Bacteria Are Smart Survivors, Including PSA

Bacteria Are Smart Survivors, Including PSA.

by Sue Edmonds

The PSA bacteria (Pseudomonas syringae pv. actinidiae) isn’t just attacking New Zealand kiwifruit vines. Now considered a pandemic, it has spread to twelve countries.

First recorded in China in 1984, attempts to curtail its spread are not working very well here, with affected vines being removed, and copper and streptomycin sprays proving ineffectual.

Fred Harvey of Te Puke, a relatively small grower who has been using biological methods focused on soil and vine health, had heard reports from Italy that things were improving there. Although his orchard has some PSA infection, he wasn’t convinced that the advice being given to New Zealand growers was the total answer. So he took a trip to a major Italian growing area south of Rome, and spent four days interviewing kiwifruit orchardists whose revised systems were showing both lowered rates of infection and increased harvests.

The orchards he visited were around 70% affected, most since 2009. But he was quickly informed that they were no longer trying to fight it, but learning to live with PSA by changing their farming methods.

All but one were now using Agrizest, a New Zealand invented product for woody plants from Indigo Ltd of Pukekohe, (readily available in New Zealand), which both stimulates growth and increases immunity to predators such as PSA. Some were conducting trials on blocks of vines, with other blocks as controls, while others were using Agrizest on their whole crops. Initially spraying had been done only after flowering, but now they were doing two sprays before flowering and two more afterwards. And each year, as plant immunity increases, harvest tonnages were increasing and the fruit was bigger and more crunchy, with higher Brix levels, and of better quality.

Where once copper sprays had been used liberally, now they only use it once a year for a winter cleanup. It has been discovered that the PSA bacteria are protected from harmful sprays because they are covered in a biofilm, both inside and outside host plants. The Italians had also adjusted the male pruning cycle, after discovering that the bacteria increased rapidly and changed to a virulent form in higher temperatures (up to 27). Looking after vine and soil health had become a major focus, with chemical fertilisers hardly being used, and replaced with seaweed, composts and fish. They also no longer do girdling on their vines. One grower had commented that these days the amount of expensive water needed for irrigation had halved both the cost of electricity and the water required, from 1000 Euros/ha to 500 Euros/ha just for the water.

After seeing their changes being effective, the growers had expressed curiosity to Harvey as to why New Zealand growers, many with multi-million dollar investments in their orchards, weren’t visiting Italy to find out how the Italians were succeeding. They were being visited by growers from other affected countries, but not from New Zealand until then.

The Italians were highly amused to hear that in New Zealand, imported pollen was being blamed for the incursion. In Italy they are convinced it was brought there in graft wood of Hort16 from New Zealand! Certainly scientific papers from Italy (Renzi et al, May 2012), recommended to Harvey by the growers, have shown that it can enter the vines through any natural openings and lesions, reach any part of the stems and trunks, and overwinter in the rootstock.

The growers impressed upon Harvey the importance of the information contained in a TED presentation by Bonnie Bassler of Princeton University on ‘How Bacteria Communicate’. Bacteria haven’t survived for millions of years without being smart. A most enlightening experience, and one Harvey considers should be compulsory viewing for all kiwifruit growers. This can be viewed at http://www.ted.com/talks/bonnie_bassler_on_how_bacteria_communicate.html.a

ENDS

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