How effective are methods to induce or facilitate the natural resistance of temperate trees to exotic Phytophthora species? A systematic review

ABSTRACT Some of the most prominent exotic pathogens in temperate forest regions belong to the genus Phytophthora. The pathogen, which is nourished by the enzymatic destruction of living plant cells, can cause mortality in >150 plant species including many temperate forest trees. However, studies have demonstrated the natural disease resistance of trees can be directly induced or facilitated using methods to deploy biochemical compounds that include foliar sprays, trunk injection or bark application, and soil amendments. This systematic review identified and analysed the efficacy of novel treatments to induce a natural resistance against different Phytophthora species in temperate trees. Results showed that treatments reduced Phytophthora infection symptoms compared to controls in all but one of the experiments reviewed. Trunk injections demonstrated the highest cumulative efficacy with a pooled effect size of 1.85 ± 0.56 (Hedge’s g ± 95% confidence interval). Foliar sprays had the second highest efficacy, with a pooled effect size of 1.11 ± 0.28. Finally, soil amendments had the lowest cumulative efficacy, with a pooled effect size of 0.61 ± 0.36. This review supports the use of treatments on trees in nurseries, urban forests, orchards, and arboreta; however, success is dependent upon the application of optimal doses.