A new foliar disease of Quercus robur (European or English oak) in Western Australia
In December 2017, a truffle orchard in WA experienced 20% tree mortality in their Quercus robur trees, displaying dieback and defoliation symptoms. Quercus ilex in the same orchard, did not suffer from the same symptoms. Diseased samples were sent to the Australian National University and to the Department of Primary Industries and Regional Development Diagnostic Laboratory Services for pathogen identification. A well-known dieback-associated fungal pathogen, Neofusicoccum australe (also known as Botryosphaeria), was isolated from diseased material. N. australe is a well-known fungal pathogen causing dieback in many hosts. However, another pathogen, a fungus called Discula quercina, was isolated also from diseased Q. robur twigs.
Sampling of Quercus robur in eastern Australia, has not detected D. quercina, however it has been detected on other tree hosts in New South Wales - pin oaks and plane trees. Symptoms associated with D. quercina infection on the Quercus robur trees included defoliation, twig and leaf dieback, leaf distortion and angular necrotic spots on mature leaves. The literature states that symptoms are usually most severe on the lower branches where humidity is high. In countries where the disease is found severe outbreaks have been known to kill nearly all the foliage on susceptible trees. The level of disease and the risk of new infections reduce when weather conditions are warmer and there is less humidity in the canopy. Also as the leaves mature they become more resistant to the pathogen. Succulent growth, however, can still be attacked at any time of the growing season when wet conditions are prevalent.
The particular orchard where this new disease was found is in a high rainfall area of WA. This situation in combination with irrigation probably contributed to maintaining near perfect conditions for the Discula infection. At this stage it is unclear whether Discula contributed significantly to tree mortality in this orchard. Any stress to trees, such as water stress from too little or too much water, can make them more susceptible to dieback caused by Neofusiccom. It is thought that the Discula pathogen may have been able to infect because of favourable environmental conditions and in combination with Neofusicoccum caused the tree mortality. It is anticipated that both fungal pathogens and the diseases that they cause can be managed with a proposed reduced watering regime.
The orchard will be re-visited in the coming spring to clarify the diseases situation there.