By K. Frost and C. M. Ocamb
Cause Pythium debaryanum and P. ultimum, soilborne fungus-like microorganisms. The disease is associated with high temperatures at digging time or in early storage and shipment and care should be taken to avoid damaging tubers at harvest. The pathogen is attracted to the tuber's skin and can attack directly. Irrigation practices do affect disease incidence. Overwatering, clearly shown by increased incidence in the first tower of a center pivot irrigation system where more water is applied, does impact disease levels. Usually major losses in the field do not occur. This disease can result in substantial damage in storage as infected tubers rot due to secondary infections by soft-rotting bacteria.
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 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.
- 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.
- 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/1000 ft row. 48-hr reentry.
- Quadris Ridomil Gold (Group 4 + 11) at 0.82 fl oz/1000 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 0.42 oz/1000 row feet on a 6- to 8-inch band width at planting or Ridomil Gold SL at 0.42 fl oz/1000 row feet in-furrow at planting or Ridomil Gold SL at 0.42 oz/1000 row feet + Orondis Gold 200 (Group U15) at 4.8 fl oz/A 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. Do not use with Polyram. 48-hr reentry.
- V-10208 (Group 22) at 8 fl oz/A in-furrow and then side-dressed after at least a 25-day interval. 12-hr reentry.
- Amplitude at 3.2 to 4.8 fl oz/100 lb seed pieces, 6 to 8 fl oz per 1,000 ft row as an in-furrow treatment, or 3 to 4 quarts/A as a chemigation treatment on 14- to 21-day intervals. Preharvest interval is 0 days. 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