Tall Fescue Online Monograph

Brown patch, a common disease of tall fescue turf, often is a problem during warm, humid, or wet weather (Fig. 26-14). Activity of the fungal pathogen R. solani may result in blighting of large areas of turf.

Several subgroups (anastomosing groups) of the fungus exist, with AG1 being the most common in tall fescue (Tisserat, 2006). Infected leaves usually develop purplish-green, then tan lesions that contribute to the bleached appearance of large, up to 0.8 m (3 ft) in diameter, distinct circular patches. Lesions may appear water-soaked and have a brown band between the green, photosynthetically active leaf tissue and diseased tissue. Severity depends on weather and turf management practices. Optimum conditions for a high frequency of infections include an extended period (>10 h) of leaf wetness and night temperatures >21°C (70°F) (Tisserat, 2006). However, brown patch can develop when night temperatures are consistently above 15°C (60°F) and leaves remain wet for 10 consecutive hours (Tredway et al., 2005). This disease is particularly damaging to intensely managed turfs receiving a high level of N fertility and growing in compacted, poorly drained soils. Weather-based, disease-warning models have been developed to predict outbreaks of brown patch in several turfgrass species (Fidanza et al., 1996; Burpee and Atkins, 1991; Palmieri, 2005). The fungal pathogen R. zeae can cause similar signs at warmer temperatures.

In Arkansas, shade-grown plants of ten cultivars of tall fescue had significantly greater injury from R. solani activity than sun-grown plants (Zarlengo et al., 1994). Safari was consistently one of the most susceptible tall fescue cultivars to R. solani in shade, while Hubbard 87 and Shenandoa' were among the least susceptible cultivars. Development of cultivars with improved resistance to brown patch or faster recovery is an important goal of turfgrass breeders. Newer cultivars have higher turf density; this characteristic can increase the hours of leaf wetness and thus brown patch incidence (Giesler et al., 1996b). Giesler et al. (1996a) demonstrated in a group of cultivars that brown patch incidence was related more to plant architecture than to cultivar, with tall cultivars having less brown patch than medium or dwarf cultivars. Recently breeders have developed dwarf cultivars with more open canopy to address this problem. Cultivars entered in the 2001 NTEP Tall Fescue Test with OD tag, such as Blackwatch (PICK-OD3-01), Magellan (OD-4), and Serengeti (GO-OD2), have brown patch data (Natl. Turfgrass Eval. Program, 2006). Turf management that affects turf density can influence brown patch incidence, including mowing height and frequency, fertilization, and time of irrigation. Different strains of brown patch are predominant in different areas, making screening for brown patch resistance more difficult for turfgrass breeders (Brilman, 2003).

Although less common than brown patch, gray leaf spot (Pyricularia grisea) also is a major fungal disease of tall fescue, first documented in tall fescue in the mid 1990s. A serious outbreak occurred in Alabama during the summer of 1999 on Rebel II tall fescue when warm temperatures, high humidity, and frequent showers favored disease development (Hagan, 2000). Symptoms usually occur at air temperatures between 21 and 35°C (70 and 95°F). The fungus also requires at least 14 h of continuous leaf wetness (Tredway, 2005). Newly established turfs usually are more susceptible to gray leaf spot than mature stands. Excessive water and N fertility may increase disease severity (Clarke and Vaiciunas, 2001). Small, round, gray-brown lesions appear on leaf blades and sheaths. As the lesions become larger, their centers often turn from light brown to gray, and purplish-brown margins form. As infection progresses, leaves yellow, then wither and die. Application intervals and modes of action of several fungicides labeled for brown patch and/or gray leaf spot control in tall fescue are presented in Table 26-4. Signs of selected tall fescue diseases, including those for brown patch and gray leaf spot, are summarized in Table 26-5.