Risk factors for kiwifruit bacterial canker disease development in 'Hayward' kiwifruit blocks

In November 2010 Pseudomonas syringae pv. actinidiae biovar 3, the cause of a severe disease, kiwifruit bacterial canker, was first recorded in New Zealand. This study examined risk factors relating to disease management, vine management and orchard layout that were associated with disease symptoms observed by orchardists in Actinidia chinensis var. deliciosa 'Hayward' orchards. A cross-sectional study using data collected via a questionnaire investigated orchard blocks that were symptom-free in March 2012. The outcome we modelled was detection of disease in the block during the study period from March 2012 to February 2013, and multivariable logistic regression was used to identify potential risk factors. Data from 194 growers were included and comprised 53 orchard blocks which remained disease free and 141 which became diseased. This cross-sectional study identified four factors that were associated with Psa symptom development. The associated factors identified in this study are not necessarily causal, but our results can be used by the kiwifruit industry to help prioritise research needs to identify processes involved in the development of kiwifruit bacterial canker in kiwifruit orchards. Priority for further research is the relationship between the timing of copper sprays, callus tissue formation and Psa mobilisation. A second priority is to determine the biological mechanism for the association between girdling and a reduction in disease risk. Use of a cross-sectional study provided a new way to investigate plant disease risk factors and this type of study could be more extensively used, especially during incursions of unwanted organisms.