By K. Frost and C. M. Ocamb
Cause Pythium debaryanum and Globisporangium ultimum (formerly P. ultimum), soilborne fungus-like microorganisms. Recently the genus Pythium Pringsheim (nom. cons.) sensu lato (s.l.) was reorganized into three genera based on DNA sequencing and molecular phylogenetics: Pythium, Elongisporangium, and Globisporangium by Nguyen et al. (2022). The Pythium-like microbes that may be present in grower fields could fall into the genus Globisporangium or Pythium species.
Resistance to the commonly used fungicide Ridomil has been reported in the Columbia Basin, much less so in Idaho. Field sampling has confirmed resistant isolates making up more than 90% of the Pythium and Globisporangium population.
Symptoms Decayed tissue is brown or gray; its uniform texture resembles a soft, watery paste. Margins of rotted tissues are sharply defined and darker in color. Tubers may develop a sour odor. They may eventually dry and mummify. Generally this disease rots the tuber from the inside out, leaving only a thin papery shell, sometimes called "shell rot." Rots to tubers are also caused by late blight, bacterial rot, and pink rot. While infection may start in the field or from harvest, these symptoms are commonly found in storage.
Cultural control
- Prevent tuber injury during harvest.
- Keep tubers as cool and dry as possible during harvest.
- Store promptly at low temperature (45°F to 50°F).
- Maintain storage at 90% to 95% relative humidity as soon as tuber surfaces dry.
Chemical control One application when plants are flowering has been shown effective in Colorado. A second application two weeks following the first application has an increased benefit. New in-furrow application at planting reduces cost over two aerial applications. Where high levels of resistance are present, use of this material is not recommended. Soil tests can determine levels of resistance in field soils.
- Elumin (Group 22) at 8 fl oz/A in-furrow and then side-dressed after at least a 25-day interval. 12-hr reentry.
- Mefenoxam and metalaxyl formulations (Group 4) are available and have good efficacy in the absence of resistant pathogen strains. Resistance to mefenoxam is a significant problem for growers in regions where potato is cropped regularly.
- MetaStar 2E AG at 1.68 fl oz/1,000 ft row. 48-hr reentry.
- Orondis Gold (Group 49 + 4) at 27.8 fl oz/A as an in-furrow application at planting. Preharvest interval is 14 days. 48-hr reentry.
- Quadris Ridomil Gold (Group 4 + 11) at 0.82 fl oz/1,000 row feet applied in-furrow using 3 to 15 gal/A of water. See label for reentry intervals.
- Ridomil Gold Bravo SC (Group 4 + M5) at 2.5 pints/A following tuber initiation when the largest tubers are the size in diameter of a nickel. Make a second application 14 days later. Under severe pressure a third application may be made 14 days later. Do not use within 14 days of harvest. 48-hr reentry.
- Ridomil Gold SL at 1.0 to 2.0 pints/A as a broadcast or band incorporation preplant or at planting. 48-hr reentry.
- Ridomil Gold MZ (Group 4 + M3) at 2.5 lb/A. Do not use within 14 days of harvest. Note: Contains an EBDC fungicide, so take care not to exceed seasonal limits. 48-hr reentry.
- Ridomil Gold Copper (Group 4 + M1) at 2 lb/A. Do not use within 14 days of harvest. 48-hr reentry.
Biological control
- Amplitude ST at 3.2 to 4.8 fl oz/100 lb seed pieces. 4-hr reentry. O
- Stargus at 6 to 8 fl oz per 1,000 ft row as an in-furrow treatment, 3 to 4 quarts/A as a soil drench on 10- to 14-day intervals, or 3 to 4 quarts/A as a chemigation drip treatment on 14- to 21-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
Reference Nguyen, H.D.T., Dodge, A., Dadej, K., Rintoul, T.L., Ponomareva, E., Martin, F.N., de Cock, A.W.M., Lévesque, C.A., Redhead, S.A., and Spies, C.F.J. 2022. Whole genome sequencing and phylogenomic analysis show support for the splitting of genus Pythium. Mycologia 114(3):501-515. https://doi.org/10.1080/00275514.2022.2045116