Filtrates from cultures of A. citri are selectively toxic to rough lemon [Citrus jambhiri] or tangerine [C. reticulata cv. Dancy] leaves. Two related toxic compounds were isolated from the A. citri cultures that are selectively pathogenic to rough lemon. Both compounds affected susceptible rough lemon when 50 .mu.l of solution (0.1 .mu.g toxin/ml) was applied to leaves; tangerine and other nonhost plants were not visibly affected at 1000 .mu.g/ml. A rough lemon selection that is tolerant to the rough lemon-infecting isolates was affected by the toxins, but symptom development was delayed. Isolates pathogenic to ''Dancy'' tangerine yielded a toxin that visibly affected the host species when applied (50 .mu.l, 1.0 .mu.g/ml) to leaves. The tangerine-specific toxin at 1000 .mu.g/ml did not affect rough lemon or other nonhost species. Several conditions favoring toxin production in culture were determined; especially notable was the stimulatory effect of ZnSO4 in the medium. Procedures are described for purification of toxins, based on solvent extractions and chromatography. Each of the toxins caused leakage of electrolytes from susceptible, but not from resistant, tissue.