QUANTITATIVE RELATIONSHIPS AMONG THAXTOMIN-A PRODUCTION, POTATO SCAB SEVERITY, AND FATTY-ACID COMPOSITION IN STREPTOMYCES

Thaxtomin A production in culture, potato common scab severity (percen tage of tuber surface infected or number of lesions per tuber), and fa tty acid profiles were determined for 78 Streptomyces isolates. Only p athogenic Streptomyces spp. (n = 17) produced thaxtomin A in culture. Thaxtomin A production in culture (mu g/mL) was significantly positive ly correlated with the percentage of tuber surface infected (A = 0.60; p = 0.017) but not with the number of lesions per tuber (R = 0.37; p = 0.17). An increase of 1 mu g/mL in thaxtomin A production correspond ed to an 11% increase in disease severity (percentage of tuber surface infected). The data indicate that quantitative information on the abi lity of a particular pathogen isolate or population to produce thaxtom in A may be critical to understanding and predicting the disease poten tial of that population. Using cluster analysis of fatty acid data, 94 % of 67 unknown field isolates grouped with other field isolates havin g the same pathogenicity (plus or minus).