Introduction:
Root lesion nematodes (Pratylenchus penetrans) are small vermiform animals that live in soil and plant roots (see Figure 1). These organisms are 0.4 to 1.0 mm in length and only visible with a microscope. The nematode has a short well-developed stylet in the mouth region which it uses to probe and feed on plant roots. After hatching from the egg, all four life stages, from the second-stage juvenile to the adult male or female, are capable of feeding on plant roots.

Symptoms:
In potatoes, symptoms of nematode damage may include reduced foliar and root growth and reduced tuber yield and quality. Field studies have indicated that tuber yield can be reduced by as much as 40%, depending on the numbers of nematodes present in the soil at planting. Tubers are also more prone to becoming misshapen. In addition, root systems damaged by nematodes are less able to tolerate moisture stress and are prone to fungal invasion. For example, root lesion nematodes and Verticillium species may act together to cause the disease complex ‘Potato Early Dying Syndrome.’

Nematode Life Cycle:
Root lesion nematodes generally require 4 to 8 weeks to complete their life cycle depending on soil moisture and temperature. The first stage of the life cycle is the egg followed by four juvenile stages and finally the mature adult. Soil moisture conditions that are optimum for the plant and soil temperatures between 200C and 250C are ideal for the development of Pratylenchus penetrans.

• Avoid using potato cultivars that are susceptible to root lesion nematode damage. For example, the cultivar ‘Superior’ is more susceptible to nematode damage than the cultivars ‘Russet Burbank’ or ‘Shepody.’ No cultivars grown in the Maritime region are resistant to root lesion nematodes.
• Soil fumigation will reduce nematode populations in soil, but these chemicals are expensive and not necessarily cost-effective.
• Avoid planting rotation crops that are good hosts for root lesion nematodes in the year prior to planting potatoes. Soybeans and red clover are both good hosts for nematodes, while annual ryegrass is less so. Plants such as marigolds may reduce nematode populations but these crops are usually of no economic value to the grower.

Nematode Assays:
Nematode damage is directly related to numbers of root lesion nematodes present in soil when the potato crop is planted. On Prince Edward Island, Pratylenchus penetrans populations greater than 2000 per kg of soil at planting usually cause tuber yield reductions in the cultivar ‘Superior.’ ‘Russet Burbank’ and ‘Shepody’ are not as susceptible to nematode damage, but counts greater than 5000 nematodes per kg of soil at planting may cause significant yield reductions. If nematodes are a problem, then field soil assays of nematode numbers should be made. These assays will enable growers to monitor nematode populations and allow the selection of an appropriate nematode management system. Instructions on sampling and handling techniques are described in the pamphlet, “Collection of Soil and Roots for Analysis of Plant Parasitic Nematodes” (Agdex 600/628, May 1994).