Evaluating Leaf Spots on Shade Trees

This year the MU Plant Diagnostic Clinic has received a large number of tree samples with spotting and blighting of the leaves, often causing early defoliation this fall. Leaf spots can be caused by many different species of fungi and bacteria, as well as insect injury. Fungi cause most of the leaf spot seen this year at the clinic. The trees and associated fungal leaf spot diseases seen this year include anthracnose diseases on several species, ash with Mycosphaerella leaf spot, buckeye with Guignardia leaf blotch, crabapple with cedar apple rust and apple scab, elm with black spot, hickory with Gnomonia leaf spot, and oak with Actinopelte (Tubakia) leaf spot. The fungi that cause these symptoms are prevalent during wet, humid conditions such as we frequently experienced this summer.

We are commonly asked what effect this damage will have on the affected tree? In many cases, these symptoms have developed in late summer. Normally these diseases don’t seriously affect the health of a tree, especially when they occur late in the season after much of the photosynthesis has taken place for the year. So, even though these spots may be unsightly and hurt your fall color display, control is often not necessary. However, if the tree has been weakened by other factors, such as transplanting or repeated defoliation, or is declining, chemical control may be needed next year if conditions are favorable again for disease development. Materials used for controlling leaf-spotting fungi include azoxystrobin, fixed copper, Bordeaux mixture, captan, chlorothalonil, thiophanate methyl and triforine. Remember to read and follow the label directions.

In most situations, nonchemical practices are most practical. To reduce the level of infection next year, remove and destroy fallen leaves and branches this fall and winter. Prune out dead twigs and branches, as well as any growth that has become overly dense and is reducing air movement. Make sure any pruning you do to oaks occurs well before early spring to prevent the open wounds from attracting the beetles that spread oak wilt disease. In addition, Actinopelte leaf spot is often more severe in oaks with iron chlorosis, so be sure to treat with iron chelates if this is a problem. While light use of fertilizers high in phosphorus and low in nitrogen to stimulate root growth next season can help promote vigor, remember to avoid the overuse of nitrogen fertilizers that can cause new leaves to be succulent and more disease prone. When designing new plantings, do not crowd plants. Overcrowding leads to poor air circulation and creates a favorable environment for these leaf spotting pathogens. See http://agebb.missouri.edu/pdc/diagnosticreports/leafspot01.htm for more information. You are also welcome to submit samples for diagnosis and management suggestions to the diagnostic clinic.

Simeon Wright, MU Plant Diagnostic Clinic, (573) 882-3019