Diagnostic Facts Diagnostic Services Michigan State University

MSU-DS15 www.pestid.msu.edu September 2004 Tar Spot Jackie Smith, Diagnostic Services ar spot gets it name from the shiny black color intensifies as the season progresses. During lesions it creates on infected leaves. Tar mid to late summer the raised, black stromata Tspot is rarely serious enough to threaten develop within the spots. The black spot grows in the health of trees, but make trees unsightly. Heavy diameter and thickness until, by late September, it infections can also cause truly does look like a spot early leaf drop. of tar. The surfaces of spots There are two species of the caused by R. acerinum that cause become convoluted and tar spots on . Other appear rippled (Fig. 1). species cause tar spots on Leaves infected with R. various woody ornamentals punctatum result in several and a few herbaceous hosts. pinhead-sized stromata causes within the yellow area large tar spots (0.5-2cm). (Fig. 2). R. punctatum causes small spots (about 1mm in diameter), sometimes called speckled tar spots. Maples BIOLOGY susceptible to one or both species include bigleaf, Rhytisma spp. overwinter mountain, Norway, red, on infected leaves that Rocky Mountain, silver, fall to the ground. The sugar and sycamore. following spring, just as Fig. 1 Silver maple leaf infected with tar spot. new leaves are unfolding, SYMPTOMS the fungal tissue in the leaves on the ground ripens. Tar spot infection occurs in early spring as the The surfaces of the spots split and spores are ejected. leaves are expanding. Though the characteristic The spores are disseminated by wind. They land on tar-like spots do not arise until mid to late summer. newly expanding leaves of a susceptible host and The first symptoms of infection are light green to start a new disease cycle. Symptoms appear 1-2 yellowish green spots. The spots enlarge and their months after infection. CONTROL warranted include young trees with limited foliage and trees in aesthetically valuable locations. Historically, significant outbreaks of tar spot occur The recommended fungicide applications for tar infrequently. In Michigan, however, we have seen spot are at bud break and twice thereafter at 7- to severely infected 14-day intervals. trees for the last F u n g i c i d e s three years. Trees recommended in moist, sheltered i n c l u d e locations, which triadimefon or allow the pathogen mancozeb. These to easily survive are the active the winter, may i n g r e d i e n t s ; b e r e p e a t e d l y products are infected. This sold under a disease is usually a variety of trade cosmetic problem names so you and does not affect will need to look t h e l o n g - t e r m at the fine print health of the tree. on the product The most effective container. m a n a g e m e n t A d e q u a t e procedure is to c h e m i c a l rake and destroy c o v e r a g e o n leaves in the fall. large trees is A p p l i c a t i o n s difficult and o f f u n g i c i d e s Fig. 2 Infected sugar maple leaf. often requires are possible but s p e c i a l i z e d control of the disease is difficult. Complete spray equipment. Contact the Michigan Nursery coverage of leaf tissue is needed for success and and Landscape Association (MNLA) at this can be difficult on mature maples. To be 1-800-879-6652 to find a landscape professional effective fungicide applications should be coupled in your area who can assist you with fungicide with good sanitation practices. applications and other disease management Landscape situations where fungicide use may be procedures.

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