A novel method for the identification and enumeration of microorganisms with potential for suppressing fungal plant pathogens

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 spread-plated 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.