Evaluation of biological control agents and conventional products for post-harvest application on potato (Solanum tuberosum L.) to manage leak

Authors: HOLLINGSHEAD, ANDREW - University Of Idaho; OLSEN, NORA - University Of Idaho; THORNTON, MIKE - University Of Idaho; MILLER, JEFF - Miller Research, Inc; SCHISLER, DAVID - Retired ARS Employee; Slininger, Patricia - Pat

Submitted to: American Journal of Potato Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/19/2020
Publication Date: 9/14/2020
Citation: Hollingshead, A.K., Olsen, N.L., Thornton, M., Miller, J., Schisler, D.A., Slininger, P.J. 2020. Evaluation of biological control agents and conventional products for post-harvest application on potato (Solanum tuberosum L.) to manage leak. American Journal of Potato Research. 97:477-488. https://doi.org/10.1007/s12230-020-09795-z.
DOI: https://doi.org/10.1007/s12230-020-09795-z

Interpretive Summary: Pythium ultimum is a fungal pathogen that infects potato tubers through wounds or openings in the skin. The disease ‘Pythium leak’ develops as a dark lesion that profusely leaks water and encourages secondary invasion by additional pathogens. Effective fungicides for its control are lacking. The fungicide Mefenoxam, applied in-furrow or as a foliar treatment, is the primary one for control of leak, but it has notoriously been inconsistent, and in the last decade, pathogen resistance has been reported. Scientists at the National Center for Agricultural Utilization Research (NCAUR, USDA-ARS, Peoria, IL) have developed a biological control agent for postharvest potatoes that is comprised of three beneficial bacterial strains of Pseudomonas. These bacteria have been effective in reducing several diseases in stored potatoes, including Fusarium dry rot (Fusarium sambucinum), pink rot (Phytophthora erythroseptica), and late blight (Phytophthora infestans). In this research a combination of three chemical fungicides (azoxystrobin, fludioxonil, and difenoconazole) was shown to significantly reduce the incidence and severity of leak in storage. The addition of bacterial tricultures alone also limited leak incidence and severity, but the combination of the bacterial triculture and the chemical trio provided the greatest efficacy in reduction of leak development. This combination of biology with chemistry offers multiple modes of action that would be unlikely to foster the occurrence of resistant pathogens in the future, and it provides growers with a potential broad-spectrum new tool for management of leak and other postharvest potato diseases.

Technical Abstract: Bacterial antagonists applied post-harvest may provide an effective way to manage leak of potatoes by competing with Pythium ultimum. The objective of this paper was to test the efficacy of various bacterial antagonists and conventional post-harvest fungicides to manage leak. Two studies were performed with three formulations of a Pseudomonas fluorescens triculture (a fresh culture, dried on Kenite 700, or dried on Attapulgite clay) applied at a rate of 3.5 mL kg-1. A third study contained treatments of a fresh culture, culture dried on Kenite, a culture dried on Kenite, rehydrated with water and then blended with a fungicide containing a three-way mixture of azoxystrobin, fludioxonil, and difenoconazole (0.033 mL kg-1); and a culture dried on Kenite, rehydrated with nutrient broth Medium 1, and mixed with the same fungicide. Other treatments included Pseudomonas syringae (0.006 g kg-1) in studies 1 and 2, while hydrogen peroxide and peroxyacetic acid (0.042 mL kg-1), phosphorous acid (4.2 mL kg-1), and a three-way mixture of azoxystrobin, fludioxonil, and difenoconazole (0.033 mL kg-1) were used in all three studies. Unwashed tubers (cv. ‘Russet Burbank’) were wounded and inoculated with P. ultimum. After 30 min of being inoculated, tubers were treated, then stored at 21 degrees C for four days prior to disease evaluation. In study one, none of the formulations of Pseudomonas fluorescens triculture significantly controlled leak compared to the inoculated control; however, the fresh triculture formulation significantly decreased leak incidence by 23.4% in the second study. In studies 1 and 2 the three-way mixture of azoxystrobin, fludioxonil, and difenoconazole (1.2 and 7.3% leak incidence, respectively) was significantly different from the inoculated control (12.0 and 36.7% leak incidence, respectively). The third study showed that the formulations of Pseudomonas fluorescens strains significantly decreased leak incidence compared to the inoculated control when a full rate of azoxystrobin, fludioxonil, and difenoconazole was mixed with the triculture formulation. The triculture of Pseudomonas fluorescens formulations mixed with a full rate of azoxystrobin, fludioxonil, and difenoconazole and rehydrated with Medium 1 had similar bacterial counts as the fresh triculture and the triculture dried on Kenite, potentially indicating a greater activity level afforded by cell revival in dilute culture medium. Pseudomonas syringae, phosphorous acid and hydrogen peroxide-peroxyacetic acid were ineffective in controlling leak in all studies. The efficacy of azoxystrobin, fludioxonil, and difenoconazole is promising for the management of leak in storage and the addition or sole application of P. fluorescens tricultures may further limit leak development, but additional research may be warranted.