Winter opportunities to reduce Psa risk
Winter is a key time to reduce orchard vulnerability to Psa-V by moving away from less tolerant varieties such as Hort16A. Grafting onto clean stumps greatly improves chances of establishment of new varieties and rapid return to a productive canopy. The Hawkes Bay orchard in the photo illustrates the value of planning ahead with Gold3 notch-grafts already well-established as the Hort16A canopy is removed.
For Hayward orchards Matua and M-Series males are known to be less tolerant to Psa-V and are generally the worst affected in high Psa-V infection years. They should be progressively removed, and stump-grafted or notch-grafted to the generally more Psa-V tolerant and highly beneficial chieftain males during winter. This change reduces Psa-V infection risk within the orchard with the flow on effect of also reducing risk of flower-bud infection through lower in-orchard inoculum levels.
Growers with Hayward on Kaimai rootstock have identified these plants as more vulnerable to leaf-spot through spring. Plants may have been introduced as replacements and can be scattered through orchards or established in rows where shelters have been removed. They add risk, as high incidence of leaf-spot provides an ongoing inoculum source for the orchard throughout the year and a strategy of replacing these is recommended.
Hort16A rootstocks, or Hort16A interstocks accidentally maintained in cut-over blocks, also add risk as they can be a source of exudate through the spring period. Growers in the Te Puke area are generally looking to reestablish these plants to remove risk. Growers grafting blocks to Gold3 are similarly recommended to ensure interstocks are removed as the influence of these on Psa risk and also future cropping effects are unknown.
When grafting, make every effort to secure budwood from the cleanest source and maximise hygiene through the grafting process to provide the best opportunity of graft success.
Also in winter take the opportunity to replant alongside unthrifty plants. A good source of healthy plants are now available through the Kiwifruit Plant Certification Scheme (KPCS) and replacing weak plants is a future investment into the overall health and cropping potential of the orchard.
Other strategies to reduce Psa-V risk before spring are found on the Psa-V Risk Compass.