Hypersensitivity of Plum Genotypes to Plum Pox Virus

The hypersensitive response (HR) of plum hybrid K4 as an active defence mechanism is directed against certain plum pox virus (PPV) strains/isolates of the type CG but not against type DI. There is no clear correlation between the HR of K4 and serological di€erences of the PPV strains D,Mand C, respectively. A total of 14 genotypes out of 21 progenies of K4 reacted necrotically and were proved to be either hypersensitive and ®eld resistant or highly sensitive and not ®eld resistant. The HR can be connected with a complete or with an incomplete virus localization in K4 and some of its progenies. The remaining genotypes were more or less sensitive and tolerant, respectively. Growth of buds from PPV-CG- infected plum cultivars on K4 di€ered in the degree of systemic virus invasion in the plant as a trait of quanti- tative virus resistance. The percentage of growing buds correlated with the level of quantitative PPV resistanc