ABSTRACT
Two important diseases of potato (Solanum tuberosum L.) that impact the quality of the crop are induced by the action of nematodes, namely rootknot and corky ringspot disease. The first disease is caused by several root-knot nematode species, while the second is caused by tobacco rattle virus (TRV) vectored by stubby root nematode species. Both diseases damage the quality of the tuber and render it unmarketable. Both dis eases are managed by controlling the nematodes. The damage threshold for both nematodes is very low. In the Pacific Northwest (PNW) of the USA where the Columbia root-knot nematode (CRN) M. chitwoodi (Golden et al. 1980), and the northern root-knot nematode (NRN) M. hapla (Chitwood 1949) threaten potato quality. Santo et al. (1981) estimated that one CRN and 50 NRN nematodes per 250 cm3 soil at planting, may lead to crop failure. Mojtahedi et al. (2001) demonstrated that 3 viruliferous stubby root nematodes per 250 cm3 soil suffice to cause visual viral symptoms on tubers. In the (PNW), a potato consignment with 10 or more percent tubers blemished due to corky ringspot (CRS) disease or any other injuries that may be considered by the processors as unmarketable. A tuber is considered a cull if six or more M. chitwoodi spots or one corky ringspot blemish are scored. Thus, a strict control of these nematodes before planting potato is essential for crop safety. Presently, soil fumigation with 1,3 dichloropropene at 180-225 ha-1 is the only method labeled by the manufacturer to control CRN and stubby
Corresponding author: Charles R. Brown
root nematodes in the PNW (Santo et al. 1997; Ingham et al. 2000). This fumigant must be shanked 45 cm deep, costs the grower $300-500 ha”1, and may not even be environmentally sound. Contrary to PNW, control of CRS in the potato fields of northeastern Florida (Weingartner et al. 1983) is not achieved by fumigation. Alternative control measures such as green manure and organic amendments have reduced soil populations of root-knot nematodes, but the residual population was high enough to require additional augmentation with some sort of nematicidal chemical treatment to achieve an economic control (Mojtahedi et al. 1993). Stubby root nematodes are less sensitive to soil amendments, than root-knot nematodes (Mojtahedi unpubl. data). The root-knot and stubby root nematode species associated with potato have a wide host range and crop rotation has not provided a reliable method of control. Rotating potato with alfalfa, wheat or com in PNW has not reduced populations of CRN and NRN nematode species enough to obviate the need to fumi gate. Host races of CRN increase on alfalfa rendering it an unsuitable choice for rotation. Rotating potato with com or wheat to reduce the impact of CRN also proved futile (Mojtahedi et al. 2002). Although alfalfa as a rotational crop may reduce CRS incidence on potato (Thomas et al. 1999), the presence of certain weeds may negate the benefit of such a practice (Boydston et al. 2002). Time of planting and early maturing vari eties may provide relief for certain growers with root-knot nematode problems. In the PNW, early maturing varieties, such as Shepody and White Rose, may escape northern root-knot nematode damage especially if the initial soil population is low. In the Central Valley of California, by early planting, potato may escape damage by the southern root-knot nematode (SRN), M. incognita (Chitwood 1949). In tropical and subtropical regions, where SRN produces 12 generations per year, escaping the im pact of nematodes is impossible. Trichodorid nematodes introduce TRV to potato tubers immediately after they are formed (van Hoof 1964), and therefore escape from CRS is unlikely. Based on information presented so far, it is clear that potato growers in hot production zones where SRN is a problem and those in temperate regions (such as the PNW and Europe) where CRN and M. fallax are problems, would benefit greatly by having varieties resistant to root-knot nematode and corky ringspot.
