Leaf blight of cotton (Gossypium hirsutum) caused by Stemphylium solani occurs in several cotton growing areas of Brazil, and causes severe yield losses especially in the State of Paraná. The inheritance of resistance to S. solani and to its phytotoxin was investigated in three cotton cultivars. Crosses including reciprocals were made between the cotton cultivars PR 94-82, PR 94-215 and CNPA Precoce 2, identified as resistant, and cv. IAPAR 71 as susceptible. Thirty days after sowing, two leaves of each plant were inoculated with S. solani, and the other two leaves of the same plant were infiltrated with the toxin containing culture filtrate of the same isolate at a dilution of 1:1. The percent of leaf area infected by the pathogen (LAI) and the percent of necrotic leaf area caused by the culture filtrate (LAN) were evaluated seven days after inoculation. Considering the reactions of 740 segreganting plants, the coefficient of correlation between the LAI and LAN was high (r=0.70). The segregating populations of the three crosses showed significant additive genetic variation for LAI and LAN. A F3 based chi-square (c2) testing for the 1:2:1 ratio for a single non-dominant gene segregation was not rejected (P=0.001) for the crosses IAPAR 71 x PR94-82 and IAPAR 71 x CNPA-PRECOCE 2. For the cross IAPAR 71 x PR94-215, the pattern of segregation including two genes and epistasis (9:6:1) was accepted by the chi2 test.
