EFFECT OF RUST ON YIELD AND DIGESTIBILITY OF PEARL-MILLET FORAGE

The effects of infection by Puccinia substriata var. indica on yield and digestibility of pearl millet forage was examined in 1988 and 1989. Dwarf hybrids Tifleaf 1 (susceptible) and Tifleaf 2 (resistant), and tall hybrids Gahi 3 (susceptible) and Tift 85DAX186 (experimental cultivar Gahi 4, resistant) were inoculated or treated with fungicide after the first harvest to establish different levels of disease. For both dwarf and tall hybrids, there were no consistent differences in yield or digestibility between the disease-free plots of susceptible and resistant cultivars. Green yield, dry-matter yield, and forage quality as measured by in vitro digestibility were negatively correlated with final rust severity and area under the disease progress curve of dwarf and tall hybrids. Dry-matter concentration was unaffected by disease. Loss of digestible dry-matter yield could be expressed by either curvilinear or linear functions of final disease severity. In three of four experiments, the rate of loss of digestible dry-matter yield was greater at low rust severities than at higher rust severities, which indicated that highly effective resistance is necessary to reduce losses attributable to rust.