Evaluation of new Oomycota fungicides to manage Phytophthora crown and trunk cankers of almond and their systemic movement after soil application

The increasing prevalence of Phytophthora crown and trunk cankers on almond in California, especially in newly planted orchards, and pathogen resistance to older fungicides warranted the evaluation of new treatments for improved management. In two orchard trials, Nonpareil almond trees grafted on two rootstocks were soil-inoculated with Phytophthora cactorum, and the soil was treated with ethaboxam, fluopicolide, mandipropamid, oxathiapiprolin, or selected mixtures that were compared to potassium phosphite (KPO3) and mefenoxam. All fungicides except mefenoxam significantly reduced the incidence of disease. Oxathiapiprolin overall resulted in the highest reduction with few or no trees dying. Fluopicolide ranked second with efficacy similar to KPO3. Ethaboxam was the least effective. Binding of oxathiapiprolin to dry soil was demonstrated in field and laboratory studies, stressing the importance of application to pre-wetted soil followed by irrigation. In greenhouse and field studies where trunks of soil-treated trees were wound-inoculated, systemic uptake of the new Oomycota fungicides as well as of mefenoxam and KPO3 into rootstocks and scions of almond trees was evident. Our studies demonstrate that oxathiapiprolin and fluopicolide can provide highly effective control of Phytophthora crown and trunk cankers in young almond orchards. The less effective ethaboxam should be used in mixture with another fungicide such as fluopicolide. Based in part on our studies, oxathiapiprolin was registered on almond in 2021, and registration of fluopicolide, ethaboxam, and mandipropamid (nursery use only) is ongoing. With a range of modes of action available in the future, resistance management can be practiced with fungicide mixtures and rotations.