Effect of inoculum, cultivar, and the biological control fungus Beauveria bassiana on damping-off caused by Rhizoctonia solani on tomato

Rhizoctonia solani Kuhn is a common soil inhabiting fungus that causes disease in cultivated plants worldwide. Considering the environmental and economic costs of chemical pesticides, development of additional disease control strategies is necessary. Disease-resistant cultivars and biological control are attractive alternatives.The main objective of this study was to evaluate 10 tomato (Lycopersiconesculrntum Mill.) cultivars for resistance to R. solani. The cultivars includedCarolina Gold, Celebrity, Hybrid 882, Mountain Belle, Mountain Delight, MountainFresh, Mountain Pride, Mountain Spring, Mountain Supreme, and Sunpride. The Second objective was to compare two forms of R. solani inoculum for ability to cause disease on tomato seedlings. Inoculum of R. solani was produced on either rice or cornmeal;sand, and added to planting medium as 0.5 to 1.0-mmsieved rice at 1%, and 4% cornmeaLsand. Infested mixes were placed in 7.6-cm diameter pots and one untreated tomato seed was planted per pot. Stand count data were collected at 7, 14, and 21 days after planting (DAP). At 28 DAP,disease incidence and severity were recorded. There was no difference in percent stand count, or disease incidence for the two inoculum types. Disease Severity was significantly higher with the cornmeals and inoculum. Pre-emergence damping off was most severe with Celebrity, Mountain Belle,Mountain Spring, and Sunpride. Cultivars that appeared to be less susceptible to damping-off were Hybrid 882, Mountain Fresh, and Mountain Supreme. There Were no differences in disease severity of tomato seedlings.The third objective of this study was to evaluate different forms of Beauveria bassiana (Baisamo) Vuemin, as a biological control agent against R.solani on tomato, cultivar Mountain Spring. The six treatments included: 0.02 g conidia per 20 seed; 0.20 g mycelia per 20 seed; 30 g alginate prills per 2 Lplanting medium; 82 g mycelia per 5 L planting medium; seed treated withconidia + planting medium treated with prills, and seed treated with conidia +planting medium treated with mycelia. The three controls were untreated seed in infested soil, and untreated seed and seed treated with methyl cellulose in infested planting medium. Seeds were coated with 0.5% methyl cellulose solution(for adhesion) before addition of conidia or mycelia of B. bassiana. All planting mix was infested with 4% w/w cornmeals and inoculum of R. solani, except the first control group. Treated planting mixes were placed in 7.6-cm diameter pots,and one tomato seed was planted per pot. Stand count data was taken at 7 and14 DAP. At 31 DAP, disease incidence and severity, and shoot and root weights were determined. Protection against damping-off was greatest with seed treated with mycelia, seed treated with conidia + planting medium treated with mycelia,and seed treated with conidia. However, seed treated with conidia alone delayed germination. Disease incidence was lowest with seed treated with conidia. Seedtreated with conidia and medium treated with mycelia had the lowest disease severity. The two treatments with the greatest shoot weight were seed treated with conidia + medium treated with mycelia and medium treated with mycelia. Seed treated with conidia had significantly lower shoot and root weights. Due to delayed germination, plants in this treatment were physiologically younger than plants from other treatments. The treatment with the highest root weight were the planting medium treated with mycelia, and the seed treated with conidia +planting medium treated with mycelia, although, these were not different from the control groups