Abstract:
Biological control of plant diseases through the addition of microbial biocontrol agents and the promotion of
earthworms can be an environmentally friendly alternative to the chemical control of plant diseases. However,
possible risks with biocontrol agents and their interactions with earthworms and other soil biota have not been
well studied. The aim of this study was to assess whether the beneficial bacterium Bacillus amyloliquefaciens and
the earthworms Aporrectodea caliginosa or Aporrectodea longa could reduce disease in oilseed rape (Brassica
napus) challenged with the pathogen Alternaria brassicae. Plant growth and productivity were measured as plant
survival, height, biomass, and flower development as well as disease index. A second objective was to assess
whether the presence of the bacterium at high concentrations would influence the survival, growth, and
reproduction of the earthworms. One outdoor and one greenhouse experiment were performed with Br. napus
plants challenged with Al. brassicae inoculated to the plant leaves in the presence or absence of Bacillus
amyloliquefaciens inoculated to the root environment and in the presence or absence of earthworms (Ap.
caliginosa or Ap. longa) added to the soil. All treatments were replicated three times. In the outdoor experiment,
inoculation with Al. brassicae reduced the growth of plants and the addition of Ap. caliginosa increased plant
height. In the greenhouse experiment, pairwise comparisons of plants challenged with Al. brassicae showed that
treatment with B. amyloliquefaciens led to significantly lower disease index than the treatment with Ap. caliginosa
plus B. amyloliquefaciens, while other treatments had intermediate disease indices. The addition of Al. brassicae or
B. amyloliquefaciens increased the survival and mass increment of Ap. caliginosa as a main effect when used
separately but not when used in combination.
This study did not give any clear indication of the usefulness of B. amyloliquefaciens for biocontrol of plant
pathogens such as Al. brassicae when growing plants in natural soil. In addition, no significantly positive effects
from the tested earthworm species were seen.
