The leaf-spotting diseases that we see at this time of year on trees and shrubs are aesthetically damaging but usually do not cause a significant effect on plant growth to cause alarm. Of course, there are always exceptions, such as the damaging dogwood anthracnose fungus or downy mildew on rose. Start by looking in disease texts, university fact sheets, or this newsletter to see what diseases a particular plant is known to host. Compare that information to the symptoms you are seeing on your plant. This preliminary work may help you determine whether you need help from the Plant Clinic, an Extension office, or an arborist.|
So far this spring, the Plant Clinic has received samples of Phyllosticta on hydrangea; Coniothyrium on viburnum; and anthracnose on sycamore, birch, maple, oak, and walnut. There is a myriad of fungi that could occur on other tree hosts. Most of these diseases are treated in the same manner.
These fungi are all around us in the environment. They are able to infect succulent new growth in the spring because the leaf cuticle is still developing and the leaf is “tender.” In addition, the plant grows slowly in cool, wet conditions, while fungi thrive in those conditions. In a warm, dry spring, the fungal leaf diseases are minimal. Additional infections occur as more leave emerges in cool temperatures and rain showers. Eventually summer temperatures no longer favor the fungus, and leaf tissue “hardens off.”
Anthracnose is the most common group of deciduous tree leaf diseases. Although the anthracnose name causes most people to think of one specific disease on one host, it is actually a term used to refer to many diseases caused by several fungi that produce fruiting structures called acervuli. The anthracnose diseases of trees may cause leaf spotting alone (ash and maple anthracnose), stem cankers (oak anthracnose), or a combination of both (sycamore anthracnose). The causal pathogens are fungi, including species of Discula, Discella, Gloeosporium, Monostichella, Kabatiella, and Colletotorichum. In terms of disease management, it is not necessary to know the exact fungus producing the symptoms. Probably the most dramatic leaf spotting and leaf drop we see in Illinois from anthracnose is on ash, sycamore, and walnut. A large quantity of leaves with brown to black spots and blotches may have fallen from infected trees this spring. With warmer weather and rain, the trees will produce a new flush of leaves and recover. That is probably happening now.
You will find many fungicides listed by host and disease in university pest-control handbooks. This information is taken directly from product labels and is intended to list products registered for use against various diseases. Keep in mind that few fungicide applications recommended on landscape plants work as rescue treatments. In most cases, the use of fungicides in the landscape is a protective barrier against infection, not a cure of an existing infection. There may not be an immediate remedy for disease infection. Disease control often includes assessing the severity of an infection (including an accurate diagnosis) initiating immediate cultural management procedures, and following through with a fungicide treatment the following year if needed. In most cases, we do not recommend the use of fungicides for anthracnose or other early season leaf spot disease management on trees and shrubs. Instead, improve plant vitality by watering in periods of drought, fertilizing in the fall or early spring with a balanced fertilizer, and removing dead wood. Try to make conditions less favorable for the fungus by removing fallen foliage, thinning plants as necessary for better air flow and quicker plant drying, and watering from below rather than above. Watering early in the day to allow plants to dry before evening is also helpful.
For further information, consult Report on Plant Disease (RPD), no. 648, “Leaf Spot Diseases of Shade and Ornamental Trees in the Midwest”; no. 621, “Anthracnose Diseases of Shade Trees”; or several other reports discussing leaf spots on specific hosts: RPDs, no. 600, black walnut; no. 601, mountain-laurel; no. 637, hawthorn; and no. 638, firethorn. These are all available in Extension offices or on the Extension VISTA Web site.