Early-season leaf diseases of trees often cause undue concern and alarm for homeowners. Master Gardeners and landscape maintenance personnel also often become very adamant about making exact fungal identifications. It is good to know that many are being very careful about making a proper diagnosis. Still, when it comes to spring and leaf spotting diseases of deciduous trees, don’t lose too much sleep worrying about an exact fungal identification. You might see Septoria on magnolia; Phyllosticta on maple; Cercospora on sweetgum; an anthracnose on sycamore, ash, oak, or maple; or a myriad of other fungi on other tree hosts. Most of these diseases will be treated the same.
Will we see these leaf diseases in 2000? Most of these fungi infect succulent new growth because the leaf cuticle is still developing and the leaf is “tender.” Early-season infection generally requires cool and wet conditions as the leaves are emerging. Because Illinois average temperatures have been much warmer than usual this year, and the weather has been much drier than usual, we don’t expect to see major problems with leaf diseases. Keep in mind, however, that we have had many showers, and if these showers coincide with cool temperatures and leaf emergence, the fungal leaf diseases will result. In fact, anthracnose has been seen on sycamore; and a case or two of maple anthracnose has been reported to the Plant Clinic this year.
Anthracnose is the most common group of deciduous tree leaf diseases. Although the term “anthrac-nose” causes most people to think of one disease, it is actually a term used to refer to diseases caused by fungi that produce fruiting structures called acervuli. This is of significance to laboratory diagnosticians but is not of much value in the field. The anthracnose diseases of trees may cause leaf spotting alone (ash and maple anthracnose), stem cankers (oak anthracnose), or a combination of both (sycamore anthrac nose). The causal pathogens are fungi including species of Discula, Discella, Gloeosporium, Monostichella, Kabatiella, and others. In terms of disease management, it is not necessary to know the exact fungus-producing symptoms. Probably the most dramatic leaf spotting and leaf drop we see in Illinois from anthracnose is on ash, sycamore, and walnut. A tremendous amount of leaves with brown to black spots and blotches may fall from the tree. With warm-er weather and rain, the trees will produce a new flush of leaves and will recover.
There are few fungicide applications recommended on landscape plants that will work as rescue treatments. In most cases the use of fungicides in the land scape serves as a protective barrier against infection, not a cure of an existing infection. Although we may not be able to provide immediate remedy for disease infection, we have time to assess the severity of an infection, follow up on accurate diagnoses, initiate cultural management procedures, and then to use a fungicide the following year if needed. In most cases we do not recommend the use of fungicides for anthracnose disease management on trees. Instead, improve tree vitality by watering in periods of drought, fertilizing in the fall, and removing dead wood. For further information, consult Reports on Plant Disease (RPDs) No. 648, Leaf Spot Diseases of Shade and Ornamental Trees in the Midwest; No. 621, Anthacnose Diseases of Shade Trees; or several other reports discussing leaf spots on specific hosts: RPDs 600 (Black Walnut), 601 (Mountain-Laurel), 637 (Hawthorn), and 638 (Firethorn). These are all available in Extension offices. RPDs 601 and 621 are also available on the Web at www.ag.uiuc.edu/~vista/pubs.html under horticulture publications.