MULTIPLICATION AND MOVEMENT OF PSEUDOMONAS SYRINGAE PV. ACTINIDIAE IN KIWIFRUIT PLANTS

Abstract

Pseudomonas syringae pv. actinidiae (Psa), the causal agent of bacterial canker of kiwifruit, is able to spread through infected kiwifruit plants. This study had two goals. The first one was to understand some of the conditions which lead to the movement of Psa in the plant. To study the role of relative humidity (RH) in the colonisation process, young potted plants of Actinidia chinensis ‘Hort16A’ were inoculated in the containment glass house at Plant & Food Research at Ruakura and kept under different RH conditions. Plants kept at an average RH of over 95% for three weeks developed systemic symptoms. Live cells of Psa were found in most of the stem tissues of the three-month-old plants. Plants which were kept at high RH for only 48 hours did not develop systemic symptoms and the bacteria did not move far from the point of inoculation. The second goal of this study was to determine whether there was a correlation between endophytic populations of Psa and symptom expression. Ninety six plants from a cross between parents of different susceptibility to Psa were stem inoculated with Psa and kept for four weeks in a glasshouse with an average RH of over 85%. Under those conditions, necrosis of a few mm to more than 20 cm in length developed from the point of inoculation. No strong correlation between endophytic populations of Psa and the length of the necrosis was found, indicating that relying on the length of necrosis alone to determine the susceptibility to Psa of a kiwifruit genotype could be misleading.