SUMMARY

Survival in the field of Xanthomonas arboricola pv. pruni was studied using two mutants marked by resis-tance to 200 µg ml-1 of rifampicin (Rifr) and rifampicin-chloramphenicol (Rifr-Chlorr). The mutants were used to contaminate branches (July 1995) and buds/leaf scars (October 1995) of peach trees cv. ‘Elegant Lady’ in two peach orchards in the district of Verona. A sus-pension of the Rifr-Chlorr mutant was also used for vac-uum infiltration (September 1995) of leaves on de-tached shoots of the same cultivar. Survival was as-sessed on the basis of periodic reisolation on appropri-ate recovery media from bud/leaf scar samples (Octo-ber 1995-March 1996) and debris of infected leaves kept on the soil (January-April 1996). The pathogen was found in bud/leaf scar samples and in infected leaf debris up to 6-7 months after contamination. The Rifr mutant was only reisolated (frequency 13%) from leaf tissue samples with typical spots collected in one of the two experimental peach orchards in the summer of 1996. The mutants were never found associated with asymptomatic leaf or flower samples (summer-autumn 1995/spring-autumn 1996). X. arboricola pv. pruni ap-parently survives in buds/leaf scars and in infected leaf debris on the soil, providing potential sources of inocu-lum for primary infections even in the absence of spring (‘black tips’) and summer cankers, never observed dur-ing the experiments