The low down on Psa in kiwifruit


Psa – you’ve likely heard of it, if not dealt with it and it is still a major challenge across the kiwifruit industry.

This bacterial pathogen – formally known as Pseudomonas syringae pv.actinidiae – causes leaf lesions (spotting), shoot dieback, flower-bud infection and secondary cankers. Levels of impact depend on things like varietal susceptibility, vine age and health, orchard inoculum levels and prevailing environmental conditions. In short, typical disease triangle risk factors.

Psa enters through both natural plant openings (stomata, hydathodes, leaves and lenticels) and man-made wounds (e.g. pruning and girdling
cuts). It impacts production through loss of canopy due to shoot and cane collapse and crop loss from male and female flower-bud infection.

Cool wet spring conditions favour disease development and early in the season infected vines can express exudate containing Psa bacteria, directly increasing inoculum load and disease risk.

Hayward leaf tissue is susceptible to Psa infection until 4 weeks after emergence and flower buds (present within 2 weeks of budbreak) are also highly susceptible. Gold3 flower buds follow a similar risk pattern.

Good Psa management requires spray protection across all vine surfaces prior to the arrival of Psa inoculum. Often leaf spotting and/or sepal
infection triggers a spray response by growers but these indicators in fact show infection periods have already been missed, as research shows that there is a 10-day lag between Psa infection and visible symptoms.

A smarter approach – and one that predicts forward risk – is needed to ensure sprays are well-timed, so that protection is in place before Psa arrives.

Shortly after Psa was first discovered in New Zealand, the industry recognised the value of developing a weather-based disease forecasting
model to help provide this forward thinking and help with the long-term management of the bacteria.

Plant & Food Research (Beresford et al) identified conditions conducive to infection and developed the model using temperature, leaf wetness and rainfall values to calculate a daily risk index (R). A threshold of 1mm of rain was determined as sufficient for multiplication and dispersal of Psa field testing, including a 9-week spring and 82-week seasonal accuracy study, which found that the model predicted Psa infection risk with high accuracy.

Today, the model is linked to a nationwide weather station network and forecasting platform which provides a daily Psa risk (R) value and risk category, for the following 10 days.

Descriptive risk categories range from negligible, through to severe and help guide grower decisions on spraying (ahead of high-risk weather) and orchard activities, ensuring pruning, girdling and thinning are completed in low-risk weather.

The Psa-V Risk Model is available at and can be viewed via desktop computer or mobile phone. To register simply contact KVH on 0800 665 825 or email

Article supplied by Kiwifruit Vine Health Inc (KVH)

KVH is a leading biosecurity organisation dedicated to supporting the New Zealand kiwifruit industry. KVH was established in December 2010 to lead the industry response to the Psa incursion. Since November 2012 KVH have been the lead organisation responsible for managing biosecurity readiness, response, and operations on behalf of the kiwifruit industry.