Comparative efficacy of bacteria, fungi, and yeasts as biological control agents for diseases of vegetative crops

The potential applications for use of bacteria, fungi, and yeasts as biological control agents of fungal diseases on vegetable crops are reviewed. The most extensively studied bacterial organisms, including Pseudomonas spp., Bacillus subtilis, and Enterobacter cloacae, have been reported to reduce many seedling diseases and root rots on several vegetable crop species. The bacteria are easy to grow and can be applied as a seed treatment or as a soil drench, and mechanisms by which they reduce diseases have been studied. The bacteria appear to protect plants against a wide range of pathogens and the potential for commercial utilization is promising. Fungal biocontrol agents, including the extensively studied Gliocladium and Trichoderma species, in addition to other weakly virulent or saprophytic fungi, have been reported to reduce infection or reproduction of many pathogens. Most of the applications of these fungi are for soilborne diseases, with a few reports of reduction of foliar fungal pathogens. The fungi are also relatively easy to grow and formulate for large-scale application and several are now available commercially. The yeast organisms have shown the greatest potential for reducing foliar diseases, especially those diseases caused by mildew fungi. Their use for postharvest disease control has not been investigated extensively and is worthy of research. All three groups of biocontrol agents require development of appropriate formulations to enhance application and survival, and they require additional study to elucidate the various modes of action against the fungal pathogens. The biocompatibility of these agents with fungicides is expected to enhance their efficacy, and there are a few examples available to demonstrate this. There are several areas in which additional research is needed, and these include the application of molecular techniques to characterize and/or modify strains, evaluation of ecological competence of the agents, and methods to enhance survival.