Control of Postharvest Fruit Rot Diseases of Kiwifruit by Antagonistic Bacterium Bacillus subtilis

Postharvest fruit rot diseases cause a severe loss of kiwifruit during cold storage, transportation, marketing and in retail stores and Botryosphaeria dothidea, Diaporthe actinidiae and Botrytis cinerea were known as their major pathogens in Korea. Bacillus subtilis SK1-2 which showed the strongest antagonistic effects on B. dothidea, D. actinidiae and B. cinerea in in vitro growth inhibition assays was selected as an antagonist isolated from kiwifruit vines in the course of searching for antagonist candidates for the biological control of the postharvest fruit rot diseases. Cell suspensions of B. subtilis SK1-2 were harvested according to the optimal growth conditions such as pH, temperature and carbon and nitrogen sources defined for its mass production and applied four or six times at intervals of 10 days onto kiwifruit vines grown in rain-proof and open-field orchards to test the control efficacies on the postharvest fruit rot diseases. The control efficacies of B. subtilis SK1-2 on the postharvest fruit rot diseases were 68.2 and 56.5% in rain-proof and open-field orchards treated four times, respectively, and 80.0 and 65.7% in rain-proof and open-field orchards treated six times, respectively, whereas those of the registered chemical fungicide thiophanate-methyl were 72.4 and 67.1% in rain-proof and open-field orchards, respectively. The novel antagonist has the potential to be developed as a potent microbial agent successfully controlling the postharvest fruit rot diseases of kiwifruit vines grown in rain-proof orchards in the future.