Nematoda, one of the largest animal phyla, is represented by over 28,000 known species that typically measure less than .1 inch long. Because they are so minute, not considered terribly significant by human standards, and there are only so many honorable souls dedicated to their identification, it’s estimated there are thousands upon thousands of species yet to be identified. Nematodes, or roundworms, are present in aquatic environments, humans, animals, soils and plant tissues. There’s no doubt they outnumber us: one out of every five multicellular organisms known to exist on planet Earth is a nematode. However, let’s focus on those species that parasitize plant roots. For us growers, they are the significant ones.
Generally speaking, soilborne nematodes reproduce sexually. Once fertilized, the females are capable of depositing over 100,000 eggs per day. Those parasitic to roots place their eggs either in the soil surrounding roots or within the root itself. The egg will then develop, and the young nematode will molt four times as it progresses through the juvenile stage to the adult stage, all the while feeding off outer or inner root layers. The egg, juvenile and adult stages of some species are capable of entering a type of dormancy (cryptobiosis), for lack of host or should cold temperatures or drought set in. But the average life cycle, from start to finish, lasts anywhere from three to six weeks.
Some of these root parasites hold strong ties to a particular plant, while others are not so selective and feed from hundreds of different hosts. Knowing these creatures parasitize crops is one thing; recognizing that they are parasitizing your crops is another. Yellow, stunted, wilting plants may lead you and others to question the adequacy of your soil amendments or watering regimen. Distinct patches of withering plants may help to clue you in. When in doubt, uproot a few plants and take a close look. Stunted or swollen roots, galls and cysts on feeder roots and root rot are a good sign that nematodes are at work.
If you think nematodes are a problem, there’s only one sure way to find out. Uproot unhealthy and healthy plants and send the roots (including feeder roots) and accompanying soil to a qualified laboratory. If nematodes are present, the lab should be able to identify the species and estimate their density. Tables on economic thresholds for specific nematode species and crops are available, and knowledge of densities will help you determine whether you have surpassed that threshold.
If the lab report confirms your suspicion, there are several steps you can take to suppress parasitic nematode populations.
Include resistant crops in your rotation plan
Nematode-resistant crops can limit or prevent parasitic nematode reproduction. With reduced populations, their impact on subsequent plantings of nonresistant crops can be significantly reduced. Considerable research money has been, and continues to be, spent on developing nematode-resistant cultivars. While many have been genetically engineered and are not an option for the organic grower, others have been developed through the more traditional method of breeding plants with heritable, nematode-resistant genes.
Resistant crops are considered the most economical and environmentally sensitive approach for managing root-knot nematodes. Although the number of resistant varieties is low, they do exist, and that number is certain to increase in the future. USDA-ARS scientists in Charleston, S.C., have been developing improved watermelon germplasm and have come up with cultivars of bell pepper (Carolina Wonder and Charleston Belle) and southern pea (Charleston Nemagreen, Charleston Blackeye and KnuckleHull-VNR) that demonstrate strong resistance to root-knot species. USDA-ARS and Clemson University scientists have also co-developed Carolina Cayenne, a cayenne-type pepper resistant to root-knot nematodes. Incidentally, this plant’s resistance is considered equal in impact to methyl bromide fumigation.
Resistant cover crops are also an option. Certain sorghum, oat, rye and black-eyed pea cultivars are known to resist root-knot nematodes. Velvet bean, when planted as a cover crop, can suppress root-knot, cyst and stunt nematodes, particularly when the mature plants are incorporated into the soil (the vines and leaves are even more toxic than the roots).
Control those weeds
Yes, yet another reason to control weeds. Most parasitic nematodes appreciate roots, whether they belong to your valued crop plants or invasive weeds. You can plant as many nematode-resistant crops as you like, but if weeds grow freely among them, all is lost. Simply put, the more weeds, the more opportunities for nematodes to reproduce.
Try not to be part of the problem
Scrutinize transplants before plugging them in. Their roots could harbor an assortment of these pests. Clean soil-prep equipment (and those muddy boots, for that matter) before each use to avoid spreading the problem from field to field. Once nematodes have established themselves, they’re impossible to eradicate.
Support soil health
You may not realize it, but countless soilborne microbes feed on parasitic nematodes. If you regularly incorporate organic matter into your soil, you will fuel their populations. Organic matter will also contribute nutrients and improve soil structure; two soil components that contribute to plant vigor and the ability to tolerate damage from nematodes.
I consider these the simpler steps toward nematode control. If warranted, there are other, more drastic steps you can take, such as solarization or including fallow fields in your rotation plan. It all depends on the extent of the infestation and, ultimately, your economic thresholds.
The author, a regular contributor to Growing, is a biologist who lives and farms in Vermont’s Northeast Kingdom.