Biological control of Xanthomonas bacterial spot of tomato with compost amended mixes and compost water extracts

Bacterial leaf spot diseases of vegetable crops can cause serious losses because available resistant varieties and chemical control procedures often provide inadequate control. Recently, it was shown that composts incorporated into soils or potting mixes and that compost water extracts applied to the foliage of plants may reduce the severity of bacterial speck of Arabidopsis. The objective of this work was to test the effectiveness of these compost treatments on the control of bacterial spot of tomato caused by Xanthomonas campestris pv. vesicatoria.; Two types of compost-amended potting mixes consistently reduced the severity of bacterial leaf spot. Unfortunately, these effects varied among batches of composts used. In contrast, vermiculite and a highly decomposed Sphagnum peat mix, which do not support high microbial activity, consistently failed to suppress this disease. Greenhouse bioassays showed that topical sprays with compost water extracts prepared from several types of composts effectively reduced the severity of this disease. Their efficacy was similar to that of acibenzolar-S-methyl, an inducer of systemic acquired resistance (SAR) in tomato. The active ingredient (s) in the extract was heat stable, filterable (0.2 muM) and did not directly inhibit in vitro growth of the pathogen. In a 1997 field trial, application of compost water extracts at weekly or bi-weekly intervals to tomato plants significantly reduced the incidence of bacterial spot on fruit. Similar results were obtained with acibenzolar-S-methyl. Composted yard wastes incorporated into field soil also reduced disease incidence on fruit. No interaction between active extracts and compost incorporated into soil on disease incidence was observed. In 1998, when the disease pressure in the field was very low, none of the treatments had significantly less disease than the control. In conclusion, both compost water extracts and compost amendments significantly reduced the severity of bacterial spot of tomato but only under severe disease pressure.