LAUSR

Abstract Cashew powdery mildew, caused by Erysiphe quercicola, is one of the main disease problems of cashew trees in Brazil. Thus, research to study the reaction of commercial cashew clones under different epidemiological stages of the disease is very important for identifying resistant clones. The objective of this research was to evaluate the reaction of commercial clones of cashew to powdery mildew by monitoring the disease during three disease cycles. The study was carried out at Embrapa Agroindustria Tropical, Pacajus Experimental Field, Ceara, Brazil, during 2012, 2013 and 2014 using eleven commercial clones of cashew (CCP 09, CCP 76, CCP 100, CCP 06, EMBRAPA 51, EMBRAPA 50, BRS 265, BRS 189, BRS 274, BRS 275 and BRS 226). Powdery mildew severity on inflorescences was estimated using a descriptive scale of severity consisting of scores ranging from 0 to 4. The transformed data were used to estimate the area under the disease progression curve (AUDPC) and the rate of infection. Based on similarities in disease severity estimates with time, clones were clustered using multivariate clustering analysis. Epidemics of cashew powdery mildew differed between the three cycles in terms of duration and severity. The clones were classified into four clusters of similar clones, defined by the graphical analysis on the basis of the powdery mildew severity. Clones BRS 274, BRS 275, CCP 1001 and BRS 226 attained lower AUDPC and clones BRS 189 and CCP 06 showed higher AUDPC. The results show evidence of partial resistance to powdery mildew among commercially grown cashew clones. Clones BRS 274, BRS 275, BRS 226 and CCP 1001 were the most resistant, while CCP 06 and BRS 189 clones were the most susceptible to powdery mildew. The AUDPC seems to be suitable for discriminating between genotypes, whereas the rate of disease progression may be used for establishing a threshold in the evaluation of other control methods, as it does not clearly differentiate between disease reactions.