Dying branch tips? You’ve got bigger problems. It might be from past droughts, root damage, or insect infestation. Check out our oak decline webpage, and consider hiring a professional arborist.
Left: Oak leaf blister in early June 2024. These blisters turned brown later in summer.
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