In this article, Dr Cherie Gambley from the Department of Agriculture and Fisheries, Queensland, provides an update on a multi-million-dollar research project that was established to address high-priority viral and bacterial diseases affecting vegetable crops.

Area Wide Management of Vegetable Diseases: viruses and bacteria (VG16086) is a strategic levy investment under the Hort Innovation Vegetable Fund. This 4.5-year project includes co-investment from the Department of Agriculture and Fisheries, Queensland (DAF, QLD); Victorian Department of Jobs, Precincts and Regions; the Northern Territory Department of Primary Industry and Resources; the Western Australian Department of Primary Industries and Regional Development (DPIRD); and the University of Tasmania. The project is supported by another specific-focused project led by the New South Wales Department of Primary Industries and similarly funded by Hort Innovation.

Dr Cherie Gambley.

The project started in early 2018 and has made good progress. Ongoing accurate diagnoses of what causes disease outbreaks in multiple districts provides valuable knowledge fundamental for development of disease management strategies. Important environmental hosts of several key viruses were identified and include cucumber green mottle mosaic virus (CGMMV) in the Northern Territory, cucumber mosaic virus (CMV) in Victoria and beet pseudoyellows virus (BPYV) in South Australia.

Progress on national mapping of weed hosts for tomato spotted wilt virus (TSWV) by growing district is ongoing. This virus has an extremely wide host range, so it is important to provide specific local information to districts rather than the whole list. Studies on strains of TSWV that break virus resistance genes in capsicum and other genetic diversity studies have started. This includes study of at least 10 different viruses and multiple Pseudomonas syringae pathovars and phytoplasma species. Results of these studies inform on vegetable breeding programs, new molecular crop protection strategies, such as products like RNAi-bioclay and diagnostics.

Monitoring activities in Queensland have identified key weed hosts for thrips in the dry tropics and confirmed the presence of both western flower and tomato thrips. Aphid monitoring has provided valuable baseline data on naturally occurring parasitoids and predators in south-east Queensland. In Carnarvon, Western Australia, key vector species detected were western flower thrips, onion thrips, green peach aphid and melon aphid. Population variation of these key insects by season was also recorded in the Queensland and Western Australian studies. Monitoring of thrips in Victoria commenced in October 2019 in lettuce crops and is ongoing. Victorian monitoring activities are in collaboration with Stuart Grigg from Ag-Hort Consulting.

Industry engagement has continued nationally. Engagement is a mix of industry forums delivered in collaboration with the National Vegetable Extension Network (VegNET), AUSVEG, and on-farm discussions during disease surveys and field trial site visits. These events are a great way to provide new information and to capture and incorporate information from growers and consultants into management strategies as they are developed.

A new industry engagement workshop that focused on educating consultants and agronomists about disease identification was delivered in September 2019 in Gatton, Queensland before moving up to Bundaberg. It will be rolled out nationally over the next 12 months. A similar workshop format will be delivered towards the end of the project to provide updated advice on management strategies and information on how to identify any newly detected diseases.

Case study: Managing potyvirus disease of zucchini in Australia

Virus disease is a major limiting factor to zucchini production and is mostly caused by one or more potyvirus species. Papaya ringspot virus-type W (PRSV-W) is the main virus found in Queensland, zucchini yellow mosaic virus (ZYMV) dominates in Western Australia and watermelon mosaic virus (WMV) occurs in all regions, but particularly in the Swan Hill area in northern Victoria.

Rapid spread of these viruses by aphids (less than one minute) frequently results in very high disease levels. Affected crops have reduced fruit set and high numbers of deformed unmarketable fruit. The rapid spread by aphids means insecticides are not effective for disease control.

In previous years, seed companies have invested considerably in developing zucchini varieties with virus tolerance. This means the varieties can produce good yields of marketable fruit even when infected. In project VG16086, we were able to set up trials of the different varieties in three different Queensland production districts.

This allowed evaluation of the varieties for virus tolerance and agronomic performance under different climates and compared new generation varieties with those previously available.

The three trials were completed in collaboration with Chris Themsen from Agreco Australia and Chris Monsour from Prospect Agriculture as well as five seed companies.From these trials, at least six zucchini varieties performed consistently well with excellent tolerance to PRSV. A further four had intermediate tolerance. All are worth evaluation by growers on their own properties to decide which varieties are best suited to their local environment and market requirements.

Chris Monsour talking to Dale Williams in the field trial.

Further activities: Queensland and Western Australia

Denis Persley from DAF, QLD and DPIRD’s Craig Webster will continue to expand this research. The best performing varieties are being evaluated against the other two potyviruses, ZYMV and WMV.

Preliminary work in glasshouse experiments in Western Australia are already show promising results, which will be further tested in the field. Additionally, the varieties will be assessed for how well they support multiplication of the three viruses. The mechanism for the observed tolerance is not known and seems in some varieties to represent immunity (that is, the virus is unable to multiply inside the plant at all). This is important as immune reactions assist in reducing the sources of virus within the environment.

Field trial site at Bowen Research Station for evaluation of zucchini varieties for tolerance to potyvirus. Images courtesy of Dr Cherie Gambley.

Key points for virus management

Virus tolerant varieties are a valuable tool for management and should be used as part of an integrated approach. Other actions that help reduce movement of the aphids into crops or between crops or reduce sources of the virus within the environment is also important. This not only helps with lessening disease impact, but it will also help to prolong the lifespan of tolerant varieties.

These actions include:

• Destruction of harvested or abandoned cucurbit crops as soon as practically possible. These crops are very important sources of virus and aphids on-farm and within the district.
• Separation of new crops from older crops, even by as little as 50 to 100 metres, can be beneficial and planting new crops upwind from older crops can also help.
• Separation of cucurbit crops using blocks of plants that don’t host the virus or the aphid, particularly if these were tall and act as a barrier for movement of the aphids (e.g. sorghum).
• Selective use of insecticides for general control of insect pests. No insecticides are registered for control of virus diseases. In general, they don’t reduce virus spread by the aphids. Worryingly, they often have the opposite effect if they disrupt natural enemies and make the plants less pleasant for the aphids to feed, therefore increasing their activity within the crop. As the aphids keep moving and tasting plants instead of settling and feeding, they also spread the virus.
• Consider decontaminating harvesting knives regularly during harvest. All three viruses can be spread by sap on equipment such as knives. A solution of two per cent virkon or similar can be used to disinfect tools.

Find out more

Project fact sheets are available to download from the Hort Innovation website.

For more information about the project, please contact Dr Cherie Gambley on 0423 200 211 or email cherie.gambley@daf.qld.gov.au.

This project has been funded by Hort Innovation using the vegetable research and development levy, co-investment from the Department of Agriculture and Fisheries, Queensland; Victorian Department of Economic Development, Jobs, Transport and Resources; the Northern Territory Department of Primary Industry and Resources; the Western Australian Department of Primary Industries and Regional Development; and the University of Tasmania and contributions from the Australian Government.

Project Number: VG16086

This article first appeared in the winter 2020 edition of  Vegetables Australia. Click here to read the full publication.