Disease Resistance and Biomass Stability of Forage Pearl Millet Hybrids with Partial Rust Resistance.

The expression of partial resistance to Puccinia substriata var. indica and its contribution to digestible biomass production in forage pearl millet hybrids were evaluated in field experiments at Tifton, GA. Inbreds Tift 383, Tift 65, and nine inbreds with partial resistance selected from the cross Tift 383 × 'ICMP 501' were crossed to Tift 23DA. The parental inbreds and hybrids were evaluated in natural epidemics in 1996 and 1997. Because of maturity differences among the lines, slope of the regression of logit rust severity on time (apparent infection rate) and area under the disease progress curve (AUDPC) calculated for a defined interval of plant growth (10 days before to 20 days after anthesis) and adjusted for initial rust severity at 10 days prior to anthesis were the most useful indicators of resistance. Inbred resistance was not a reliable predictor of hybrid resistance when evaluated by either variable. Hybrids were evaluated for biomass production in 1996 and 1997 in a split-plot design, with hybrids as main plots and nontreated or chlorothalonil fungicide-treated as subplots. Differences existed among hybrids for AUDPC and for digestible dry matter yield (DDMY) and its components. Over all hybrids, the response between DDMY and final rust severity was described by logarithmic regression. Two clusters of hybrids were identified by cluster analysis of disease-related data from both experiments. The cluster of susceptible hybrids tended to have a lower DDMY and were less stable over year × treatment environments than the cluster of partially resistant hybrids. Lodging in nontreated plots in 1997 primarily occurred in susceptible hybrids. Although resistance was expressed in certain hybrids, greater levels of partial resistance are needed to provide adequate protection against DDMY losses.