IDENTIFICATION AND COMPARISON OF DIFFERENT STRAINS OF APPLE CHLOROTIC LEAF SPOT VIRUS AND POSSIBILITIES OF CROSS PROTECTION

Apple chlorotic leaf spot virus was first described on apple trees in 1959. Isolated from Prunus in 1966 it was, for a long time, considered as a latent virus. In fact, in the last years, the virus was demonstrated to be responsible for various diseases, inducing intra and inter-incompatibilities, prune bark split and false plum pox. Furthermore, the virus is widespread, and roughly 35% of the Prunus plants tested in the last 4 years at the station appeared to be naturally infected.

This paper reports the results obtained from the comparison between strains of CLSV from apple and Prunus. These comparisons were made by noting the symptoms on a wide range of woody and herbaceous indicators, and by the serological technique.

Possibilities of interactions between four purified strains of CLSV were evaluated on two indicators, and the results were recorded with regard to the appearance of line pattern on Malusplatyca rpa and bark split on Prunus domestica P 707.

Results greatly depended on the host plant and the nature of the strains. On Malusplatyca, no cross protection was observed, and line pattern symptoms did develop whatever the strain combination was. On the contrary, inoculation of P. domestica P 707 with a mild strain (unable to induce bark split symptoms) always prevented the plant from developping symptoms when surinfected with a strain normally inducing severe bark split.

Our recent results show a new case of cross protection and they also point out, once again, the great diversity of the virus strains and their economical importance.Routine testing for plum pox virus confirmed a great similarity between plum pox symptoms and those associated on a few plum cvs. with some CLSV strains. But, if this economical importance is partly due to the severity of symptoms it also results from the high level of infection in several Prunus sp.