Root lesion nematodes, Pratylenchus spp., are native to the U.S. and have a very wide host range, so they are common in potato fields. Root lesion can be introduced by any means that moves soil, but are not a concern until population densities reach critical levels. It is common for multiple species to occur in the same field with the predominant species varying by production region. Root lesion is well-adapted to agricultural conditions so the condition is not likely to die out or be eradicated by human action.
Root lesion destroys root tissue and interferes with tuber bulking. Nematodes use stylet sucking mouthparts to feed on roots, causing localized lesions that can progress to girdling and root death. The impact of nematodes on yield depends on their population density. Some species, particularly P. penetrans, interact with Verticillium dahliae to cause the potato early dying disease (PED). In combination with verticillium, low population densities of root lesion are damaging because the nematode changes the response of the potato plant to the fungus. A synergistic interaction for root lesion and verticillium has been shown for foliar symptoms, leaf life span, specific gravity and yield of many potato cultivars.
Root lesion nematodes are animals with a simple life cycle. Juveniles hatch from eggs and undergo two molts to become sexually mature. They navigate through the soil matrix in water films, following temperature and chemical gradients to locate roots. Juveniles and adults enter and move freely inside roots, exiting when conditions become unfavorable. Females lay eggs continuously so root lesion can be found in soil, on the surface of roots or within root tissue. Their fecundity is low relative to other nematodes, so knocking population densities down usually provides season-long protection of potato.
Root lesion has a flexible life span because of its ability to desiccate. In the lab, one generation lasts four to six weeks. By slowly drying the soil, we’ve extended one generation to more than six years, with the nematodes in a living but dormant state until soil moisture was reinstated. A similar phenomenon occurs in the field; nematodes sheltered in dead root fragments of previous crops can survive for several years.
The remarkable ability to suspend life, coupled with a very wide host range, gives root lesion a staying power that makes them a site factor with the potential to cause chronic disease.
Not every infested field has a problem, but all potato growers should consider themselves at risk for a nematode problem. Plant stunting is best observed as the canopy starts to close. As is the case with other nematode diseases, the stunted areas will be patchy within the field. But the hallmark symptom—lesions on roots and stolons—is indistinguishable from damage caused by other pathogens, so it is not surprising that root lesion is often overlooked when diagnosing potato problems. Fields with a history of declining yields or those at a yield plateau should be checked for nematodes, particularly when other crops in the rotation disappoint as well.
The only way to assess the damage potential due to root lesion nematodes is to submit a sample to a nematode testing lab. Most labs use a sieving method to recover nematodes from soil and an incubation method that collects nematodes as they exit roots. Incubation assays are important for root lesion because most of the population occurs in the living roots of the current crop or dead fragments of crops past. It’s important to keep samples from overheating or drying too quickly so the nematodes arrive at the lab alive and able to move.
Soil testing to determine nematode population densities is important because damage is related to nematode pressure when the potato crop is planted. For samples collected the year prior to potato, the lab may factor in an expected overwinter mortality. Thresholds for management decisions vary by lab and production region, and many labs factor verticillium into the equation because of the synergistic interaction between it and root lesion. It is not necessary to test for nematodes every year, but it is important to remember that root lesion nematodes have a very wide host range and reproduce between potato crops.
There are no potato cultivars resistant to root lesion nematodes, so soil fumigation remains the gold standard for control. Multiple products share the attributes of being lethal and applied in time to reduce nematode inoculums at planting—key factors for reducing nematode pressure. The soil matrix is deep and complex so it is impossible for any material, including fumigants, to kill all nematodes. This makes it all the more important to follow label directions and fumigate when nematodes are active. Soil fumigation is preferred for fields with a history of PED because it is highly effective for both nematodes and verticillium.
In cases where the only target is nematodes, it may be more cost effective to use a product aimed specifically at them. Nematicides kill root lesion and other plant parasitic nematodes, and nematistatic compounds stun or paralyze them, but the end result is similar—reduced early season pressure from nematodes. A number of products have a good track record for increasing yields in nematode-infested fields, but they are more likely than fumigants to be associated with bounce-back effects that lead to high nematode densities at harvest.
Cover and biofumigant crops can be effective for nematodes, but it requires finesse. Nematodes reproduce while crops grow, so the timing of incorporation is important—it is possible for biofumigation to increase root lesion. Almost all cover crops are hosts, so it’s important to choose wisely. We’ve achieved yields comparable to fumigation using forage pearl millet cover crops in on-farm experiments, but the best choice will vary according to the predominant nematode species and production region.
When designing a nematode management plan it’s important to remember that the potential for yield loss depends on nematode population densities at planting.
Keeping root lesion in check throughout the rotation pays twice—better performance of rotation crops and potato. Root lesion nematodes may be permanent residents in potato fields, but they don’t have to be a spoiler for a great potato crop.