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The effect of the terpenoids gossypol, 6-methoxygossypol, 6,6′-dimethoxygossypol, gossypolone and apogossypolone on growth of fungal soil pathogens was investigated. The compounds were tested at a concentration of 100 μg ml−1 in a Czapek Dox agar medium at 25°C. Gossypol, gossypolone and apogossypolone demonstrated strong growth inhibitory activity (≥90%) against Pythium irregulare, Pythium ultimum and Fusarium oxysporum. These same terpenoids provided good growth inhibition against most Rhizoctonia solani isolates. Methylated gossypol derivatives generally yielded reduced growth inhibition against the tested fungi compared with gossypol. Dose–response effects of gossypol, gossypolone and apogossypolone were determined over a concentration range of 5–100 μg ml−1 against P. irregulare CR1, P. ultimum ATCC 56081 and R. solani CR15. At lower concentrations, gossypol proved to be a more potent growth inhibitor of P. irregulare (ED50 = 4 μg ml−1) and P. ultimum (ED50 = 13·2 μg ml−1) than the other tested compounds. Rhizoctonia solani CR15 was more resistant to growth inhibitory effects of all tested terpenoids (ED50 = 35–43 μg ml−1).Significance and Impact of the Study: This work demonstrates that gossypol is an effective natural antimicrobial agent against a wide range of potential fungal pathogens of cotton. Relative to gossypol, methylated gossypol derivatives that are also found naturally in root tissue were less effective at inhibiting the growth of soil fungal pathogens. However, by virtue of their significant concentration in root tissue, they still may contribute to cotton defence.