Bacillus thuringiensis C25 suppresses popcorn disease caused by Ciboria shiraiana in mulberry (Morus australis L.)

ABSTRACT

The mulberry tree is an important crop for silkworm farming and for the health care industry. In Asia, the annual productivity of mulberry fruits is greatly reduced due to popcorn disease mainly caused by Ciboria shiraiana, a sclerotia-forming fungal pathogen. To date, the development of efficient biocontrol agents (BCAs) against this disease has been hampered by the recalcitrance of C. shiraiana to in vitro culturing methods. Here, we established alternative in vitro antifungal assays that directly monitored the effects of BCAs on the growth of C. shiraiana apothecia and further reported that Bacillus thuringiensis C25 suppressed popcorn disease in field conditions. Initially, from mulberry drupelets showing the popcorn disease symptoms, we confirmed the infection of C. shiraiana and observed its morphology in asexual stage. Then, apothecia of C. shiraiana were induced from the sclerotia collected from the disease-infested orchard. Two bacterial isolates, Enterobacter sp. C5 and B. thuringiensis C25, strongly suppressed the elongation and fresh weight accumulation of apothecia stalks, the width of hymenium, and ascus and ascospore formation of C. shiraiana. In addition, both bacterial isolates degraded the ultrastructure of hymenium cells in C. shiraiana apothecia. Ultimately, treatment of mulberry trees with B. thuringiensis C25 mitigated the incidence of popcorn mulberry disease under field conditions. In conclusion, B. thuringiensis C25 is the first reported BCA shown to efficiently control mulberry popcorn disease. Our results also support that B. thuringiensis exerts diverse biocontrol roles in addition to insecticidal behaviour.

KEYWORDS:

• Apothecia
• Bacillus thuringiensis
• biocontrol
• Ciboria shiraiana
• mulberry popcorn disease

Acknowledgements

K. Kim was supported by Chonbuk National University.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was carried out with grant support from the Cooperative Research Program for Agriculture Science & Technology Development (PJ009411), RDA, Korea and the National Research Foundation of Korea (NRF) funded by the government (MEST) (No. 2011-0020202, NRF-2012R1A1A2042533), Korea.