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

  • M. Nyoni
  • E. Lötze
  • M. Mazzola
  • J. P. B. Wessels
  • A. McLeodEmail author
Original Paper


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.


Phosphonates Phytophthora cactorum Pythium irregulare Apples Phosphite Phosphorous acid 



We acknowledge the South African Apple and Pear Producer’s Association (SAAPPA), the Technology and Human Resources for Industry Programme (THRIP) for financially supporting the research, and without whom this research would not be possible. We are appreciative of Marieta Van der Rijst (Agricultural Research Council, Biometry Unit, Stellenbosch, South Africa) for statistical analyses of the data.

Supplementary material

13313_2019_647_MOESM1_ESM.doc (178 kb)
ESM 1 (DOC 178 kb)


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Copyright information

© Australasian Plant Pathology Society Inc. 2019

Authors and Affiliations

  1. 1.Department of Plant PathologyStellenbosch UniversityMatielandSouth Africa
  2. 2.Department of Horticultural ScienceStellenbosch UniversityMatielandSouth Africa
  3. 3.Tree Fruit Research LaboratoryUSDA-Agricultural Research ServiceWenatcheeUSA
  4. 4.ProCropLynedochSouth Africa

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