Ross Pannell Writes About Phytophthora

We have been on the “HEARTWOOD” property for about 5 years, arriving at the end of Feb 2014. Consequently, we found ourselves straightaway in our first harvest, with pretty much minimal knowledge of walnuts. But as our a son- in-law famously said – “How hard can it be?” YEH – RIGHT!!

Probably one of the biggest challenges and as yet not completely solved has been the effect that Phytophthora has on some of the trees. It is quite devastating to see 15 year old ( and younger) trees gradually succumb to the point where they are just standing skeletons.

Once they are felled it is easy to see the extent to which they have been infected with the whole trunk blackened.

The property used to grow apples among other things and this is considered to be a source of infection.

We were advised by a local Ag chemical firm to try using Phosgard, which contains phosphorous acid, applied with syringes developed for the avocado industry, directly injecting a solution into the trunk. This appeared to work on some trees as new growth started, but after a while this collapsed, presumably due to the trees’ root system being unable sustain the required nutrients. There was also a problem with some trees being damaged due to excess pressure applied to the syringes.

We also foliar sprayed the trees twice a year, once when full canopy had developed, about November, and another after harvest in early to mid-May before leaf drop. A neighbouring walnut farmer has applied the second spray just before harvest to ensure maximum foliar uptake with a complete canopy, and I think we will follow suit this year.

A search of the internet pointed to a bacterium which had been beneficial in controlling and potentially eliminating the Phytophthora by parasitizing the disease in the root zone.
I found that a local company was producing Streptomyces Lydicus and so after discussions with the manufacturer and our agronomy consultant, John McKendry I decided to try it.

It was also considered that the Streptomyces would possibly be effective against walnut blight. So I tried using it in place of the copper spray for blight over the whole orchard. Results might be mixed as our Rex look quite healthy, but the Meyric and Vina have dropped a lot of blighted fruit, so possibly we will use copper as well next season.

Along with the residual Streptomyces foliar spray falling on the orchard floor, all trees were given a soil drench of the bacteria and phosgard mid spring.

Although it may take some time for the streptomyces to colonise in the root zone the expert opinions are that it will work in the long run.
I guess that’s what walnut farming is – a waiting game!

Ross Pannell,
“Heartwood”, Appleby, Nelson.

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New Zealand Walnut Industry Group