Extension Educator, Horticulture
Our cool wet spring has exhilarated trees to flourish with new growth, but the cool wet also encouraged a flourish of leaf-fouling fungi. When leaves stay wet for long periods, they are more susceptible to fungal diseases. Each year we see some spotting and browning of tree leaves; however, this spring's cool temperatures coupled with long periods of wet leaves from rain or dew is perfect for fungal pathogens to thrive.
This year maple trees, particularly silver and red maples along with their hybrids, are showing leaf problems. Often owners report "blighted" or blackened leaves. Stephanie Porter at University of Illinois Plant Clinic reported a flurry of questions concerning leaf spots on maple leaves in Illinois and Indiana. After investigating samples at the Plant Clinic her findings revealed not just one fungal disease on the leaves, but multiple pathogens.
Anthracnose was the main culprit. Sycamore (or maybe it should be "sickagain") suffer from this fungal disease to some degree each year. This year seems to be maple's turn at anthracnose. Other trees such as ash, oak, elm, and walnut can also get anthracnose.
Maple anthracnose is characterized by irregularly shaped brown to black spots or blotches that follow the veins of leaves. Leaves are often distorted. Leaf symptoms are variable depending on the maple species. Entire young leaves of Japanese maple may become blackened and shriveled. Purple to brown streaks may develop along the veins on Norway maple. Small to large, round to irregular, green brown or red brown areas develop along or between the veins of sugar, Norway, and Japanese maples. If numerous, the lesions may merge, affecting the entire leaf. Severely infected leaves may fall prematurely. Less infected leaves may remain all season.
UI Plant Clinic also found Venturia maple spot and Maple leaf blister. Forebodingly the pathogen that causes Venturia maple spot is similar to the pathogen that causes apple scab and the pathogen that causes maple leaf blister is similar to the pathogen that causes peach leaf curl. In our office we have also seen several samples of peach leaf curl this spring.
So what is the bottom line on these "blighted" maple leaves? The good news is healthy trees that lose their leaves early will almost always produce new leaves within several weeks. As the season progresses into warmer drier periods, the mature leaves are fairly resistant to infection. Leaf spots make the tree unsightly until a new flush of leaves brings healthy growth.
As for disease management once infection occurs it is too late to use fungicides. Generally the best management option is to promote good tree health: water during drought periods; remove dead or dying branches; and fertilize if necessary in fall. Apply 2 to 4 inches of wood chip mulch to the soil around a tree. Ideally it should extend to the dripline or outer branches of the tree. Do not pile mulch next to the tree trunk.
For additional information and management on tree leaf spots, check out the following fact sheet: http://web.extension.illinois.edu/forestry/publications/pdf/forest_health/UIUC_Leaf_Spot_Diseases_of_Trees.pdf
After last season's drought some trees may have additional problems. We also saw leaf damage from the hot dry winds that desiccated young, succulent leaves. If trees have green plump buds on the twigs, they should releaf. Try scratching the twigs with your thumbnail. It should look green and moist under the outer layer. If buds and branches are dry and dead, it may be time to go tree shopping.
In addition samples (with pictures if possible) can be mailed or taken to the UI Plant Clinic S-417 Turner Hall, 1102 S. Goodwin Ave. Urbana, IL 61801 Phone: 217-333-0519. Check their website for proper packaging and fees and lots of great info. http://web.extension.illinois.edu/plantclinic/