Evaluating different approaches in the application of phosphonates for the control of apple root diseases

Authors: NYONI, M - Stellenbosch University; LOTZE, E - Stellenbosch University; Mazzola, Mark; WESSELS, J.P. - Procrop; MCLEOD, A - Stellenbosch University

Submitted to: Australasian Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2019
Publication Date: 6/6/2019
Citation: Nyoni, M., Lotze, E., Mazzola, M., Wessels, J.B., McLeod, A. 2019. Evaluating different approaches in the application of phosphonates for the control of apple root diseases. Australasian Plant Pathology. 48(5):461-472. https://doi.org/10.1007/s13313-019-00647-x.
DOI: https://doi.org/10.1007/s13313-019-00647-x

Interpretive Summary: Control of soil borne pathogens and parasites is crucial to the successful establishment of new orchards on old orchard sites. Certain of these soil-borne pathogens not only inhibit the growth of newly planted trees on old orchard sites but can also cause significant damage in established orchard systems. Among these, disease caused by oomycetes including species of Phytophthora are the most harmful to orchard productivity and tree survival. Phosphonates have been shown to have activity against certain oomycetes including Phytophthora and Pythium species. When applied to plants, the phosphonate breakdown product, phosphite ions, which have activity toward the pathogens can be translocated to various plant parts and provide protection against disease development. Studies were conducted to determine which method of phosphonate application resulted in the greatest quantity of phosphite accumulation in plant roots. The relationship between phosphite root concentration and disease severity were assessed. Different phosphonate application approaches differed in their efficiency of delivering phosphite to the roots of apple trees. Use of phosphonates as foliar sprays was the most promising approach for applying phosphonates to young apple trees. Phosphonate treatments resulted in a significant reduction in root infection by Phytophthora cactorum and Pythium irregulare relative to the non-treated control. However, the quantity of the active ingredient phosphite detected in apple roots was in general not indicative of the suppression of P. irregulare and P. cactorum in apple tree roots. Future research should also assess whether phosphite in apple tree roots has a direct toxic effect towards P. cactorum and P. irregulare, or whether phosphonates induce a resistance response in tree roots challenged with these pathogens. This will provide a better understanding of the significance of root phosphite concentrations in the oomycete-apple system.

Technical Abstract: Phosphonate fungicides are registered on various tree crops in South Africa for the management of oomycete root rot pathogens, but not on apple trees. The study investigated several phosphonate treatments previously evaluated independently by technical advisors in South Africa. A replicated orchard trial was conducted in non-bearing asymptomatic orchards; tree roots were infected by oomycetes but foliar symptoms were absent. Phosphonate foliar-, trunk paint- and soil drench treatments were equally effective, and consistently resulted in a significant reduction in Phytophthora cactorum and Pythium irregulare root DNA quantities relative to the non-treated control. The latter was not always true for phosphonate trunk spray treatments. Trunk paint applications applied at an annual dosage of 40 g phosphorous acid/tree yielded significantly higher root phosphite (breakdown product of phosphonates) concentrations than the soil drench and trunk spray applications; the latter were applied at lower annual dosages of 7.5 g a.i./tree and 20 g a.i./tree, respectively. Foliar sprays applied at a low annual dosage (1.8 to 3.0 g a.i./tree) often outperformed the soil drench and trunk spray treatments in root phosphite concentrations. No clear association was evident between root phosphite concentrations and pathogen suppression. Root phosphite typically peaked at 8-weeks post-treatment for winter applications, and between 2- to 4-weeks for summer applications. A rapid decline in root phosphite was evident over the 12-week summer period, but not for winter applications. Monitoring root growth in the untreated control plots showed that root growth was continuous but that it peaked in summer, with reduced growth in winter.