Rust Disease of Callery Pear. (Photo: MU-PDC)

With a cool, wet spring behind us, we have observed a heavy presence of foliar disease problems across Missouri's deciduous shade trees. While infections occur during a damp spring when leaves are first emerging, the fallout continues to persist into the summer months. As the mid-summer heat rises, homeowners are still noticing thinning foliage, leaf distortion, and premature leaf drop.

One major disease that causes leaf spots, lesions and defoliation of deciduous trees is Anthracnose, which is a general term covering a broad group of pathogenic fungi that attack a wide spectrum of shade trees and shrubs, notably sycamore, maple, oak, ash, elm, walnut, and flowering dogwood. These fungi overwinter in fallen leaf debris, twigs or small branch cankers, and infect the plants during the spring when the splashing rain and wind carry the spores upward. The symptoms start with small, dark brown circular or irregular-shaped spots, and quickly develop into large necrotic lesions, leading to significant defoliation. With less energy collected for the trees due to the loss of leaves, the canopy looks thin and ragged, and highly vulnerable to summer drought stress.

Leaf Spot and Rust Diseases of Buckeye. (Photo: MU-PDC)

Other prevalent foliar diseases in Missouri this year include fungal diseases such as rust disease, leaf blight, leaf blotch, leaf spot and powdery mildew, as well as bacterial disease such as fire blight, and bacterial canker disease.

In the most cases, foliar diseases that are discussed above don't result in tree death but can cause aesthetic issues. Although it might be too late to treat them with fungicide, the trees likely will survive and leaf out later in the season or next spring. Management right now should focus on reducing stress and disrupting the overwintering of the pathogens to protect next spring's budbreak. The most critical step in controlling these diseases is sanitation by removing the fallen leaves and branches this fall to remove the primary source of inoculums. Homeowners should also maintain proper irrigation for the tree if there is inadequate rainfall this summer. Lastly, use chemicals as the last resort. If the homeowners observe complete defoliation of their high-value trees in consecutive years, to treat anthracnose, fungicide with active ingredients (such as copper, chlorothalonil, mancozeb, or thiophanate-methyl) must be applied preventatively in the spring at budbreak and early leaf expansion. Liquid copper fungicide and antibiotics can be used to control bacterial diseases in the spring.

Although the symptoms of spring foliar diseases for deciduous shade trees may look similar, they can be caused by abiotic factors such as drought or chemical damage, or by infection of some severe diseases such as Dogwood Anthracnose, Oak wilt, Bacterial Leaf Scorch or Phytophthora Crown and Root Rot. Differentiating between cosmetic leaf spot diseases, environmental stress related leaf scorch, and destructive vascular diseases can be challenging. Missouri residents can submit high-resolution digital photographs or plant samples directly to the MU Plant Diagnostic Clinic for laboratory testing and diagnosis, as well as customed management recommendations.
For more information on sample submission, please visit the MU-PDC website.