Cause:
The northern (Meloidogyne hapla) and Columbia (M. chitwoodi) root-knot nematodes are among the most important prob-lems in potato production. Both nematodes blemish tubers and may render them unmarketable. M. chitwoodi is more important than M. hapla, causing more severe tuber damage. Both are sedentary endoparasites (see Nematodes), and only the second-stage juveniles (the infective stage) and adult males (which may be rare) are present in soil. The disease spreads principally by infected tubers (seed or commercial) and reused irrigation water. M. chitwoodi continues to develop above 41oF, but M. hapla is not active below 50oF. M. chitwoodi can migrate up from 3 ft deep and cause economically significant tuber damage.
Symptoms:
Root-knot nematode infection rarely causes above-ground symptoms in potatoes. External symptoms of M. chitwoodi are distinct pimple-like bumps and lumps on tubers. M. hapla usually produces a more general swelling on the tuber or none at all. Both species produce brown spots within the vascular ring, within 0.25 inch of the tuber surface. Brown spots are about 0.04 inch (1 mm) in diameter and are evident only when the female nematode begins egg production. The female's white body and egg sac containing eggs (egg mass) produced outside the body often can be seen with a hand lens in the center of the brown spot. Unless several nematodes are in the same area, the spots' borders are distinct. The spots should not be confused with symptoms of heat necrosis, which produces spots of similar size but more diffused. Tubers infected with late season infection of M. chitwoodi may not show visible external or internal symptoms at harvest but may develop them in storage.
Note the bumps on the exterior of this tuber.
Notice the brown spots within the infected tuber.
Symtoms of Meloidogyne chitwoodi before and after peeling.
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. Nematode populations naturally decline through the winter. Thus, low populations in fall may not be detectable in spring. This is critical because low M. chitwoodi populations (less than one in 250 cu cm of soil) can cause severe economic losses. Sampling depth may be important, especially following deep-rooted crops such as alfalfa and field corn. Take soil 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 live and
dead nematodes may be distinguished. Control strategies may depend on the species of root-knot nematode, so request species identification when you submit the sample.
Cultural
control:
Use only certified seed inspected for nematodes.
Do not return tare dirt from potato or other crops to your farm.
Avoid moving soil from infested farms or fields by cleaning machinery and equipment.
Avoid using waste ditch water for irrigation. Settling ponds reduce nematode spread; nematodes settle to the bottom and eventually die. So, siphon water from near the water surface.
Use nonhost crops such as cereals in rotation with potato to suppress or minimize nematode population increases. Also, short-season crops such as wheat or sweet corn reduce the number of generations of M. chitwoodi compared to long-season crops such as field corn.
Weed control within rotation crops is critical because many weeds are suitable hosts for root-knot nematodes.
Plan an early harvest in fields infested with M. chitwoodi, even if using proper control measures.
Avoid storing tubers harvested from a field infested with M. chitwoodi.
Green manure crops such as sudangrass ('Trudan 8' or 'Sordan 79') and rapeseed ('Humus') effectively suppress nematode populations. These are grown after short-season crops such as wheat or sweet corn, in early August. Sudangrass is incorporated in fall before frost, rapeseed in spring 3 to 4 weeks before planting potatoes. M. chitwoodi has been adequately controlled when chemical controls have been used along with these green manure crops.
Chemical
control: No control technique ever eliminates nematodes from soil, and they eventually infect tubers. However, these recommendations should reduce the disease's impact.
Soil fumigation in fall is highly recommended, but spring fumigation is possible if criteria for fumigation conditions are met. Generally, soil temperatures and moisture for fumigation are better in fall than spring. In addition, nematode populations are more active in fall and therefore more susceptible.
Metam sodium 32.7% (Vapam HL, Soil-Prep, Nemasol) at 50 to 100 gal/A can reduce nematode populations. In certain fields, sprinkler-applied (1 inch of water) metam sodium cannot adequately reduce nematodes 2 to 3 ft deep in soil. For best control, apply metam sodium as a broadcast spray at a depth of 14 to 16 inches using a Noble plow or similar shanks with spray nozzles attached. Follow by metam sodium applied on the surface in inch water or sprayed on the surface and incorporated by discing 4 to 6 inches deep. 48-hr reentry.
Telone II at 20 gal/A for mineral soils, 25 gal/A for
muck soils. 5-day reentry. Restricted-use pesticide.
Telone C-17 at 30 gal/A for mineral soils and at 41 gal/A for muck soils. 5-day reentry. Restricted-use pesticide.
Mocap 10 G at 60 to 120 lb/A to control M. hapla and suppress M. chitwoodi. For best results with M. chitwoodi, use Mocap 10 G at 120 lb/A in combination with soil fumigation. Mocap protects the treated area from migrating nematodes for 5 to 6 weeks. Broadcast Mocap as close to planting time as possible and incorporate immediately in the top 4 to 6 inches (6 inches is better) of soil by rototilling or discing. 48-hr reentry.
Combine preplant soil fumigation and nematicide
application for high populations of M. chitwoodi located deep in soil.
Telone II at 20 gal/A plus Mocap 10 G at 120 lb/A applied broadcast and incorporated 4 to 6 inches deep by rototilling or discing just before planting. 5-day reentry. Restricted-use pesticide.
Telone II at 15 gal/A followed by metam sodium at 40 gal/A applied by sprinkler in inch water or broadcast and incorporated 4 to 6 inches deep. 5-day reentry after Telone II application and 48-hr reentry after metam sodium application. Restricted-use pesticide. Metam sodium applied this way also controls the early-dying disease (Verticillium wilt).
Content edited by:
Russ Ingham, Phil Hamm, and Cynthia M. Ocamb on
January 1, 2009
