Evaluation and Heritability of Resistance to Sugarcane Red Rot and the Effects of Environmental Stress on Disease Severity.

Heritability of resistance to red rot, caused by Colletotrichum falcatum, was studied using progeny from 40 crosses among 24 parental clones of sugarcane. Significant differences in susceptibility were detected, although high levels of resistance were rare in the breeding and selection populations of the Louisiana sugarcane cultivar development program. Narrow-sense heritability estimates determined by mid-parent-offspring regression for four different disease traits ranged from 0.19 +/- 0.04 to 0.31 +/- 0.05. Potential genetic gain by selection for resistance, using a 10% selection intensity, ranged from 14 to 37% of the mean. The rot index provided the highest heritability estimate and the most potential genetic gain from selection. Low broad-sense heritabilities were estimated among years for the disease traits in the parent population. These results indicated that the population level of red rot resistance can be increased by careful choice of parent clones and cross-based selection. Genotype by year interaction, however, greatly affected evaluation. The effects of environmental stress on disease severity also were investigated. Greenhouse and field studies demonstrated that red rot severity can be increased by the occurrence of drought conditions during the initial growth processes of vegetatively propagated sugarcane. The effects of oxygen deprivation or poor drainage and Pythium root rot on development of red rot and spring shoot population of sugarcane were evaluated under controlled and field conditions. Red rot severity was not increased by previous oxygen deprivation. In field experiments, inoculation of stalks with C. falcatum before planting resulted in the reduction of shoot populations the following spring. Poor drainage resulted in additional reduction in shoot populations developing from inoculated stalks. The detrimental effect of poor drainage on shoot population from inoculated stalks was alleviated by application of the fungicide metalaxyl. Pythium root rot, caused by Pythium arrhenomanes, reduced initial root system and growth of shoots in greenhouse experiments. The combination of P. arrhenomanes and C. falcatum inoculation increased dead-bud percentage in one of two cultivars and red rot severity for both. The results suggest that spring shoot population reductions associated with poor drainage are affected by the combined effects of red rot and Pythium root rot