A.D. Mowat, S.M. Hoyte, A.W. Holmes, P.A.G. Elmer, T.R. Reglinski, S.A. Miller, S.J. Saunders
An incursion of a virulent biovar of Pseudomonas syringae pv. actinidiae (Psa biovar 3 or Psa-V) bacterial canker in New Zealand has caused significant economic impact for the kiwifruit industry. In the worst case, this pathogen has resulted in the loss and replacement of whole vines. The phenotypic expression of susceptibility of kiwifruit vines to a given strain of Psa is a function of genetics, environment and management. The mineral composition of cultivated kiwifruit vines can be influenced by management practices, such as chemical fertilizer application, as well as by incorporating composted organic matter in the soil. For other crops, the susceptibility of plants to fungal and bacterial pathogens can be influenced by plant nutrition. Nitrogen is one of the major nutrients applied to kiwifruit. Does nitrogen influence the susceptibility of kiwifruit plants to bacterial canker? To answer this question we aimed to falsify the following three null hypotheses. Susceptibility to Psa is not influenced by rate of nitrogen, source of nitrogen, or by interaction with nitrogen and genotype. Seedlings of Actinidia deliciosa ‘Hayward’ and A. chinensis ‘Hort16A’ were grown in sterile pumice supplied with one of six nutrient-based treatments. The first, a control, was based on a standard nutrient solution suitable for kiwifruit seedling growth. The second and third treatments were based on low and high rates of nitrate applied with the standard solution, respectively. The fourth and fifth treatments were based on low and high rates of ammonium applied with the standard solution, respectively. A sixth treatment, based on plants treated with a foliar ActigardTM and the standard solution provided a positive control. At the four-leaf stage, seedlings were stab inoculated with a high concentration of Psa. Symptom expression was monitored over time in detail and then summarised as a binary score for plants without or with any sign of symptom expression within a treatment. The key finding was that the symptom expression for A. deliciosa ‘Hayward’ seedlings treated with a high rate of nitrate was the same 25 days after Psa inoculation as the seedlings treated with ActigardTM, but was significantly lower than those seedlings treated with the standard nutrient solution. For A. deliciosa ‘Hayward’ seedlings, symptom expression was significantly higher for the high ammonium treatment than for the ActigardTM, low ammonium and standard nutrient solution treatments 13 days after inoculation. For A. chinensis ‘Hort16A’ seedlings, no significant differences in survival were observed between nitrate, ammonium, ActigardTM or the standard nutrient treatments.
Mowat, A.D., Hoyte, S.M., Holmes, A.W., Elmer, P.A.G., Reglinski, T.R., Miller, S.A. and Saunders, S.J. (2015). EFFECT OF NITROGEN SOURCE ON THE SUSCEPTIBILITY OF TWO KIWIFRUIT SEEDLING GENOTYPES TO BACTERIAL CANKER. Acta Hortic. 1095, 161-167
DOI: 10.17660/ActaHortic.2015.1095.20
Actinidia deliciosa, A. chinensis, Pseudomonas syringae pv. actinidiae, ammonium, nitrate

Acta Horticulturae