The fungus Rhizoctonia solani Kuhn is considered the most important cause of cotton damping-off in Brazil. Treatment of seeds with fungicides is the most efficient and economical procedure to control this disease. The performance of fungicides depends, among many other factors, of the pathogen population in the soil. This work was carried out, under greenhouse conditions at Embrapa Agropecuária Oeste, Dourados, MS, with the goal to determine the effect of treatment of cotton seeds with fungicides to control damping-off at different inoculum densities of R. solani in the soil. Seeds of cultivar DeltaOpal treated or not with different fungicides were sowed 3 cm deep in sand contained in plastic flats. Seeds were placed in individual and equidistant wells. Inoculum of the fungus was homogeneously distributed in the substrate. The fungus was grown for 35 days on autoclaved oat seeds and then ground to powder using a mill until reach the size of 1mm. Four inoculum densities were tested: 1 g; 2 g; 3 g e 4 g/plastic flat with dimensions of 56x35x10 cm. The effect of the fungicide treatment on initial and final seedling emergence, as well as, in the control of post and pre-emergence damping-off was observed. Treatment of seed with a combinations of fungicides gave the best results in controlling damping-off as compared to the use of each fungicide alone. The interaction fungicides x inoculum densities, was significant indicating that the efficiency of the fungicides was somehow affected by the fungus densities. Fungicides had the best performance at low inoculum densities (1g and 2g/flat). As for fungi populations of 3g and 4g/flat, the efficiency of all fungicides tested decreased sharply, with a reduced control of the disease. The fungicides used in this study did not have any phytotoxic effect on cotton seedlings.
soilborne pathogen; fungus density; Gossypium hirsutum
