Presence of Pseudomonas syringae pv. actinidiae in buds of asymptomatic Actinidia chinensis var. deliciosa plants in late autumn and winter

In bacterial diseases, the latency period, during which there are no visible disease symptoms, can be prolonged depending on the conditions unfavorable for the growth of pathogens. As for Pseudomonas syringae pv. actinidiae (Psa), the causal agent of the bacterial canker of kiwifruit (BCK), the ability to survive several years in asymptomatic plants was recently highlighted. The Psa pluriannual latency in susceptible plants raises serious problems to control BCK for which it is necessary to consider defense strategies other than those based on spray treatments. Phytopatogenic bacteria can be hosted in buds of asymptomatic plants and their survival is regarded as a factor facilitating the overwintering of certain diseases caused by P. syringae. To clarify the bud epidemiological role, the Psa early detection in dormant buds (≈300) was carried out. In late autumn, 10 asymptomatic A. chinensis var. deliciosa plants were selected in an orchard located at the edge of a safety area that in the current year did not show BCK symptoms. At the beginning of dormancy (mid-December) and of “bleeding” (end of February), the buds were removed from a one-year old shoot. The Psa presence was determined by direct isolation and PCR analysis. At the end of the two surveys, the plant phytosanitary state was monitored to relate the orchard situation with the effective Psa absence/presence as pointed out by the early detection on bud samples. Psa was present in buds from 1 out of 10 and in 4 out of 10 plants in the first and in the second survey, respectively.