Root-knot nematodes occur throughout the world and are primarily important in tropical and subtropical climates. There are over 60 species described with new ones are continuously being identified. In temperate climates on potato, the species of most concern is Meloidogyne chitwoodi or the Columbia root-knot nematode. In the U.S.A., it is found in the Pacific Northwest (WA, OR, ID, and northern CA), the San Luis Valley of CO, and UT. M. chitwoodi has caused export embargo of seed tubers from these States to some countries such as Mexico. M. hapla or the northern root-knot nematode predominates in northeast and north-central U.S., and east and central Canada. M. incognita or southern root-knot nematode is found in the southern U.S.
Columbia root knot nematode (m. chitwoodi)
Eggs are laid inside roots and may number up to 1000. Reproduction does not require males. The second juvenile stage emerges from the egg and is the infective agent penetrating roots. Juveniles will repeatedly infect roots and may infect tubers as well. They enter tubers through the lenticels (stomates or pores). After the fourth juvenile stage, adults emerge from 'giant cells' that are developed for feeding juveniles. The males are wormlike and females are pear-shaped, about 1.5 mm (0.06 inch) long. M. chitwoodi likes sandy soils and soil temperatures of 68-77 F for reproduction. This nematode may complete three to five generations in a season in western U.S.A.
Tuber defect caused by northern root-know nematode (M. hapla)
Symptoms and Damage:
Root-knot nematodes attack tubers and cause blemishes making tubers unmarketable for fresh markets and reducing processing quality for chips and fries. Tubers infected with M. chitwoodi often appear bumpy but this appearance does not occur with infection by M. hapla. In addition to external defects, internal necrosis may occur between the skin and vascular ring. These necrotic spots are caused by egg-laying females. Galls are produced on roots and tubers. In the case of M. chitwoodi pimple-like galls appear on tubers but not on roots. Vine symptoms are rare but severly infected plants may be stunted, yellowish and wilt under conditions of low precipitation.
M. chitwoodi readily moves through the soil vertically as much as four feet making it difficult to control. Interaction by M. chitwoodi with soil pathogens has been noted in warm climates but little is documented in temperate ones. Due to a wide host range, crop rotation has limited effect. Control of host weeds year to year helps suppress population build up. M. chitwoodi likes grasses, corn and cereals. Some green manures such as canola and sudangrass used before potato planting can reduce tuber infestation. Research to develop resistant potato varieties in North America is ongoing. Short season varieties are less affected by these nematodes since infection of potato plants is later in the season. Long maturing varieties are more affected. Soil fumigants may be used and nematicides are available.
Nematodes in Potato: