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High inoculum  levels in Psa-affected flower buds

High inoculum levels in Psa-affected flower buds

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27 Oct 21 Grower News

High inoculum levels in Psa-affected flower buds

Samples of infected Hayward flower buds taken from a site with extensive leaf-spotting and flower-bud browning were tested and showed Psa-V levels of 3.3 x10⁷ on the surface of the flower buds.

This inoculum level is very high and similar to levels used experimentally to inoculate in potted plant trials. The result illustrates potential levels of inoculum in canopies with severe leaf-spotting and the value of effective spray programmes.

Trials continue this season to identify spray programmes that provide efficacy in minimising Psa-V infection through spring. These provide evidence of the value for growers of maintaining a proactive programme to protect vines from leaf and bud infection.

A short video demonstrating the effectiveness of spray applications in Psa-V management compares the health of vines sprayed and unsprayed rows and is available on the ZESPRI Canopy here.

As leaf spot can continue to be an inoculum source, with the edges of spots producing inoculum when high-risk weather conditions favour Psa-V multiplication, continuation of copper spray programmes beyond fruit set are recommended . Copper should be applied at summer rates in response to high-risk weather. An application is recommended following male pruning to protect wounds. Frequency of sprays can reduce as temperatures increase and moisture levels fall reducing Psa-V infection risk.

The KVH Psa-V Risk Model can be used as a guide for growers in planning timing of orchard activities.


KVH investigates reports of unusual symptoms to identify and manage any biosecurity risks.

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