A novel method for the identification and enumeration of microorganisms with potential for suppressing fungal plant pathogens
O. P. Rupela · S. Gopalakrishnan · M. Krajewski · M. Sriveni
Abstract This paper describes a method that allowed
counting of both the total culturable and antagonistic
microorganisms in a given source such as compost.
Fusarium solani, used as the test fungus, was spreadplated
on quarter-strength (1/4) potato dextrose agar
(PDA), its surface was exposed in a laminar flow for 4 h
and then another layer (2–3 mm thick) of 1/4 PDA was
poured over it, on which an appropriate dilution of a
compost sample was spread-plated. Microorganisms in
the compost samples appeared first, and were counted as
total culturable organisms. Plates were further incubated
until F. solani grew through the upper layer of PDA
(generally in 4–8 days) and covered the whole plate
including most of the microbial colonies, except for a few
which had a halo around them. These were counted as
antagonistic, and they were isolated and purified for
further studies. The population of bacteria in the six
specific compost samples (called Biodynamic or BD
preparations by organic farmers) ranged from 3.45 log10
(in BD502) to 8.59 log10 (in BD504) per gram of
materials. The population of antagonistic bacteria was
counted for three of the six compost samples, and ranged
from 3.24 log10 (in BD502) to 6.90 log10 (in BD500). Of
the 67 bacterial isolates showing a halo that were
assembled from different sources, 17 suppressed at least
1 of the 4 plant pathogenic fungi against which these were
evaluated using the dual culture method.