The wet weather has caused a fair amount of disease on new growth this spring. The Plant Clinic has received a number of samples and questions about turf showing leaf spots. Samples have indicated the presence of a disease that used to be known as Helminthosporium leaf spot. That disease has been reclassified into several leaf diseases, and pathologists now refer to “Helminthosporium types” when discussing leaf spots in the genera Bipolaris, Drechslera, and Exserohilum.

These fungi are now active and may be found throughout Illinois. The symptoms vary, depending on fungal species, grass species, weather conditions, and cultural conditions. In general, look for small spots or lesions varying in color from reddish brown to purplish black. Lesion centers are often tan and typically have a dark reddish brown border, giving an “eyespot” appearance. In wet weather, the lesions may merge, yellowing the turf or causing tip dieback.

These leaf spot diseases favor dry periods alternating with prolonged periods of cloudy, moist weather and moderate temperatures. The diseases progress quickly when grass is cut too short, turf is slow growing, or fertility is low. Excessive shade and excessive use of nitrogen also encourage leaf spot diseases. Turf specialist Dr. Tom Voigt points out that applications of the phenoxy herbicides can also enhance turf leaf spots. Some phenoxy herbicides include 2,4-D, mecoprop, and 2,4-DP. Other stresses may contribute to disease severity.

Cultural control measures are usually effective. In cases for which such measures are not adequate (such as at some golf courses), chemical controls may be used as protectants. Chemicals labeled for use in Illinois are listed in the Illinois Commercial Landscape and Turfgrass Pest Management Handbook, 1998-1999. Correct mowing practices will help control leaf spots. Mow all turfgrasses at the recommended maximum height for the species. Mow frequently enough so that no more than one-quarter to one-third of the leaf surface is removed at one time. If a lawn has been fertilized, it may be necessary to mow every three or four days during warm, wet periods.

These leaf spot diseases can be suppressed with proper fertilization; however, it is important to avoid high levels of nitrogen. Contact Tom Voigt for a set of guidelines on how much fertilizer to apply to a home lawn and when to to make the applications. Another source of turf information is the University of Illinois Center for Turfgrass Science Web site (http://www.turf.uiuc.edu).

Information is also available on leaf spot disease resistance by some bluegrass cultivars adapted to Illinois. ‘Marion’ was one of the first Kentucky bluegrass cultivars developed for resistance to leaf spot diseases. Of course, that cultivar may have susceptibility to other diseases that are a concern in your area, so look at the total resis make the applications. Another source of turf information is the University of Illitance package before deciding on a cultivar. Resistance levels will vary somewhat by location. For more information, consult Report on Plant Diseases No. 405.