Influence of soil factors,fertilizers and manures on pathogenicity ofRhizoctonia solani onVigna species

Rhizoctonia solani caused maximum mortality of mung bean seedlings at 20°C, and the disease incidence decreased with increase of temperature; 30° was optimum for mycelial growth of the fungusin vitro. The fungus grew best in nutrient broth of pH 5.5 but infected mung bean and pea seedlings more severely in neutral and alkaline river sand than in the sand adjusted to acidic reaction. The disease incidence was higher in adequately moist sandy loam and less in soil under moisture stress. Incidence of cowpea seedling rot was higher in heavy-textured loam and silt loam soils than in light-textured sandy- and loamy sand. Addition of montmorillonite and kaolinite in the sandy soil increased the disease incidence, but these clays reduced fungus growth in culture. More seedling rot occurred in the sandy soil fertilized with urea, potassium nitrate, monocalcium phosphate, or potassium dihydrogen phosphate while soil application of ammonium nitrate, potassium chloride, or potassium sulphate decreased the disease. In tests with combined soil application of N (as urea), P (as monocalcium phosphate) and K (as potassium chloride), disease incidence was more in all combinations having P. Among the six micronutrients tested, only boron reduced the disease incidence significantly both in presence and absence of NPK fertilizers. Farm-yard manure and biogas sludge aggravated seedling rot but their water extracts decreased it. Humic acid, extracted from farm-yard manure, increased the disease incidence but was inhibitory to fungus growth in culture. Green manure also resulted in more disease.