Table of Contents

Mould Research - A project started in late 2019

Background

Many growers were disappointed with quality issues that arose from the 2019 harvest. It’s been established that nearly every supplier in the South Island had a portion of their crop affected by a mould that hadn’t manifest itself so widely before.

It became obvious that as an industry we need to have a better understanding of this and so at the NZWIG committee meeting on the 21st of November 2019  it was decided to fund a scientifically based research project about the problem.

The research is being undertaken by a company called Plant Diagnostics Limited who are specialists in this field, and Heather North, along with members of your committee, will oversee the project.

The research will involve;

  1. A literature review to find out what is already known about fungi that can infect walnuts.
  2. Further analysis of the 2019 season’s nuts and cracked kernel to understand more about the types of fungi present.
  3. Field tests of the developing nuts from mid December onward to see if fungi are present during the growing season.

In addition, Walnuts NZ Co-op will be asking suppliers whether they would be willing to participate in a survey to understand more about orchard conditions that may either encourage or discourage the development of such moulds. Differing levels of mould occurrence in the crop from different orchards was observed so the researchers would like to see whether this has any correlation with factors such as canopy density, soil type, irrigation practices or blight spray regime.

This research may be carried out over several seasons and the initial outcome is expected to be greater understanding of the mould types and lifecycles and what conditions favour their growth. This should give us a foundation for learning how to manage it in future, if the problem arises again.

If you follow the affairs of NZWIG you will know that we have substantial reserve funds and about 17% of these may be used for the project. If it becomes apparent that this needs to be an ongoing project further financial assistance from the Government agencies will be applied for.

Of necessity the research will need to be thorough. It will run over a long period and so be some time before  results are reported.

NZWIG Committee, deciding on the research project. Meeting also attended by Frank Brenmuhl, Chair of the board of Walnuts NZ Co-op, and Heather North, who is on the Co-op board as well as being NZWIG’s Research Leader.

Update On Walnut Mould Project – March 2020

By Heather North

(This article was published in the Autumn 2020 newsletter)

(Progress since the previous update sent out 11 December 2019)

Prompted by the unusual levels of moulds found in the 2019 walnut crop, NZWIG is funding and managing a project to investigate what fungi may be causing this, and why. The identification and lab work is being carried out by Plant Diagnostics Ltd. Walnuts NZ Co-op is collaborating on the part of the work looking into any on-orchard factors that might be important in the development of moulds. Below, we have summarised where we are up to with each of the objectives. Following that are some notes on the findings to date from the lab work and our initial thoughts on what these findings might mean. 

Objective 1 (NZWIG): Literature survey on published information on walnut mould
Not yet begun – we will discuss this objective with Plant Diagnostics soon.

Objective 2 (NZWIG): Lab analysis of nuts from 2019 harvest
Complete – samples known to have high levels of mould (with a range of visible symptoms) were selected and taken to Plant Diagnostics on 10 December, and these were analysed to identify the fungi present. The samples included 100 in-shell nuts from one Canterbury grower, and five cracked samples from the Walnuts NZ Co-op factory (from the crops of several different growers). Plant Diagnostics has provided us with a report.

Objective 3 (NZWIG): Lab analysis of nuts currently developing on trees
In progress – this objective aims to track the presence (or not) and development of moulds through a growing season, with samples to be taken on four dates from five orchards around Canterbury, and analysed at the lab. Samples: (1) at nutlet stage (pre-shell-hardening) – was taken 14/15 December; (2) post-shell-hardening – was taken 25 January; (3) at the beginning of nut-fall, choosing nuts where the husk is cracking but the nut is still hanging on the tree – to be taken in the first week of April; (4) dried nuts post-harvest. Plant Diagnostics has done the analysis and provided us with reports from the two samples taken in so far.

Objective 4 (Walnuts NZ Co-op): Survey of orchard management and environment factors
In progress – different levels of moulds were found in the 2019 crops from different orchards, and we want to see if we can relate these differences to any particular orchard management or environmental factors. We developed a survey that covers factors including rainfall, soil type, orchard canopy density, shelterbelts/ventilation, orchard floor maintenance, irrigation and scheduling, fertiliser/nutrient applications, blight management, and harvesting, washing and drying practices. So far, ten growers have been surveyed, with one more possible survey to be conducted prior to harvest.

