SELECTION OF MICROORGANISMS FOR BIOLOGICAL-CONTROL OF SILVER SCURF (HELMINTHOSPORIUM-SOLANI) OF POTATO-TUBERS

Few management strategies exist for silver scurf, an important posthar vest disease of potatoes. In this study, the microbiota of 47 agricult ural soils and 7 tuber samples was screened for biological control age nts of silver scurf. Soil or periderm samples were transferred to sepa rate samples of gamma irradiation-sterilized field soil enriched with potato periderm. After incubation, the samples were assayed for biolog ical suppressiveness to silver scud using a whole-tuber/infested soil assay. Over 430 isolates of bacteria, yeasts, and actinomycetes were r ecovered from tubers and soil associated with the 12 most suppressive soil samples. Thirteen strains were selected for further study on thre e different strains of Helminthosporium solani, including one that was resistant to thiabendazole. Microbial strains that significantly inhi bited H. solani (P less than or equal to 0.05) in at least one experim ent were identified as Pseudomonas putida (PM1), Nocardia globerula (S 244), and Xanthomonas campestris (P76). Colonization studies with rifa mpicin-resistant strains of putative biological control agents reveale d that long-term colonization of the tuber surface was not necessary t o reduce disease symptoms. Highly variable levels of conidiophore prod uction prevented selection of the single most suppressive strain. Poss ible sources of variability in biological control are discussed, inclu ding physiological age of the tuber, tuber infection in the field, and uneven free moisture in the storage.