Only 10% of all nematodes that we know are plant parasites/ pathogens and a lot of them live in marine sediments. One of the common nematodes that affect animals are the dog heartworm (Dirofilaria immitis). Nematodes are found in deep sea vents, underground caves and many other extreme environments. Nematodes are also diverse in terms of their nutrition. Plant parasitic/ pathogenic nematodes derive their nutrition from plants and animal parasitic nematodes from their animal hosts. Some nematodes are very specific about their hosts while some are generalists. The soybean cyst nematode prefers soybeans, cowpeas, sweet clover and dry beans while it does not reproduce well on peas, all from the legume family. Whereas, the root-knot nematodes affect several crops from several different plant families. 

There are free-living nematodes or those that do not always live in association with another organism. These nematodes could feed on bacteria (bacteriovores), fungi (fungivores) or other smaller nematodes (predatory).  Since I am a plant pathologist, I am mostly concerned with plant parasitic/pathogenic nematodes. Here is an introduction to some of the naughty nematodes that cause plant diseases.

Once the nematodes hatch after two rounds of molting within the eggs, their vermiform stage is called 'J2' or second-stage vermiform juvenile. These J2s are the infective stage as they swim towards soybean roots and enter into them. Once they are inside, they reprogram root cells that transport nutrients (Phloem) to set up their own feeding cell called Syncytium. The syncytium then functions as a nutrient sink and reroutes all the carbon fixed by soybeans to feed the nematode. The nematodes molt twice again as they feed, passing through juvenile stages 3 and 4 or J3 and J4, maturing into adult males or females. Adult males still retain their vermiform or worm-like shape while the females take a lemon shape or become globose. Females remain attached to the sycytia as they protrude out of the roots and males move out of the roots to fertilise the females. The females (yellow) grow and mature into cysts (brown) which subsequently fall down into the soil at the end of the season.

Once again, unlike the SCN, all mobile stages of the lesion nematode can invade roots (recall that only J2 stage is infective in SCN). Because of the way these nematodes feed and move, they could lay their eggs within the roots or outside in the soil. Therefore, these nematodes have more flexibility in completing their life cycle than sedentary nematodes like the SCN. Because these nematodes do not have to establish intimate feeding sites and become cozy inside the roots, they do not have special adaptations for certain crops and hence, have a broader host range. Carrots, soybeans, corn, wheat, tomatoes, potatoes and several other plants can host the lesion nematodes. In our teaching laboratory, these were maintained on mint as the lesions are very clearly visible. Some labs sustain cultures of this nematode on sterile carrot discs!

Just like cyst nematodes, these are endoparasitic and sedentary. In plain English, that means that they like to settle in one spot inside the plant roots, set up a feeding cell and complete their life cycle using that feeding cell, without moving around in the roots. The way these RKNs move through the roots is also slightly different from how the cyst nematodes do. Meloidogyne entrolobii or the Guava root-knot nematode is a serious new pest for the United States, especially in sweet potato growing areas like the Carolinas, Mississippi, Texas, and Florida. It is in fact an invasive quarantine pest, as crops that are resistant to other root-knot species are not resistant to this particular species. 

While cyst nematodes, root-knot nematodes and lesion nematodes are some of the most important nematodes in the pan-American context, the warmer parts of the United States, especially areas with sandier soils, are breeding grounds for many other kinds of nematodes. 

American dream often includes a nice home in the suburbs with sprawling lawns. When these lawns are down in the southern parts of the country, not just humans, but nematodes also like to party in those lawns throughout the year. Apart from a root-knot called Meloidogyne graminicola, there are three other nematodes that love to feed on grass and make bald patches on green lawns. They are the lance, stubby root and sting nematodes.

When I was interning at University of Florida with Dr. Billy Crow, Dr. Don Dickson and Dr. Zane Grabau, We went to a bunch of fields - an artichoke farm and a napa cabbage farm were the two striking ones. When we analysed the soil samples and the roots that we plucked out from these fields, we could see more than one type of nematode as possible problems. It is indeed possible to have multiple nematodes in sandy soils and warmer weathers. Yellow, stunted plants or bald patches in a field are good symptoms indicating nematode issues.

Not all nematodes are long, thread-like microscopic creatures. It should be evident by now, from looking at cyst and root-knot nematodes. But, there are other quite misshapen nematodes or nematodes of unique shapes that can misshapen the economies of entire farms and orchards. 

In short, it is believed that there is at least one problematic nematode for every dear crop that we want to grow. But, fortunately, we know enough to manage them and protect our crops from severe economic losses. While we are more interested in nematodes that cause damages and losses, it is nice to know that only 10% of all the nematodes that we know of do so. For the most abundant animals on the planet, that is a small percentage. It is also humbling to me that even within that 10%, we might not have captured the knowledge of the complete diversity. On the flip-side, there are so many other nematodes in the soil that are probably helpful and we know less about. To me at least, these creatures are amazing to look at, study and control.