Isolation, evaluation and identification of rhizosphere actinomycetes with potential application for biocontrol of Valsa mali

Apple valsa canker (AVC), caused by Valsa mali (Vm), is a serious disease worldwide. In this study, a total of 193 actinomycete cultures were isolated from the rhizosphere of healthy wild apple trees in the Qinling Mountains in China. Six of these isolates had a strong antagonistic activity towards V. mali (inhibition rates >50%). The hyphal growth and conidial germination of V. mali were significantly inhibited by these isolates. The strain SL01 could induce the protoplasm leakage from V. mali hyphae. In a detached twig inoculation experiment, the fermentation broth of the six isolates showed significant inhibiting effects on disease development by V. mali. Among the six candidates, strain SL01 showed the best potential for biocontrol of AVC based on the inhibitory effects on hyphal growth, conidial germination and disease development by V. mali. Strain SL01 was identified as Streptomyces longissimus based on morphological, biochemical, physiological characteristics and 16S rDNA sequence analysis. In field studies, the SL01 strain showed an average of 90.3% inhibition on AVC recurrence in a two-year experiment. This study indicated that the actinomycete strain SL01 has the potential for biological control of AVC.