Podridão radicular de Rhizoctonia em soja: fatores edáficos que interferem na incidência e efeito da calagem

Abstract

Rhizoctonia root rot (RRR) is a disease caused by the fungus Rhizoctonia solani that causes the death of soybean plants in patches over croplands. The control is difficult since the pathogen is a natural inhabitant of the soil and has a wide range of host plants. It is known that edaphic factors can affect the occurrence of diseases in plants, however, there is limited information about which edaphic factors are essential for the occurrence of RRR in soybean. Furthermore, as there are no soybean cultivars with high genetic resistance and chemical control with fungicides is restricted to seed treatment, other control methods such as cultural control could be used to manage the disease. Thus, the objectives of this work were i) to determine which physical, chemical and biological attributes of the soil correlate with the occurrence of RRR in soybean, and ii) to verify the effect of application of limestone rates on the incidence of RRR and soybean yield. Initially, four fields were selected with different soil classes (Acrisol, Cambisol Ta, Cambisol Tb, Nitisol) and history of occurrence of RRR in soybean in the municipality of Soledade, Rio Grande do Sul, Brazil. In the first study, soil samples were collected between March and June 2020 at two locations in the field (inside and outside the patch). For chemical and physical haracterization, samples were stratified in 0-5, 5-10, 10-15 and 15-20 cm layers, while for biological analysis the samples were collected at a depth of 0-10 cm. In total, 18 chemical attributes, 16 physical attributes and two biological attributes were evaluated. The variables chemicals most correlated with the location within the patch were Al, H+Al, Al saturation and Cu content. However, the variables with the highest correlation at the site outside the patch were Mg, Ca, pH, SMP, CTC and base saturation. As for the physics attributes, there was no consistency in the data regarding the location inside or outside the patch. There was no significant difference (p > 0.05) between the sites for bacterial and fungal colony-forming units. The second study consisted of an experiment carried out under field conditions in the 2020/2021 crop season. The treatments were arranged in a factorial scheme of 4 fields x 2 sites in each field (inside and outside the patch) x 4 doses of limestone (0, 3, 6 and 9 t ha-1). The experimental design was in complete blocks with four replications. The incidence of RRR inside the patch was on average 40%, while outside the patch it was 1.2%. The reduction in the incidence of the disease was 3.9%, 3.7%, 3.6% and 1.8% for each 1,000 kg of limestone applied to the Acrisol, Cambisol Ta, Cambisol Tb and Nitisol, respectively. The highest soybean yield was 4713 and 3349 kg ha-1 for the Acrisol and Nitisol with application of 6.6 and 6.2 t ha-1 of limestone, respectively. Soybean yield in the Cambisols increased linearly with liming. The productivity improvement reached 121 and 172 kg ha-1 for each 1000 kg of limestone applied in the Cambisol Ta and Cambisol Tb, respectively. Thus, liming reduced the incidence of RRR in all fields and promoted better plant nutrition, increasing soybean productivity. Liming is, therefore, a practice that contributes to soil fertility and plant disease control.