Mechanisms of earthworm fermentation products reducing apple replant disease

Objectives We studied the effect of earthworm homogenate prepared by wet grinding fermentation on microbial composition and hazardous compounds related to continuous cropping of apple (Malus hupehensis) and apple replant disease (ARD).

Methods We grew apple seedlings (Malus hupehensis) in pots filled with soil collected from a 27-year old apple orchard. Earthworm (Eisenia foetida) homogenate prepared by wet grinding and fermented with addition of Bacillus subtilis and Aspergillus niger was applied into soil. After the apple seedlings were grown for 4 months, the growth status of seedlings, total microbial population, hazardous compounds content and the pathogenic bacteria population were assessed.

Results The earthworm fermentation products significantly increased the number of bacteria and actinomycetes, and reduced the number of fungi. The four harmful fungi (F. oxysporum, F. proliferatum, F. solani and F. moniliforme) closely related to continuous cropping of apple decreased by 65.6%, 13.9%, 84.2% and 24.1%, respectively. The earthworm fermentation products significantly reduced the contents of phloroside, phloretin and p-hydroxybenzoic acid in the soil by 74.2%, 48.9% and 67.0%, respectively. Earthworm fermentation products also significantly promoted root growth, root respiration rate and antioxidant enzyme activity. The root respiration rate, SOD, POD, and CAT activities were increased by 69.4%, 81.2%, 81.1% and 161.5%, respectively. Compared to control, the biomass of apple seedlings increased significantly, the plant dry weight was 2.58 times of that in CK, the photosynthetic efficiency increased by 46.5%, the contents of chlorophyll a and chlorophyll b were 1.38 times and 1.65 times of those in CK, respectively.

Conclusions The application of earthworm fermentation products could effectively increase the population of bacteria and actinomycetes, and decrease that of fungi and hazardous substance contents in soil. Thus, it is effective in promoting the growth of young apple trees as well as reducing the harm caused by the apple replant disease.