Qualitative and quantitative assessments of the reaction of lucerne plants to verticillium wilt

A glasshouse assessment of reaction of lucerne (Medicaga sativa L.) plants to infection (wilt score) with Verticillium albo-atrum Reinke et Berthold was influenced by the particular combination of fungus isolate and host cultivar. Within any particular cultivar an increase in wilt score was associated with increased depression of growth when plants were inoculated with strongly pathogenic isolates but not with weakly or moderately pathogenic isolates. For plants inoculated with a strongly pathogenic isolate, growth of symptomless or slightly affected plants was reduced to as little as 26% of that of uninoculated plants with a cultivar of low resistance such as 'Rere' and to between 77 and 93% for plants of 'Vertus' which has good resistance to wilt. Scoring plant reaction on the basis of visual symptoms, for the purpose of screening for resistance, was generally as good as using quantitative growth assessments and offered the advantages of speed and leaving the plants intact. Factors which must be controlled to achieve reliable screening include light, temperature, pathogenicity of the fungus, and time of assessment. Relative reaction of lucerne cultivars to wilt was generally consistent and it was possible to distinguish three groups of cultivars: (1) 'Vertus' - good resistance; (2) 'Pioneer 524', 'Wairau', 'WL311', and 'WL318' - slight to moderate resistance; and (3) 'AS13R', 'Rere', 'Saranac', and 'Washoe' - low to slight resistance.