Biology, epidemiology, and biological and chemical control of Phytophthora vignae Public Deposited

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  • Phytophthora vignae, causal agent of stem and root rot of cowpea (Vigna unguiculata), was reported for the first time in Sri Lanka. The pathogen was found in cowpea field soils from 3 of 5 geographic regions sampled. Only one site however, had plants exhibiting disease symptoms. Of the eight cowpea varieties grown in Sri Lanka, four were shown to be relatively resistant; all other legumes inoculated were completely resistant. Two morphologic and physiologic races of P. vignae were identified among the 24 isolates recovered, based on differential pathogenicity on cowpea varieties. Bacteria isolated from field soils, and other known bacterial biocontrol agents, inhibited P. vignae in culture, but only three Sri Lankan isolates considerably suppressed the disease in greenhouse tests. Volatile substances produced by most bacteria inhibited mycelial growth and sporangial production by P. vignae. The increased pH of the exposed medium suggested the involvement of ammonia. Volatile inhibitors were produced by these bacteria in soil, but only with added substrate; Strain DF-3101 also reduced oospore germination in soil. Cowpea plants inoculated with the VA mycorrhizal (VAM) fungus Glomus intraradices in P. vignae-infested soil were larger than non-mycorrhizal plants, but only at low levels of the pathogen. VAM colonization was reduced at high levels of the pathogen, and root infection by the pathogen was reduced by VAM. The fungicides metalaxyl, fosetyl-Al, Banrot, and Manzate-200DF reduced in vitro mycelial growth, but at different concentrations. Sporangia formation and germination, and oogonia formation by P. vignae, was reduced significantly by metalaxyl and fosetyl-Al. In greenhouse tests, metalaxyl, even at low concentrations, reduced disease; Fosetyl-Al was effective at high concentrations; Manzate-200DF was effective as a soil drench but not as a foliar spray; Banrot effectively reduced disease at 50 mg a.i./L. Exposure of a bacterial biocontrol agent to these fungicides in vitro did not affect its capacity to subsequently produce volatile inhibitors, but exposure to 10 ug/ml of metalaxyl and 50 ug/ml of Manzate-200DF reduced its capacity to subsequently inhibit mycelial growth of P. vignae.
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