Nematodes are small worms that live in diverse environments, including oceans, rivers and soil. They are adapted to nearly all climates and can be found anywhere from hot African deserts to bitter cold Antarctica and everywhere in between. While most nematodes are harmless, some species are specialized parasites that can cause significant harm to crops. One of the main nematode problems in potatoes grown in the Northwest is the Columbia root-knot nematode (Meloidogyne chitwoodi), known as CRKN for short.
CRKN infects potato roots and tubers and causes the formation of galls, small swellings at the root or tuber surface. When peeled, infected tubers show brown spots in the flesh, a telltale sign of nematode feeding (Figure 1). Even though CRKN seldom causes drastic yield losses in terms of tuber size, the tuber symptoms that develop due to CRKN infection can make entire shipments unmarketable, even at relatively low levels.
CRKN was first found in the Columbia Basin in the late 1970s. Today we know that it is widespread in the western United States and some other potato-growing areas in Europe and South America. To limit its distribution, CRKN has been declared as a quarantine pest by a number of regulatory agencies. Its control is mostly based on synthetic pesticides, which can be a costly strategy. CRKN infects a wide range of crops, which makes its control challenging. In fact, it is just as happy with corn, carrots, wheat and alfalfa—crops that would fit into a rotation with potatoes but don’t help to reduce CRKN numbers. And to make control even more challenging, a wide range of weeds are also on the CRKN menu.
An accurate diagnosis goes a long way
Not all nematodes are bad news. On the other hand, it is essential that quarantine pests such as CRKN are properly diagnosed to avoid unnecessary costs. The life cycle of CRKN lasts about four weeks, depending on soil temperature and moisture levels. The most common life stage found in the soil are second-stage juveniles in search of a new host. This life stage is tiny—barely thicker than a hair—and pretty much impossible to see without a microscope (Figure 2). Once inside a plant, the juveniles undergo consecutive molts and develop into adult females or males. The symptoms found on the tuber surface are caused by adult females. The swellings or galls are their feeding sites.
As far as diagnostics and control go, CRKN can make life complicated. There are some populations of CRKN that are known to overcome resistance genes found in a wild potato species (Solanum bulbocastanum) (Figure 3). Others don’t exactly fit into the original species description that was published in 1980 and that forms the basis for CRKN identification. To remedy this, Dr. Axel Elling and Danny Humphreys, a doctoral student in Dr. Elling’s lab in the Department of Plant Pathology at Washington State University in Pullman, are taking a closer look at CRKN. How variable is this nematode? Where does it come from? These are some of the questions they are trying to answer. Asking these questions is simple, but answering them is more like putting together a giant jigsaw puzzle.
Do all nematodes look alike?
It has been long known that there are at least three variants of CRKN that differ in their ability to infect certain plant species. Most recently, a fourth variant was identified by Dr. Charles Brown and his colleagues from the USDA-ARS in Prosser, Wash. The tricky part is sorting out who is who and how everyone is related to each other—that is exactly the point where Humphreys and Elling come into the picture.
At present, we know of four distinct CRKN variants in terms of pathogenicity. The question is how variable different CRKN populations are and if this variability can be exploited to figure out where this worm came from in the first place. Is it native to the northwestern United States, or has it come from somewhere else? As long as we don’t know how variable CRKN is, it is very difficult to develop a good resistance against it.
“In effect, we are developing a CRKN family tree,” says Elling. If we have enough samples, we can more or less trace the ancestry of CRKN populations in areas like Idaho, Washington and Oregon and compare it to CRKN populations found abroad. This is important because it helps with developing better diagnostic tools and strengthens potato breeding programs aimed at developing resistance against CRKN.
Humphreys has studied the morphology of CRKN juveniles, females and males using advanced microscopy. He discovered that CRKN can be a lot more variable than previously thought—an obvious problem for diagnostics. Importantly, he found a couple of morphological details that could in fact make CRKN diagnostics easier. There are certain features that are not found in closely related nematode species with which CRKN is easily confused. Humphreys’ study has the potential to significantly improve CRKN diagnostics, says Elling. But the work does not stop there. You can only go so far by looking at a nematode. There is a wealth of information at the molecular level, most importantly its genetic material DNA.
Humphreys and Elling analyzed the genetic relationships of different CRKN populations using molecular tools and found that there are certain traits that are more common in some CRKN populations than in others. This is a good starting point for a larger study. At present, they are developing better molecular markers that might be able to capture a lot of the molecular variability in CRKN. The challenge is to get enough samples. Most potato fields are fumigated, which brings down CRKN numbers to a level that is too low for this study. The best way to find CRKN is actually to look carefully at tubers. If they have galls and brown spots, chances are they are infected with CRKN.
If you want to contribute to this project and send some infested tubers, please contact Dr. Axel Elling (elling@wsu.edu) for more information. The more samples are part of the study, the more useful the results will be. What’s in it for you? Better CRKN diagnostics and a better shot at breeding CRKN-resistant potatoes.