Pathogenicity differentiation of isolates of Phytophthora capsici to pepper

The studies on pathogenicity differentiation of isolates in Phytophthora capsici Leonian on pepper were carried out to explore the relationship between pathogenicity differentiation and isolate localities,the correlation between the pathogenicity and mycelium growth rates,and to provide more information for disease-resistant cultivar breeding and integrated management of the disease.Twenty-three isolates of P.capsici were obtained from the diseased peppers collected from Hefei,Huainan,Hexian,Qianshan,Yuexi,Nanjing and Qionglai by means of tissue isolation.The pathogenicity of the isolates from different areas was investigated by applying mycelium block stem wound inoculation on pepper seedlings and leaf in vitro inoculation,respectively.The cluster analysis of the pathogenicity data was done by unweighted pair group method with arithmetic averages(UPGMA) and the relationship between pathogenicity and geographical origin of isolates were analyzed.The mycelium growth rates of the isolates were tested and the correlation between the pathogenicity to pepper and growth rates was investigated.The results showed that all the tested isolates caused pepper blight on the stems and leaves inoculated,but there was significant difference in the diameters of the stem lesions and the severity caused by different isolates.Based on the average diameters of the stem lesions,the pathogenicity of the isolates could be classified into three categories,strong,intermediate and weak.The pathogenicity tes-ted with stem wound inoculation in vivo was positively correlated to that tested with leaf in vitro inoculation(r=0.576 5,R2=0.332 4).However,the pathogenicity of P.capsici to pepper seedlings had little correlation with growth rate(R2=0.013 7).It was suggested that there was significant differentiation in pathogenicity of isolates in P.capsici on pepper.The isolates could be classified into three types of pathogenicity,strongly,intermediately and weakly pathogenic.However,the pathogenicity difference was not obviously related to the localities of isolates,and had little relation with the mycelium growth rates.