Plant Disease March 2004
Volume 88, Number 3
The American Pathological Society (APS)
Interpretive summary

Differences in Etiology Affect Mefenoxam Efficacy and the Control of Pink Rot and Leak Tuber Diseases of Potato
Raymond J. Taylor, Bacilio Salas, and Neil C. Gudmestad, Department of Plant Pathology, North Dakota State University, Fargo 58105. Plant Dis. D-2004-0114-01R, 2004 (online). Accepted for publication 18 November 2003.

Pink rot and leak are important soil-borne diseases of potato caused by Phytophthora erythroseptica and Pythium ultimum, respectively. These pathogens differ in the way they infect potato tubers. Pythium ultimum requires a wound for infection to occur, while P. erythroseptica is able to directly penetrate undamaged tuber tissue and cause disease. Although certain cultural practices may be employed to control these “water rot” diseases, mefenoxam is the only effective fungicide currently available. Mefenoxam concentrates in the periderm and outer tissue layers of the potato tuber. Any damage to the tuber could disrupt this chemical barrier, allowing a wound pathogen such as P. ultimum an unprotected infection site. This study was undertaken to examine the factors affecting the development of pink rot and leak in potato tubers and the efficacy of mefenoxam in controlling these diseases. Mefenoxam was applied to replicated field plots in-furrow at planting, in-furrow at planting with a follow-up sidedress, as a single foliar application when tubers were 7 to 8 mm in diameter, and as two foliar applications when the tubers were 7 to 8 mm in diameter and 14 days later. Several different application rates were used with each method. The effectiveness of the fungicide was determined after harvest by challenge-inoculating healthy tubers with the pathogens. Mefenoxam was more effective in controlling pink rot relative to leak over all application methods. The greatest level of pink rot control (89%) was attained with the in-furrow at planting and sidedress application. This method provided only a modest level of leak control (35%). Foliar applications of mefenoxam resulted in 10 to 50% control of pink rot, but leak was not controlled by either foliar application method. The use of mefenoxam to control pink rot appears to be well founded. Since the combination of mefenoxam applied in-furrow and as a banded sidedressing after emergence was the only method that reliably controlled leak, serious consideration should be given to this method when planning control strategies for this disease.

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