Reduction of bacterial leaf spot severity on radish, lettuce, and tomato plants grown in compost-amended potting mixes

Compost-amended substrates offer the potential for management of diseases caused by soilborne as well as foliar plant pathogens. In this study, the efficacy of composted pine bark mix fortified with the biocontrol agent Trichoderma hamatum 382 (FCPB) against bacterial leaf spot of radish, lettuce, and tomato under controlled environment conditions was evaluated. Plants grown in the FCPB mix and inoculated with bacterial leaf spot pathogens were less severely diseased than plants grown in commercial peat mix or vermiculite. In some cases, plants were also grown in a composted cow manure mix or in a steam-treated compost-amended greenhouse soil, and these plants were also less severely diseased. Infected radish and tomato plants grown in these compost-amended substrates also harbored significantly smaller populations of Xanthomonas campestris pv. armoraciae and Xanthomonas campestris pv. vesicatoria, respectively. The disease-suppression effect of the FCPB mix was lost by autoclaving and restored by reinoculating the autoclaved FCPB mix with nonautoclaved FCPB mix. However, the disease-suppression effect varied among the batches of FCPB mix used. In contrast, vermiculite and a highly decomposed sphagnum peat mix consistently failed to suppress these diseases. These results suggest that producing vegetable seedlings in FCPB mix or compost-based substrates may provide initial protection against bacterial leaf spot pathogens.