Fig. 24-1. (Left) Pustules with urediniospores of Puccinia graminis subsp. graminicola (stem rust) on tall fescue leaves; (center) stem rust pustules on leaf sheath covering stem of tall fescue; (right) uredinial pustules of stem rust on young leaf of tall fescue in early spring.
Tall fescue is susceptible to infection by certain populations of P. graminis subsp. graminicola (Urban, 1967), whose host range also includes orchardgrass (Dactylis glomerata L.) and ryegrass (Lolium spp.) (Pfender, 2001a). Stem rust (Fig. 24-1, left and center) can cause severe yield losses, up to 40%, in seed crops of tall fescue. Stem rust severity and damage are generally more severe in first-year tall fescue seed fields than in older, established stands.
The pathogen produces several types of spores, but it is only the urediniospores (whose rust color gives the disease its name) that are involved in epidemics in commercial seed production. The summer epidemic begins when spores produced from overwintering infections (Fig. 24-1, right) are blown by wind to fresh leaf tissue. Infection requires several hours of leaf wetness during the night, followed by continued leaf wetness at the beginning of the morning light period. Probability of infection increases with temperatures between approximately 2 and 28°C; therefore, infection hazard is greatest when a warm, wet night is followed by a morning with persistent dew on the plants (Pfender, 2003). Infection cycles also occur more rapidly in warm than cool conditions because the latent period (amount of time between infection and the production of new spores from the infection) decreases from 69 d at 3.5°C to only 8.5 d at 26.5°C (Pfender, 2001b). The overall speed and severity of stem rust epidemics thus depend largely on spring and summer weather. Because of this strong influence of weather on epidemic onset and development, severity can differ significantly among years. Research is in progress to develop a weather-based warning system for stem rust epidemics of perennial ryegrass (L. perenne L.), a disease similar to that on tall fescue.
In addition to the effects of spring and summer weather on epidemic development, the amount of inoculum that successfully overwinters has a great impact on the earliness, and hence the eventual severity, of the summer epidemic. Although the overwintering biology of tall fescue stem rust has not been investigated, results of research on stem rust of perennial ryegrass indicate that significant overwintering is more likely on seedlings planted early during the preceding autumn than those planted late in the autumn (Pfender, 2004a). This suggests that disease risk can be diminished by avoiding early autumn planting.
Tall fescue cultivars vary somewhat in susceptibility to stem rust (Welty and Barker, 1993), and germplasm with a significant level of rust resistance has been identified (Barker and Welty, 1997; Barker et al., 2003). However, no current commercial cultivars grown for seed (see Chapter 23) can be produced reliably without chemical protection from stem rust. Good protection can be obtained with triazole or strobilurin fungicides, and a chlorothalonil fungicide may be combined with the first application of systemic fungicide during the season. Depending on earliness and rate of epidemic development, one to three applications may be required on seed crops during the growing season to prevent losses from stem rust (Pscheidt and Ocamb, 2001). Studies on stem rust of perennial ryegrass show that a major proportion of the damage from this disease occurs when infections spread from the leaf sheath to the enclosed inflorescence as it emerges from within the sheath (Pfender, 2004b). Control of this secondary disease spread to the inflorescence is critical for disease management, and the different fungicides are differentially effective in achieving this result (Pfender, 2006). Assuming fungicide activity against tall fescue stem rust is similar to that against perennial ryegrass stem rust, the strobilurin fungicides have a longer time window of effectiveness against this internal disease spread than do the triazoles. To preserve the long-term usefulness of strobilurin fungicides, however, it is essential to guard against development of fungicide resistance in the pathogen, by rotating or tank-mixing the strobilurin with triazole fungicides.
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