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Isolation and Characterization of Rhizobacteria from Composts That Suppress the Severity of Bacterial Leaf Spot of Radish

First, second, fifth, and sixth authors: Department of Plant Pathology, The Ohio State University/OARDC, 1680 Madison Ave., Wooster 44691; and third and fourth authors: Department of Food, Agricultural, and Biological Engineering, The Ohio State University/OARDC, 1680 Madison Ave., Wooster 44691

Composts can induce systemic resistance in plants to disease. Unfortunately, the degree of resistance induced seems highly variable and the basis for this effect is not understood. In this work, only 1 of 79 potting mixes prepared with different batches of mature, stabilized composts produced from several different types of solid wastes suppressed the severity of bacterial leaf spot of radish caused by Xanthomonas campestris pv. armoraciae compared with disease on plants produced in a nonamended sphagnum peat mix. An additional batch of compost-amended mix that had been inoculated with Trichoderma hamatum 382 (T₃₈₂), which is known to induce systemic resistance in plants, also suppressed the disease. A total of 11 out of 538 rhizobacterial strains isolated from roots of radish seedlings grown in these two compostamended mixes that suppressed bacterial leaf spot were able to significantly suppress the severity of this disease when used as inoculum in the compost-amended mixes. The most effective strains were identified as Bacillus sp. based on partial sequencing of 16S rDNA. These strains were significantly less effective in reducing the severity of this disease than T₃₈₂. A combined inoculum consisting of T₃₈₂ and the most effective rhizobacterial Bacillus strain was less effective than T₃₈₂ alone. A drench applied to the potting mix with the systemic acquired resistance-inducing chemical acibenzolar-S-methyl was significantly more effective than T₃₈₂ in several, but not all tests. We conclude that systemic suppression of foliar diseases induced by compost amendments is a rare phenomenon. Furthermore, inoculation of compost-amended potting mixes with biocontrol agents such as T₃₈₂ that induce systemic resistance in plants can significantly increase the frequency of systemic disease control obtained with natural compost amendments.

Additional keyword: Raphanus sativus.