"Helminthosporium" leaf spots.
4 (1 = rare 5 = annual)
1-4 (1 = very little damage 5 = plants killed)
All cool- and warm-season turfgrasses are susceptible.
Leaf spots appear on the leaves from early spring to autumn as small, dark-brown, or purplish red spots. The spots enlarge and develop light-colored centers, with purplish black borders. Infected leaf blades or entire plants may yellow, turn brown, wither, and die. Girdled leaf blades may drop prematurely. Crown and root rots usually appear in warm to hot weather as a reddish brown to black decay of the crown, rhizome, and root tissues. Infected areas may have a general brownish cast. The turf is thin and weak, and may have a drought injured appearance or be killed, resulting in round to irregular patches that enlarge during the summer.
Stem, crown, and root rot occurs when feeder roots are shallow, few in number, or absent. Plants often wilt during midday, even when soil moisture is abundant. Entire stands of turfgrass may be completely destroyed by severe crown and root rot; this "melting-out" of the turf stand is favored by wet weather.
Though there are many different fungal species involved which are active over a wide temperature range, the control measures and fungal biology and symptoms are sufficiently similar, which makes it practical to discuss them together. These diseases are favored by prolonged leaf wetness and a variety of poor cultural practices (see management section below) and other factors which cause the turfgrass to be stressed.
Resistance is the key to control of Helminthosporium diseases and many resistant turfgrass varieties are available. However, the resistance to these diseases is diminished when turfgrass is subject to stress. The following practices are particularly important to reduce stress and promote steady growth: 1) Irrigate deeply in the morning, but as infrequently as possible without causing stress, 2) Increase the mowing height as high as the turf species and use will allow, 3) Apply fertilizer only as needed to promote moderate growth, but do not overstimulate the turf in the spring and early summer, 4) Keep the thatch layer below 1/2 inch thick, and 5) Minimize the use of growth regulator herbicides in disease-prone areas.
If disease can not be adequately controlled by cultural practices, a number of effective fungicides are available. However, control is impossible after late spring or summer when disease is already severe. The use of fungicides will be most practical and effective when applied preventively in the spring to disease-prone areas.