The hidden enemy: Parasitic nematodes

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

Often the things we fear the most are things we don’t understand or can’t see. Does this include parasitic nematodes? Sure, parasitic worms sound quite frightening as we usually don’t see them, only the symptoms of the diseases they cause. In fact, nematodes can cause very serious diseases of both animals and plants. The World Health Organization would list at least a couple of species of nematodes among the 10 most important pathogens of humans, worldwide. If you’ve ever seen a photograph of a person infected with Wuchereria bancrofti, the causal agent of elephantiasis (a serious disease resulting in grotesque swellings of arms, legs and other body parts), you’d have a very good understanding of the insidious nature of nematode infestations.

Well, what does this have to do with the production of vegetables in Michigan? All species of vegetables have at least one nematode parasite and some of these pathogens can cause serious yield losses. We often refer to plant-parasitic nematodes as “the hidden enemy” because they are microscopic worms and since they can’t be seen with the naked eye, their presence often goes undetected. The only way to know for sure if a field is infested with plant-pathogenic nematodes is to collect samples of soil or plant tissue and submit those samples to a nematode diagnostic lab for analyses. Fields going into vegetable production should always be sampled for nematodes, preferably in the fall, the year before vegetables are grown. Management strategies and tactics can be planned or implemented at that time.

Some vegetables are planted early in the spring, so in these cases, it is too late to sample fields for nematodes and implement control tactics prior to planting this year (this is called problem avoidance.) However, producers of vegetables should scout fields early in the year for nematode damage. Nematodes will feed on plants very quickly after roots are formed, so symptoms may appear as soon as a week or two after plant emergence or transplanting. Feeding by northern root-knot nematodes, Meloidogyne hapla, will result in the production of small swellings on roots called galls. However, feeding by other nematodes will not result in characteristic symptoms. Circular or elliptical areas of stunted, yellow or dead plants are often the result of parasitism by plant-parasitic nematodes. Growers should note these areas and submit samples to MSU Diagnostic Services for confirmation of nematode problem diagnosis.

Unfortunately, if nematode problems are detected early this growing season, growers can do very little to alleviate the symptoms they observe. However, once fields are infested with plant-parasitic nematodes, they will remain infested indefinitely. Therefore, records of nematode infestations should be kept to minimize the risk to future plantings.

It’s critical to remember, the only way to positively diagnose a nematode problem is to submit a sample to a nematode lab for analysis. If this is done routinely, unexpected crop losses of undetermined causes can be minimized. Reduce the fear, let the enemy reveal itself. Nematodes may not be so scary after all.  

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