ABSTRACT
Stemphylium solani, which causes a new leaf blight of cotton, was suspected of producing a phytotoxin. Studies were conducted to examine the relationship between the reaction of different cotton cultivars and of some unrelated host species to the pathogen and its toxin-containing culture filtrates. Seven single spore isolates of S. solani from cotton and their toxin-containing culture filtrates were used for leaf and root bioassays. An isolate of S. solani from tomato was also used for comparison. The phytotoxic effect was isolate dependent. Culture filtrates of five isolates killed 40 to 60% of the cotton seedlings when incubated for 4 days at 10-1 dilution. At 10-2 dilution, the culture filtrates of most of the isolates affected the development of the root system but failed to kill any seedling. The phytotoxic effect of the culture filtrate was not degraded by autoclaving. A high correlation coefficient between the percentage of the leaf area infected (LAI) by S. solani and the percentage of the necrotic leaf area (LAN) by the culture filtrate was observed when one of the aggressive isolates and its culture filtrate were tested against adult plants of 38 cotton cultivars (r = 0.86). Cultivars CNPA T-1180-23, CNPA-PRECOCE 2, PR 94-215, and PR 94-82 demonstrated resistance to the pathogen as well as insensitivity or moderate sensitivity to its toxin. Cultivars showing intermediate reaction to the pathogen also showed intermediate reaction to its culture filtrate. Similarly, the highly susceptible cultivars Paraná 3, PR 93-129, and PR 94-216 also were highly sensitive to the culture filtrate. Of the 18 plant species belonging to 18 genera, eight were susceptible to the pathogen. With two exceptions, susceptible hosts were also sensitive to the culture filtrate, whereas nonsusceptible hosts were insensitive. A component of the culture filtrate was regarded as a pathogenicity factor.
