The bacterial pathogen Pseudomonas syringae pv. actinidiae (Psa) is the causal agent of kiwifruit canker, a serious disease of Actinidia species occurring in many countries around the world. It was first discovered in New Zealand in 2010 and continues to be a serious threat to the New Zealand kiwifruit industry. It has been observed that tissue age, particularly leaf age, could affect the susceptibility of kiwifruit to infection by Psa. In the field it has been observed that the younger leaves of kiwifruit vines are more susceptible to Psa. However, the influence of kiwifruit leaf age on Psa infection and symptom development has not been systematically studied. New Zealand scientists investigated the influence of leaf age on infection of Actinidia species by PSA using two approaches: (a) inoculation of potted 'Hayward' and 'Hort16A' kiwifruit plants and (b) inoculation of whole leaves in a detached leaf assay. Whole plants and detached leaves were spray inoculated with Psa biovar 3 (haplotype NZ-V; Psa-V) and maintained in a saturated environment. Flecking was evident on some of the leaves 8 days after inoculation. Many of the flecks later became necrotic spots with halos similar to the Psa leaf infections that have been observed in the field. With both methods, a higher percentage of leaves that were 1–3 weeks of age at inoculation had flecking and spotting more so than leaves of other ages. Leaves that were 7 weeks or older did not show any symptoms of infection by Psa. Overall, leaves of 'Hort16A' showed slightly more flecking and spotting than 'Hayward'. Scientists conclude that leaf age has a significant effect on infection and symptom development by Psa in both kiwifruit cultivars tested. Leaves from 1–3 weeks of age were particularly susceptible, this information on the relationship between leaf age and susceptibility to Psa not only can help in PSA management 'on orchard' but also can be useful to develop disease risk models. Source - http://www.freshplaza.com