Greenhouse and field evaluations of potassium phosphonate: the control of Phytophthora foot rot of black pepper in Vietnam

Abstract

Phytophthora foot rot of black pepper caused by Phytophthora capsici is a major disease of black pepper throughout production areas in Vietnam. The disease causes collar, foot and tap root rots and eventual death of the infected vine. Potassium phosphonate was evaluated for the control of this disease in greenhouse and field trials. In greenhouse trials three-month-old vines treated with phosphonate by soil drenching (10–20 g a.i./l) and then inoculated with P. capsici mycelium (2% v/v soil) had significantly less foot rot compared to vines grown in non-treated soil. In field trials mature vines were treated with phosphonate at 50–100 g a.i/pole soil drenching or 10 g a.i./l by root infusion. After 10 days root, stem and leaf specimens were removed for bioassay by inoculation with 5 ml of P. capsici zoospores suspension (10⁶–10⁸ spores/ml). Soil drenching with phosphonate inhibited the colonisation of pathogen on excised leaf, stem and root tissues, significantly more than phosphonate root infusion. Our study provides further evidence supporting the efficacy of potassium phosphonate in the management of black pepper foot rot caused by P. capsici. The excised leaf and stem bioassay used in this study is a rapid and useful technique for testing the efficacy of systemic fungicides in controlling this disease.

Keywords:

• bioassay
• phosphonate
• Piper nigrum
• Phytophthora capsici

Acknowledgements

The senior author acknowledges the award of a Vietnamese Government Scholarship. This work was partially funded by the Australian Centre for International Agricultural Research (ACIAR). We gratefully acknowledge the support of the Fruit Research and Development Centre in Thua Thien Hue province. We thank Prof. David Guest for helpful discussions and comments on the bioassay approach and experimental design, Mr. Tran Ha for provision of experimental sites on his farms, Dr. Tom Bishop for his assistance in statistical analysis and Mr. P.V. Tan for assistance in running the GraphPad software.