Effects of Pesticides on Red Rot of Sugarcane

Red rot disease of sugarcane, caused by Colletotrichum falcatum, can be an important constraint to crop production. In Louisiana, red rot primarily affects seed-cane and is more severe when billets (stalk sections) are planted rather than whole-stalks. At planting application of seed-treatment chemicals, particularly a combination of fungicide and thiamethoxam insecticide, has improved stand establishment and increased yields in billet plantings in Louisiana. However, information has been lacking on the effect of the treatments on disease development. Greenhouse experiments were conducted in 2020 and 2021 to evaluate stalk rot symptom severity and initial plant growth for billets dip-treated with a combination of azoxystrobin and propiconazole fungicides, thiamethoxam insecticide, a combination of both fungicides and insecticide, or left untreated. After 6 weeks, stalk rot severity was assessed as the number of nodes rotted, node passage, and the extent of internode rot. Plant growth was assessed for each billet as the number and length of shoots and shoot and root weight. Disease trait results varied for some treatments between experiments. The reductions in disease severity recorded for different treatments were similar for billets inoculated or exposed to natural inoculum. Disease severity was consistently reduced by the combination treatment while reductions resulting from treatment with fungicides and insecticide alone were variable. The effects of chemical treatments on plant growth after 6 weeks in the greenhouse were minor; however, there was evidence of disease adversely affecting germination, particularly under natural inoculation, where germination was reduced by one third. The treatments that reduced disease severity prevented this reduction. The results provide evidence that reductions in disease severity accounted for the stand establishment and yield improvements observed for treated billets in field experiments