Optimized fermentation of Myxococcus fulvus WCH05 enhances biocontrol of pear fire blight in the field.

Myxococcus fulvus WCH05, isolated from desert soil in Manas County, Xinjiang, China, exhibits strong predatory activity against Erwinia amylovora and has potential for biocontrol of pear fire blight. To further explore its biocontrol potential and evaluate its efficacy in pear orchards, this study optimized the fermentation conditions of strain WCH05, targeting increased cell dry weight. A combination of single factor experiments, and response surface methodology (RSM) was employed to optimize the fermentation medium (carbon source, nitrogen source, and inorganic salt) and fermentation parameters (temperature, pH, shaking speed, working volume, and inoculum size). The biocontrol efficacy of strain WCH05 against pear fire blight was subsequently evaluated in potted Pyrus betulifolia seedlings under greenhouse conditions and in pear orchards over two consecutive years. The optimal fermentation conditions were determined as follows: soluble starch 8.0 g/L, yeast extract 4.0 g/L, MgSO4 1.2 g/L, initial pH 7.0, working volume 40% (100 mL/250 mL), agitation speed 200 r/min, incubation temperature 30 ℃, inoculum size 7%, and incubation time 72 h. Under these optimized conditions, the cell dry weight reached 3.25 g/L, a 2.5-fold increase compared to the unoptimized condition (1.30 g/L). In greenhouse trials, application of the WCH05 fermentation broth significantly reduced shoot blight incidence (P < 0.05), achieving protective efficacy of 86.2% (7 days post-inoculation, dpi) and 82.6% (14 dpi), which were superior to the efficacy achieved with the unoptimized broth. In field trials conducted in Korla City, Xinjiang, in 2023, the control efficacy against pear fire blight exceeded 81.5%. In 2024, field trials conducted in Korla City and Zhangye City, Gansu Province, showed control efficacies ranging from 81.0% to 90.9% and 82.5% to 89.7%, respectively. Notably, strain WCH05 consistently exhibited higher control efficacy than the antibiotic Kasugamycin in both years of field trials.