Root-lesion nematodes (Pratylenchus spp.) are the most common nematodes attacking mint and cause substantial damage. P. 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 are migratory endoparasites.  Females of P. penetrans lay about 1 to 2 eggs/day for about 35 days, with a maximum of 68 eggs laid by one female. Eggs are laid singly or in clusters in both soil and roots. 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 with some preference for the dense root-hair zone. Young feeder roots are generally selected, with a reduction in attack as tissues age. Feeding by P. penetrans produces lesions on roots, which initially appear as water soaked areas at the root surface. These sites later become yellow, and eventually develop dark brown centers. Discreet brown lesions of necrosis usually appear in two to four weeks. Field symptoms of damage generally occur as circular to irrgular patches, perhaps 30-150 feet in diamater, that have thin stand and stunted plants. Mint often has a reddish color. Presence of root-lesion nematodes can be detected by looking for reddish-brown lesions on roots of groundsel.