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 differences of the PPV strains D, M and C, respectively. A total of 14 genotypes out of 21 progenies of K4 reacted necrotically and were proved to be either hypersensitive and field resistant or highly sensitive and not field 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 differed in the degree of systemic virus invasion in the plant as a trait of quantitative virus resistance. The percentage of growing buds correlated with the level of quantitative PPV resistance.