By R. E. Ingham and C. M. Ocamb
Potato (Solanum tuberosum) - Corky Ringspot
Cause Paratrichodorus spp. and Trichodorus spp. are migratory ectoparasites found only in soil. Their presence alone will not impact potato production. However, certain species (including P. allius, the predominant species in the PNW) can vector the Tobacco rattle virus. The virus causes corky ringspot (CRS) in potato tubers, resulting in extensive damage to tuber quality, which can lead to crop rejection.
Symptoms No symptoms are produced in the absence of the virus.
Sampling Take soil samples with sufficient lead time to implement management procedures if necessary. For example, after soil fumigation it may be several weeks before potatoes can be planted. Nematodes can often be found deeper in the soil profile and migrate upward later so sampling to a depth of two feet is often advisable. Fall sampling at harvest of the crop preceding the potatoes (to be planted the following spring) is an excellent strategy. If samples contain stubby-root nematodes, soil can be further tested for Tobacco rattle virus by bioassays on indicator plants.
Cultural control Crop rotation is ineffective because many crops and weeds are hosts for the virus and nematode. Resistant varieties are available but not adapted to the Pacific Northwest.
- Soil fumigation gives commercial control of the problem for 1 year. Nematodes persist in the second and third foot of soil, so it is important that these areas are warm and moist enough to allow good fumigation.
- Metam sodium (Sectagon 42, Vapam HL) at 40 gal/A does not always control symptoms from CRS but is more effective when shank-injected than when applied through irrigation. Telone plus metam sodium is an excellent combination. Restricted-use pesticide.
- Telone II at 9 to 18 gal/A for mineral soils and muck soils. 5-day reentry. Restricted-use pesticide.
- Telone C-17 at 27.5 gal/A for mineral soils. 5-day reentry. Restricted-use pesticide.
- Additional benefit has been obtained after soil fumigation when a nematicide has been applied broadcast and mechanically incorporated before planting.
- Mocap EC at 1 to 12 gal/A mechanically incorporated by rototilling or discing just before planting. Broadcast Mocap as close to planting time as possible and immediately incorporate in the top 2 to 4 inches of soil. Best results are obtained with deep (6 to 8 inches), even incorporation. Generally, Mocap EC should be applied following fall or spring application of a registered fumigant nematicide. In the Columbia River Basin, it is strongly recommended that Mocap EC be applied following fall or spring application of a registered fumigant nematicide. Under severe infestation or long growing season conditions, Mocap EC may not adequately protect tuber quality. 48-hr reentry; 72-hr reentry where average rainfall is less than 25 inches/year.
- Return XL (Group 1A) at 34 to 68 fl oz/A. See label for treatment programs. 48-hr reentry. Restricted-use pesticide.
- Apply Vydate C-LV where soil samples indicate less than 50 stubby-root nematodes per 250 cc of soil. Precede Vydate with a shank-injected fumigant if stubby-root populations exceed 50 per 250 cc of soil. For optimum performance, use 2.1 to 4.2 pints/A in-furrow at planting or 2.1 pints/A between 30 and 40 days after planting Caution Significant damage can occur if first Vydate application is not made before 40 days after planting. For best results, all applications other than in-furrow should be made via chemigation. See manufacture's label for details on application through sprinkler irrigation systems. Always buffer Vydate injection solution to a pH of 5.0 or lower. Do not apply within 7 days of harvest. Restricted-use pesticide.
- MeloCon WG for nematode suppression. See label for specific application types and timings. 4-hr reentry. O