Sorghum (Sorghum bicolor): a study of the resistance to crop pests in Brazil

Abstract

The insect-plant interactions involve survival mechanisms, selection of plant resistance genes and the breaking of this resistance by insects. These interactions result in the development of defense mechanisms in plants which exert selection pressure on insects, characterizing coevolutionary processes. This thesis brought together three articles on the sorghum crop aiming at resistance to the main pests. In the first article, the objective was to evaluate the susceptibility of sorghum bmr-6, a mutant genotype with a lower concentration of lignin, to the pests Diatraea saccharalis and Spodoptera frugiperda. Experiments were carried out in laboratory and greenhouse, evaluating the development of these pests in sorghum biomass hybrids bmr 007, 008, TX635 and their respective conventional isogenic genotypes (without the bmr gene). The lignin content was higher in non-bmr hybrids. The lowest survival of S. frugiperda was verified in the hybrid BR008 both bmr and non-bmr. Injury scores by S. frugiperda in sorghum in greenhouse were high (>7) in all hybrids. For D. saccharalis, there was no significant difference for larval survival in the laboratory, but in the greenhouse the hybrid BR007 both bmr and non-bmr provided greater survival. The second article aimed to evaluate the influence of D. saccharalis infestation on grain production in commercial hybrids. For this purpose, grain sorghum hybrids were planted in three crops: March 2018 (2018/1) (CROP SEASON 1), September 2018 (2018/2) (CROP SEASON 2) and March 2019 (2019/1) (CROP SEASON 3). In the experiment, six treatments were used, being the three hybrids of grain sorghum and the same three hybrids with chemical treatment with the product Altacor® (150 ml ha-1), under natural infestation, in four replications. The parameters evaluated were: length of galleries (cm), plant height (cm), length (cm) and weight (g) of panicles, infestation intensity and productivity. The infestation intensity of the three crops seasons was higher when not treated with insecticide and resulted in lower productivity. Regarding the difference between crops seasons, the planting of the 1st crop season showed lower infestation intensity. The hybrid DKB 590 had the lowest tolerance level, and with insecticide production increased 1.5 times in crop season 1 (149.77%). In crop season 2, hybrid BRS 373 showed the smallest increase in grain yield with control (29.79%), followed by AG1090 with 33.13%, and again DKB 590 (40.79%) with greater dependence on chemical control to increase productivity. Thw crop season 3 had the lowest yield among the three crops, and the hybrid AG1090 responded better to chemical control with a 105.90% increase in yield, followed by DKB 590 with 54.26% and BRS373 with 38.19%. The third article evaluated the susceptibility of 30 grain sorghum hybrids to S. frugiperda, D. saccharalis and Diceraeus melacanthus in a greenhouse. For fall armyworm, plant evaluation was performed 7 and 14 days after infestation (DAI). For D. saccharalis at 40 DAI and for the green-bellied stink bug at 12, 19 and 26 days after infestation. Through cluster analysis, it was possible to separate grain sorghum hybrids into groups according to the levels of resistance to each pest. The BRS373 hybrid stood out as moderately resistant to S. frugiperda; AG1090, 80G20, BRAVO, BRS373, AG1615 and IG220 were the most promising for D. saccharalis; and for D. melacanthus, the hybrids 50A40, A9735R, JADE, ENFORCER, BUSTER, 50A10 and IG244 were the most indicated. It is concluded that sorghum is susceptible to the three species studied here, the selection of hybrids is specific for each species, the lower lignin content in sorghum does not make the plant more susceptible to D. saccharalis and S. frugiperda, and grain sorghum it can be very sensitive to D. saccharalis attack, decreasing productivity by up to 1.5 times depending on the hybrid, planting time, lack of monitoring and adequate control.