Susceptibility in Soybean to Red Crown Rot and Characteristics of Virulence in Calonectria Crotalariae.

Susceptibility in soybean to red crown rot was investigated and virulence in Calonectria crotalariae was characterized. The following areas were examined: (i) variability and stability in virulence among C. crotalariae isolates, (ii) disease reaction in soybean cultivars in a greenhouse inoculation technique, (iii) disease development on hosts of different ages at time of inoculation, (iv) cultural characteristics and presence of dsRNA in isolates, and (v) the involvement of a toxin(s) in the C. crotalariae/soybean system. A wide range in virulence of isolates of C. crotalariae was observed, and soybean isolates were more virulent on soybean than were peanut isolates. Virulence of the fungus was stable. Evidence for physiologic specialization was not recognized in this system, but possible host specialization was observed. A range of responses among cultivars to a highly virulent isolate was detected using a greenhouse inoculation technique. This technique was rapid, efficient, provided consistent results between greenhouse and field tests, and identified the least susceptible cultivars. Quadratic and linear relationships were described between plant age and lesion length or perithecia production in older plants (10-40 days old) and seedlings (4-10 days old), respectively. Differences in lesion length and perithecia production that were observed on seedlings were similar to relative levels of susceptibility in cultivars in greenhouse and field tests. Variability in disease severity, production of microsclerotia and perithecia, and mycelial growth was observed in isolates of C. crotalariae. However, isolates did not contain detectable levels of dsRNA. Disease severity correlated positively with production of both microsclerotia and perithecia. Isolates of C. crotalariae from soybean demonstrated greater virulence and production of microsclerotia and perithecia than did isolates from peanut. A trifoliate assay with half-strength culture filtrates detected toxic metabolites produced by C. crotalariae. At least one of these toxic metabolites was heat stable. The cultivar least susceptible to red crown rot also showed the least sensitivity to culture filtrates. The toxic metabolites are secondary determinants of virulence and play a role in symptom development. Results suggest that toxic metabolites of C. crotalariae may be involved in red crown rot of soybean