RESEARCH & EVENTS
Investigating the impact of water deficit on trunk disease
B. J. Oswald 1, 2, E. S. Scott 3, L. Carvalhais 2, M. R. Sosnowski 1, 3 and O. A. Akinsanmi 2
1
South Australian Research & Development Institute, SA. 2 The University of Queensland, Centre for Horticultural Science, QAAFI, Brisbane, Australia. 3 School of Agriculture, Food and Wine, The University of Adelaide, SA.
PHYTOPHTHORA and Botryosphaeriaceae species were the most commonly isolated root and trunk pathogens found in all major almond growing regions in surveys conducted as part of Hort Innovation project AL22002. Phytophthora acerina and Diplodia seriata, causal pathogens of Phytophthora and Botryosphaeria diseases, were particularly common. Subsequent research has been endeavouring to better understand what factors influence severity of disease caused by these pathogens. Experiments were established in greenhouses at the University of Queensland in Brisbane during February 2024 and 2025. Potted cv. Nonpareil trees grafted to Nemaguard rootstock were watered to 100, 50 and 25 % of their water requirements for six weeks, to simulate water deficit stress. The two pathogens, P. acerina and D. seriata, were then inoculated into wounds made in the trunk. Due to differences in pathogen aggressiveness the P. acerina experiment was incubated for eight days, whereas the D. seriata experiment was incubated for three months. At the end of the experiments, average lesion lengths of 85-216 mm( P. acerina) and 48-141mm( D. seriata) were recorded on trunks( Fig. 1). Trees that were deficit-irrigated to 25 % of their requirements had the shortest lesions caused by P. acerina( Fig. 2). Conversely, D. seriata caused the longest lesions in trees that were deficit-irrigated to 25 %, although
Figure 1. Lesion formed in an almond trunk inoculated with P. acerina eight days earlier.
only significantly so in one of the two experiments. Results suggest that Phytophthora acerina disease severity is reduced in young trees under deficit irrigation, possibly due to the pathogen’ s requirement for wet conditions, while disease caused by D. seriata was exacerbated by water-deficit irrigation. The increase in disease severity caused by D. seriata in trees under deficit irrigation may be due to the tree being compromised under water-deficit stress. The experiments also highlighted the aggressiveness of P. acerina. Individual lesions caused by P. acerina reached up to 358mm in a week, while D. seriata inoculations resulted in lesions up to 218mm after three months. This rate of disease progress could be devastating, particularly for young trees. The aggressiveness of P. acerina in these experiments could have been exacerbated by consistently warm greenhouse temperatures, which ranged from 24 to 32 ° C in 2024 and 24 to 29 ° C in 2025. The greater average temperature in the first experiment could also explain why lesions were significantly longer in experiment 1 compared to experiment 2 for both pathogens. Further controlled environment experiments assessed the pathogenicity of P. acerina and D. seriata at 10, 15, 20, 25, and 30 ° C using detached branches from five almond cultivars. Significantly longer lesions were recorded for P. acerina and D. seriata at 30 ° C than all other temperatures. This confirms that P. acerina and D. seriata progress faster in warmer environments. Ongoing research is investigating other factors contributing to Phytophthora and Botryosphaeria diseases. Experiments are evaluating cultivar and rootstock susceptibility, the effect of pruning on wood carbohydrate content and potential relationships with disease severity. These studies aim to improve our understanding of key almond pathosystems and inform more effective management strategies for the industry.
Figure 2. Average lesion lengths caused by Phytophthora acerina and Diplodia seriata in two water-deficit stress experiments. For each experiment, columns with the same letter are not significantly different.
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