Effect of farmyard manure and green manure crops on populations of mycophagous soil fauna and Rhizoctonia stem canker of potato

Effects of organic soil amendments on populations of mycophagous springtails and nematodes and on Rhizoctonia solani stem canker of potato were investigated in two field experiments each lasting two years. The organic amendments consisted of three green manure crops (white mustard, forage rape and oats), and farmyard manure (FYM, alone or in combination with white mustard). In Year 1, before the application of soil amendments, experimental fields were infested with R. solani by growing a potato crop from seed tubers severely infested with black scurf. In the next year potato was grown again as test crop. In Experiment 1, there was a moderate degree of Rhizoctonia stem infection in the test crop. Organic amendments reduced the disease severity and increased populations of mycophagous soil organisms. The greatest reduction in disease severity was found when FYM application was combined with white mustard or when oats was grown as green manure crop. In Experiment 2, Rhizoctonia stem infection was so severe that emergence of potatoes in the test crop was reduced. Again, soil fauna populations were increased by farmyard manure combined with mustard and also when oats was grown as green manure crop. The disease severity was only slightly reduced by the former treatment, and significantly by the latter one. FYM+white mustard increased the springtail populations and had no effect on mycophagous nematodes, whereas oats increased the numbers of mycophagous nematodes tenfold. The results from both experiments support the hypothesis that stimulating the populations of mycophagous soil mesofauna can contribute to a reduction in Rhizoctonia disease severity in potato.