Root-lesion nematodes (Pratylenchus spp.) are the most common nematodes attacking mint and cause substantial damage. Pratylenchus penetrans appears to be the only species on mint in the Willamette Valley, and while both P. penetrans and P. neglectus have been found on mint in Central Oregon, P. penetrans appears to be increasing in frequency. Root-lesion nematodes have very broad host ranges. Pratylenchus penetrans probably infests every crop which might be grown in rotation with peppermint and probably most weeds found in mint stands as well.

Root-lesion nematodes are migratory endoparasites (Fig. 2). Females of P. penetrans lay about 1 or 2 eggs/day for about 35 days, with a maximum of 68 eggs being laid by one female (Mamiya, 1971). Eggs are laid singly or in clusters in both soil and roots. Second stage juveniles hatch after eggs have incubated for 9 (30 C) to 25 (15 C) days. Males are required for reproduction by P. penetrans but not by P. neglectus.

Root-lesion nematodes prefer to invade roots 3 to 13 mm behind the root tip (Freckman and Chapman, 1972) with some preference for the dense root-hair zone (Townshend, 1978). Young feeder roots are generally selected, with a reduction in attack as tissues age (Olthof, 1982). Female and J3 appear to enter roots more often than males (Olthof, 1982). The nematodes select a potential feeding/invasion site by rubbing epidermal cells with the lips and stylet (Kurppa and Vrain, 1985). After a cell has been selected, the stylet is thrust into the cell at rates up to 140/minute, making several holes in the cell wall (DiEdwardo, 1960). The nematode then pushes its head through the perforated cell wall. After penetration, the nematode injects saliva into the cell before ingesting the cell contents (Kruppa and Vrain, 1985). Penetration of the root is complete within 6-12 hours, and most nematodes reach the mid-cortex by 18-24 hours, after inoculation (Oyekan et al, 1972). Discolored areas on the root can be observed within 90 minutes after nematode arrival (Mountain and Patrick, 1959). The innermost root cells (endodermis) appear to be a barrier to invasion (Oyekan et al., 1972, Townshend, 1963b) although they may become discolored after invasion of epidermis and cortex by the nematode. Some endodermal invasion occasionally occurs in some plants after prolonged feeding. Eggs may be observed within root cortex (Fig. 24) five days after inoculation (Townshend 1963b). Root-lesion nematodes migrate between soil and roots continuously, and the population reduction in one habitat is often associated with an increase in the other (Bird, 1977). In mint, rhizomes are invaded as well as roots, but only a small proportion of the population is ever contained within the rhizome (Merrifield, 1990).

Feeding by P. penetrans produces lesions on roots which initially appear as water soaked areas at the root surface (Townshend, 1978). These sites later become yellow and elliptical, and eventually develop dark brown centers as nematodes continue to feed. Further feeding and migration produces galleries within the cortex, and discreet brown lesions of necrosis usually appear in two to four weeks (Potter, et al 1960) but may form as fast as 96 hours after invasion (Mountain and Patrick, 1959). Lesions form due to production of phytotoxic substances such as high concentrations of phenolic compounds. The necrosis (tissue breakdown) which causes the lesions often appears to advance ahead of the migrating nematode. The entire circumference of roots may be girdled when too many nematodes invade a root in the same small area (Townshend, 1963a). The combination of structural damage from feeding and physiological damage due to production of phytotoxic compounds reduces the amount of functional root length, restricting adequate water and nutrient uptake for the plant. In addition, the necrotic lesions provide opportunities for invasion by other microbial pathogens in the soil.