Studies On The Resistance To Alfalfa Powdery Mildew And The Resistance Mechanism

(Department of plant protection, Gansu Agricultural University, Lanzhou, 730070 China ) Abstract: Resistance of nine alfalfa cultivars to powdery mildew was studied by means of natural infection in the field and artificial inoculation in the laboratory. The results showed that the resistance to powdery mildew among the alfalfa cultivars was significant different. Sitel, Derby and Derful were susceptible; Algongium, Ameristand201 and Qingyang were resistant. Moreover, the level of resistance to the disease between field natural elaluation and laboratory artificial inoculation was significant positive correlation.The relationship between the stomata of leaves and the resistance to powdery mildew in alfalfa cultivars was studied. Statistical analysis indicated that the stomatal density and length were not correlated with the resistance to powdery mildew in eight alfalfa cultivars.The effection of fertilizer on the incidence and disease index of alfalfa powdery mildew was studied in the field. There were three kinds of fertilizer that were Sakerful (N: P: K: S=15:15:15:18), superphosphate and rhizobium. The results demonstrated that the incidence and the disease index of alfalfa powdery mildew with Sakerful were lower than those of others. The Sakerful different application level impact significantly on the degree of this disease occurrence. The incidence and the disease index of alfalfa powdery mildew with 75kg/hm2 were lower than those of 45kg/hm2 and 105kg/hm2.The variation of some biochemical index of alfalfa cultivars such as the content of chlorophyll, the activity of peroxidase (POD) and activity of phenylalnine ammonia-lysae were analyzed by testing alfalfa seedling whose leaves have been inoculated and uninoculated. The results were as follows: the content of chlorophyll of the control, the activity of PAL of the control were not different among cultivars, while the POD activity was significant different in the control leaves. The content of chlorophyll in the susceptible cultivars was lower than that of resistant ones in 20-27d after inoculation. The activity of POD in the susceptible cultivars increased more than that of resistant ones in 7-13d after inoculation, while the increasing rate of the activity of PAL in the susceptible was less than that of resistance ones in 7-10d after inoculation. It was suggested that the change of the content of chlorophyll, the activity of POD and the activity of PAL in the inoculated leaves were closely correlated with the resistance of varieties.