What causes symptomless Psa to become symptomatic?
A recent Psa research project has helped our understanding of how and where Psa can survive in symptomless kiwifruit plants.
Inoculation of leaves and stems, of Hayward, Gold3, Hort16A, Green 11 and A .arguta showed that Psa could multiply within these plant tissues, but often without the plant expressing symptoms. The bacterial load could also become high, in fact as high as levels found in symptomatic plants, and the Psa was able to travel at least short distances internally. Differences in metabolites and genes in symptomless plants were found, with further work needed to understand the underlying pathways involved. It was found that the physiological state of the plant and its rate of growth seemed to control symptom expression, with more actively growing plants more likely to express Psa symptoms.
Practically, results from this work point to the likelihood that late summer and autumn infections are most likely to result in Psa being present in plant tissues without the plant showing visible symptoms. It follows that late season application of control products are likely very valuable through this period despite no obvious symptoms being seen.