Observations and thoughts to date
It is too soon to draw final conclusions – we will have to wait until we have the full season of lab results from Objective 3 (this year’s developing walnuts), and put this together with what we learn from the other objectives. However, here are some key findings to date from the lab analysis, along with some early thoughts on what they might mean in practice for us as growers:

  1. The great majority of fungi identified in the 2019 crop (mature walnuts) were environmental fungi often associated with food spoilage, i.e. fungal species that are naturally present everywhere and, under certain conditions, may invade the walnuts and live and feed on them. The most common was a Penicillium species, followed by an Alternaria species, with species of Mucor, Fusarium, Gliocladium and Trichoderma also present in smaller numbers of cases.
  2. In contrast there were only a few samples of the 2019 crop containing fungi that are plant pathogens, i.e. organisms that actually cause plant diseases. The main pathogen present was a Phomopsis species, with one sample containing Botrytis.
  3. Fungi were found inside the closed (in-shell) nuts as well as in the cracked samples from the factory. It is thought that fungi can go anywhere that air can go, e.g. if the shell is permeable to air, then fungi can probably get in too.
  4. Fungi were found in nuts from the 2019 harvest that looked healthy (symptom-free) as well as those with visible moulds, rots, spots and discolouration caused by the fungi.

Thoughts: Though it would be possible that poor drying or storage conditions could be a factor in the development of moulds caused by the environmental fungi, this does not seem likely to be the key cause in the 2019 harvest because: (a) the moulds were so widespread across most growers’ crops, and (b) overall, as an industry, the drying and storage methods used during the 2019 harvest would have been very similar to those used in previous years when little mould was observed.

  1. Almost all the species present in the first sample of the current year’s developing crop (collected in December 2019) were plant pathogens, with Phomopsis species the most common, along with quite a number of samples containing Xanthomonas campestris pv. juglandis (the bacterial organism that causes walnut blight). Only one sample contained an environmental fungus (Alternaria species). It was mainly walnuts showing visible symptoms on the husk (e.g. black lesions or spotting) that were analysed, but several with only very minor symptoms were also analysed and these yielded no fungi or bacteria.
  2. In the second sample of the current year’s developing crop (collected late January 2020), most of the species present were again plant pathogens, though several environmental fungi were also detected. Plant pathogens were Xanthomonas campestris pv. juglandis, Phoma species, Phomopsis species and Colletotrichum acutatum species complex. The environmental fungi present were Alternaria species and Cladosporium species. There were three further fungi that could not be identified using the normal method of placing the walnut tissue on agar plates to encourage fungal growth, and these will need to be identified by DNA sequencing.
  3. Also to note from the second sample of the 2019/20 crop was that the large majority of the walnuts that had visible symptoms (whether black lesions, physical damage or brown lesions) contained fungal or bacterial species. In contrast, three of the four healthy (symptom-free) samples analysed had no fungal or bacterial species present.

Thoughts: Could the initial damage be done by plant pathogens during spring/summer growth of the walnuts, making the damaged walnuts more susceptible to later invasion by environmental (food spoilage) fungi? The spring of 2019 was unusually humid, leading to high disease pressure from bacterial blight in most walnut-growing areas. Could the resulting lesions have created an easy pathway for food spoilage organisms to come in? It is too early to draw this conclusion but, if we found this to be the case, then a practical response from growers is to be particularly careful with blight control and protection of the walnuts from any other form of damage. We still have some questions relating to this hypothesis, however – we don’t yet know if nuts with lesions/damage caused by plant pathogens go on to have higher levels of food spoilage organisms and/or to have higher levels of visible symptoms caused by those food spoilage organisms.

We will aim to provide a further update to growers in the months following harvest, once we have all four samples of 2020 walnuts analysed.
Heather North

Literature Review Walnut Kernel Decay 

A survey was conducted in the 2019/20 growing season to determine the microbial diversity on walnut husks and kernels as they developed throughout the season and after harvest. This followed a report of an increase in kernel moulds detected in nuts from the 2018/19 growing season following wet spring conditions. This literature review was conducted to link the findings of the survey to the potential causes of walnut kernel mould.

Click here to read the full review. 

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New Zealand Walnut Industry Group