Optimizing Controlled Environment Assessment of Levels of Resistance to Yam Anthracnose Disease Using Tissue Culture-derived Whole Plants

Studies were conducted to determine the timing and frequency of disease assessment required to effectively identify levels of resistance to yam anthracnose using tissue culture-derived whole plant inoculation assay. The effects of inoculation methods (paint brush and spray), and disease scoring methods [individual leaf area (ILA) and whole plant area (WPA)] were also assessed. Spray inoculation resulted in rapid infection and higher variations among yam genotypes, leading to earlier discrimination of genotypes than with the paintbrush method. Both the ILA and WPA scoring methods showed variation among yam genotypes, and association between the two methods gave a high positive correlation (r > 0.90). However, the WPA was faster and had the advantage of detecting differences in reactions of yam genotypes to less aggressive pathogen isolates to which the ILA method showed no variation. A single disease evaluation at 7 days after inoculation was as good as the area under the disease progress curve (AUDPC) and the disease progress rate (Rd) derived from multiple evaluations. However, a significant time–genotype interaction, suggests a need for more than a single assessment for effective comparison of genotypes. AUDPC derived from two assessments (5 and 7 DAI) was better than AUDPC from three assessments (5, 7 and 9 DAI) in separating genotypes reactions to a less aggressive pathogen isolate. This study showed that the use of spray inoculation method, the WPA scoring method, and AUDPC derived from two assessments (5 and 7 DAI) provided best conditions for evaluating yam genotypes for levels of anthracnose resistance with the tissue culturederived whole plant assay.