The epiphytic life of Pseudomonas syringae pv. actinidiae on kiwifruit and on other cultivated and spontaneous plants.

Bacterial canker of kiwifruit is the most destructive disease of cultivated Actinidia spp. The causal agent is the Gram-negative bacterium Pseudomonas syringae pv. actinidiae (Psa). Several surveys have been carried out in kiwi orchards in Romagna (northern Italy) and proved that Psa may have a significant epiphytic life, mainly on flowers, pollen and leaves. On the fruits of affected orchards Psa was detectable until early August. In Romagna, kiwifruit is cultivated in areas where also other fruit crops are grown, often side by side. A two-year survey confirmed that Psa is present as a leaf epiphyte in neighbouring stone fruit orchards (apricot and plum), when an affected kiwi orchard is nearby. Several other spontaneous plants and weeds present in kiwifruit orchards have been checked for the epiphytic presence of Psa. Stinging nettle was constantly associated with the epiphytic presence of Psa, thus providing good conditions for bacterial growth and survival. Psa was also detected on, or isolated from other weeds, like amaranth, common mallow, purslane, dandelion and smartweed. The time range during which Psa was detected/isolated on other plants was the same as Psa could be traced on kiwi plants. These results suggest that Psa, like other pathogenic and non pathogenic pseudomonads, might spend a significant lapse of time on non-host plants. The role in the epidemiology of the bacterial canker of kiwifruit of such epiphytic population is not known. As a good practice for kiwifruit orchards, control of weeds, especially in case of Psa presence, is particularly advised.