By R. E. Ingham, P. B. Hamm, and C. M. Ocamb
Potato (Solanum tuberosum) - Verticillium Wilt (Potato Early Dying)
Cause Pratylenchus neglectus is the most common root-lesion nematode found in the potato-growing areas of the Columbia Basin, eastern Oregon, and Idaho. P. penetrans is generally in a potato field that formerly grew tree fruits or mint but are beginning to be found more commonly in other situations. They are migratory endoparasites usually residing in roots but able to move in and out of roots. Both species have very wide host ranges, especially cereals and grasses. Studies in a loamy sand show that preplant soil populations of P. penetrans at 1,000/250 cu cm of soil reduced 'Russet Burbank' potato yields, but P. neglectus did not at the same level. However, these nematodes' most important effect is that they can interact with pathogens associated with potato early dying, including the Verticillium wilt fungus. Plants infected primarily with P. penetrans are more susceptible to invasion by the early-dying disease pathogens, including the Verticillium wilt fungus (Verticillium dahliae) and the soft-rot bacteria (Pectobacterium carotovorum subsp. carotovorum (syn. Erwinia carotovora subsp. carotovora). Pratylenchus penetrans can significantly increase the severity of potato early dying.
Symptoms These nematodes are not known to blemish tubers, and symptoms produced are not diagnostic. Root systems generally are reduced in size with few feeder roots and are darker in color. Short dash-like lesions can be found on roots and stolons. These areas can grow in size over time. The darkening generally is caused by secondary fungi and bacteria, which enter roots through wounds made by the nematode. Plants damaged by root-lesion nematodes may have long healthy roots near the soil surface but lower roots are severely damaged.
Sampling Sample soil any time before planting potatoes provided the soil is not frozen or too dry or wet. However, it is best to sample in late summer or early fall, when nematode populations are higher, than in early spring. If plants are present at the time of sampling, roots should be collected as well for a separate nematode extraction since they may harbor large populations not accounted for with a soil-only sample. Take samples before and after soil fumigation to determine the treatment's effectiveness; however, do not take samples within 2 weeks after treatment. Indicate whether samples are pre- or post-treatment, so that live and dead nematodes may be distinguished.
- Weed control is critical because many weeds are suitable hosts for root-lesion nematodes.
- Green manure crops such as sudangrass ('Trudan 8' or 'Sordan 79') may suppress populations of root-lesion nematodes. They can be grown after short-season crops, such as wheat or sweet corn, in early August. Incorporate sudangrass in fall before frost. Do not cut and let regrow-cutting removes nematode-suppressing ability. If cutting for hay, incorporate remaining vegetation as soon as possible.
Chemical control Chemical control is recommended particularly when high levels of P. penetrans are found during soil sampling. In the absence of potato early dying, it is not economical to control root-lesion nematodes with fumigation. Early dying is wide spread in the Columbia Basin and metam sodium use is recommended in that area. Fumigating for potato early dying or root-knot nematode also controls root-lesion nematodes.
- Metam sodium (Sectagon 42, Vapam HL) at 40 gal/A using a thin-shank injector rig with shanks spaced 5 inches apart. Use at 40 gal/A injected into sprinkler system, using 0.5 to 1 acre-inch of water, to control both nematodes and early dying disease. See label for reentry interval. Restricted-use pesticide.
- Mocap 15 G at 40 to 80 lb/A applied broadcast and mechanically incorporated 4 to 6 inches by rototilling or discing just before planting. 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.
- Telone C-17 at 24 gal/A for mineral soils and at 30 gal/A for muck soils. 5-day reentry. Restricted-use pesticide.
- MeloCon WG for nematode suppression. See label for specific application types and timings. 4-hr reentry. O