Screening of soybean cultivars resistant to black root rot (Macrophomina phaseolina). Summa Phytopathologica

ABSTRACT The screening of genotypes resistant to black root rot in the field has high cost and high variability, since a large number of external factors can affect the behavior of plants and even of the fungus, according to the place and the year of cultivation. Screening of varieties resistant to the rot caused by Macrophomina phaseolina at greenhouse presents greater uniformity and stability of results once the conditions are controlled. However, it is recommended that the results obtained in a protected environment present good correlation to the results obtained in the field in order to be adopted as a screening protocol. Thus, this study aimed to compare the efficiency of screening methods at greenhouse and check their correlation with the behavior of soybean varieties in a field test. The evaluated screening methods were mycelium disc on cut stem; stem puncture with colonized toothpick; soil infested by inoculum produced in rice grains (1g, 5g and 10g per kg of soil); and irrigation with suspension of microsclerotia (3x104 and 6x104 CFU/mL) onto seedling roots. The field trial was set in an area with history of the disease occurrence, and the soil was infested with sorghum seeds colonized with M. phaseolina in the sowing groove. Plants inoculated on the stem, at greenhouse, were subjected to two humidity conditions: humid chamber for three days after inoculation and absence of humid chamber. The disease severity was determined based on the length of the lesion, for the method of mycelium disc on cut stem, and on a scale of notes referring to the root lesion, for the methods of stem puncture with colonized toothpick, suspension of microsclerotia and soil infested by inoculum. For the latter, the height of plants was also evaluated. The methods of stem cut and colonized toothpick were the best to discriminate genotypes, while the absence of humid chamber provided greater severity of the disease. In general, BMX Apolo was more susceptible to the disease, while GDM15I029 and BMX Elite showed more resistance. There was a reduction in the height of plants inoculated by soil infestation, especially for the higher inoculum concentration. Inoculation with inoculum suspension was efficient only at the highest inoculum level, and allowed to differentiate genotypes for severity, showing that BMX Tornado was more susceptible than GDM15I029. There was a significant correlation with the field data only for the method of mycelium disc on cut stem, with (rs = 0.84) and without humid chamber (rs = 0.80). The results suggest that screening for resistance to black root rot at greenhouse using the method of cut stem is efficient and representative of the behavior of soybean genotypes under field conditions.