Verticillium wilt in Western Australia in summer 2019
By Dr Liz Dann
What is Verticillium wilt?
- Verticillium wilt is a fungal disease of avocado and a broad range of other crops including potato, tomato, grapes, stone fruit, nuts, cotton, strawberry, and some weeds including nightshade.
- The characteristic symptom is a rapid wilt of young trees, or single branches in older trees, followed by desiccation of leaves (Figures 1 and 2). Young trees may die.
- Streaky browning of the vascular system in young wilted stems about 1cm thick is apparent when a 1mm deep shaving is made with a sharp knife (Figure 3).
- Trees may recover, usually with warmer temperatures when growth of the fungus is arrested, and new vigorous growth may occur below the affected parts of the branch within several weeks (Figure 4).
How to manage it
- There are no effective fungicide treatments.
- Prune out the dead wood once dieback has ceased, or remove the entire dead tree, and remove completely from the orchard. Do not chip and use as mulch.
- Fumigation of infested sites (eg, where tree had died and been removed) prior to re-planting may be effective.
A bit more about the disease, and occurrence this year in WA
Verticillium wilt is usually not a major problem in avocados. It not seen every year, however, the occurrence in young trees in south-west Western Australia this summer has been fairly high. In a couple of the cases of which I am aware, trees have been planted into ex-potato or grapevine ground. While the disease is of minor importance in grapes, spuds are an excellent host of Verticillium. The fungus can survive in soil for many years as “microsclerotia” (Figures 5 and 6), which are very small (<1mm), compact masses of thick-walled, pigmented cells which have budded off from the fungal mycelium.
These microsclerotia germinate under the right conditions and in the presence of root exudates and infect the roots. The fungus then produces very small spores, which are easily transported to upper parts of the tree in the vascular tissues with high sap flow in spring and summer. This causes damage to the tree’s vascular system, and the tree produces tyloses, or gums, which plug the vascular system, blocking the sap flow and causing wilt. I was able to isolate the fungus from the areas of vascular discolouration (Figure 3) from samples of wilted branches from four young trees in two separate orchards collected in late February. Microsclerotia then form in these senescing branch tissues, completing the disease cycle, so it is important to remove the prunings so that the source of inoculum is reduced.
From our experience with cases in the eastern states, Verticillium wilt is mostly seen earlier in the season, eg August and September, and has been associated with damage to roots. See Talking Avocados Summer 2009/10 Volume 20 (pages 32-33) and for those who have registered and are signed in, you can also read more in the Growing section of the Best Practice Resource, click here. More can also be found in the Verticillium wilt of deciduous fruit trees fact sheet from Agriculture Victoria.
Discussions with a potato grower confirmed that Verticillium in that crop is commonly seen at this time of the year in south-west WA. A link between compromised root systems (eg, from previous disease or rootbound trees at planting), cannot be ruled out in the recent West Australian cases. The fungus is very slow growing in culture and further testing will be undertaken to confirm species, although it is likely to be Verticillium dahliae.
Please contact Associate Professor Elizabeth Dann, Queensland Alliance for Agriculture and Food Innovation (QAAFI) by emailing firstname.lastname@example.org or call 07 3443 2455. Growers are welcome to contact me if they would like more information, or if they want me to try to confirm whether they have Verticillium wilt or not.
The Improving avocado orchard productivity through disease management (AV16007) project has been by Hort Innovation, using the Avocado Fund research and development levy and contributions from the Australian Government.
This article was prepared by Dr Elizabeth Dann for the 22 March 2019 edition of the Guacamole and the Autumn 2019 Talking Avocados magazine.
Date Published: 22/03/2019