Doenças foliares em cultivares de alfafa (Mendicago sativa L.) em Lavras, Minas Gerais

Abstract

It was aimed to verify the chiefleaf diseases which occurs in alfalfa, in Lavras-MG, state of Minas Gerais, to evaluate the susceptibility of alfalfa cultivars to those diseases and to correlate the occurrence and severity ofthe diseases with the local climatic conditions. The severity ofthe diseases was evaluated by the percentages of defoliation and of leaf área covered with symptoms. So, a randomized block trial in split plot schene in time, whose factors were 35 alfalfa cultivars in the plots and 11 evaluation times which correspond to the months in the subplots. 22 evaluations were accomplished during the experimental period (September 97 to October 98), 11 of them being performed at 14 days after cut and the other ei 1 at 28 days after the cut. For evaluation ofthe defoliation, ten stem of alfalfa were collected, in a random manner, per plot, determiningthe percentage ofdefoliated knots accordingto the formula: [(number ofdefoliated knot /total number ofknot on the stems) x 100]. The severity ofthe diseases was observed on the same stems, by estimating the percentage ofthe leaf área infected through a diagramatic scale proposed by Thal e Campbell (1987b). The data was expressed in percentage and submitted to the analysis of variance. The contrasts between means compared by ScottKnotfs test at 5%. The chieffungai diseases which occurred in the 35 cultivars in Lavras, MG, and the periods of greatest severity were: leaf spot (Leptosphaerulina briosianà), which occurred throughout the year but with greater severity in the coldest periods ofthe year, its being the disease which caused the worst damages to the cultivars, summer black stem and leaf spot (Cercospora medicaginis), which presented greater severity at the end of summer and beginning of fali and rust (Uromyces striatns\ which only in the evaluations ofNovember showed higher severity even so with a low index of leaf symptom. The highest percentages of defoliation were obtained in the coldest months of the year, but the values never overtook 5% even for the cultivars most susceptible to the diseases. In the evaluations done at 14 days after cut, both the indexes ofdefoliation and severity ofdiseases were lower than at 28 days. The cultivars most resistant to leaf spot and defoliation were Crioula, P30 and Costera SP INTA. As to resistance to summer black stem and leafspot, standing out the cultivars Costera SP INTA, MH15, SW 9210 and MH4. The most susceptible were P30, Valley Plus, Crioula and Maricopa. In the grouts ofthe cultivars most resistant to rust stood out the cultivars MH 15 and MH 4 and in the group of the most susceptible ones the cultivars Esmeralda SP INTA, Valley Plus and El Grande. In general, the cultivars mostresistance to diseases were Crioula, P30 e CosrteraSP INTA.