بررسی شدت آلودگی ارقام گلابی به بیماری آتشک با استفاده از سیستم استاندارد USDA

The fire blight with the bacterial causal agent, Erwinia amylovora (Burrill) winslow etal. is one of the most important diseases of the pome fruits that causes the economical losses to quince, pear and apple productions, respectively, in some parts of country. To determine the infection severity of the 43 raturally infected pear cultivars in collection of karaj horticulture research division and also for studing the reaction of these cultivars against disease, the evaluation was performed by the USDA standard system for these cultivars in collection. In this study, although the most of cultivars had been infected on the natural conditions, but disease severity was significantly different between them. By the USDA system, pear cultivars were divided in the 2 classes. Percentage of pear cultivars in very susceptible and moderately susceptible classes were 81.4 and 18.6 respectively. Also for dividing the pear cultivars, beside of USDA method, SPSS software and the cluster analysis were also by the UPGMA method and cultivars were divided into 3 clusters, but it seems that the classification of cluster analysis did not conform with USDA system. Correlation of I.V.S in the artificial tests and the disease severity by natural infection was very significant (r= -0.83).