Different plant stresses above and beyond those of soil fertility, moisture holding capacity, and drainage that are usually emphasized at harvest can also restrict the productivity of crops. One of those stresses that has the potential to be present in every farm field is created by the feeding and other activities of plant nematodes. Autumn is the preferred time to collect soil samples for plant nematode analysis. Justification for the previous statement and additional information about plant nematodes are provided in the following paragraphs.
We will try in the following paragraphs to express our appraisal of the "Corn Nematode Problem" as we believe it currently exists. Back in the 1970's and 1980's North Central nematologists did a lot of work on the "problem". And yes, they did determine then that corn nematodes, primarily lesion or Pratylenchus spp., can reduce grain yields. Plant nematodes can do that by adversely affecting root mass and root quality, limiting the ability of the plant's roots to take up water and nutrients. The lesion nematode, which is native and widely distributed in the soils of the Upper Midwest, is particularly important as a "corn nematode" because it lives, moves, and reproduces within the outer layers (i.e. epidermis and cortex) of plant roots causing those plant cells to become discolored and diseased. Nematologists determined the significance of plant nematodes by applying various nematicides and insecticide/nematicides at planting in bands over the rows to replicated plots. At-harvest data obtained from such plots have supported the conclusion that Minnesota's lesion nematodes can reduce grain yields by up to 20 bu/acre under certain conditions. Such effects only occurred on the fertile non-irrigated soils of southern Minnesota if there had been a significant dry period while the crop was actively growing. Lesion nematodes can apparently cause yield reductions almost every year with irrigated and especially non-irrigated corn growing in sandy soil. We believe that plant nematodes are most likely to be pathogenic (cause disease) if the plant is already being subjected to other stresses like those of moisture shortage or root pruning caused by insects. Or, in other words, stresses that we consider to be most significant components of a potential "nematode problem" are those that further compromise the plant's ability to take-up adequate amounts of water and nutrients.
We have never seen pronounced or even readily visible above-ground symptoms (reductions in plant height, leaf canopy, etc.) that could be attributed to the activities of Minnesota's lesion nematodes. However, a few native plant nematodes can cause visible aboveground symptoms on corn. One of those is the needle nematode, a Longidorus sp. That nematode is restricted probably due to its large size (up to 5 mm or 1/5th of an inch compared to the much smaller lesion nematodes that are slightly less than 1 mm in length) to porous sandy soils; particularly the coarse dark sands that often border the Mississippi. The needle nematode, in addition to its large size, is also a bit unusual because it apparently can only get the nutrients that it requires from the roots of grasses. And its effects are also unusual because they are quite striking. Stunted corn plants that the needle nematode has fed on can be found adjacent to apparently entirely healthy plants growing in the same row. Other plant nematodes typically have much wider host ranges. Lance nematodes, Hoplolaimus sp., occasionally occur in sufficient numbers in localized spots ("infection centers") where they can visibly stunt corn, soybeans, and alfalfa.
So, if you are going to study corn nematodes anywhere in Minnesota or wanted to determine if a portion of a field had the potential to have a "corn nematode problem", we would recommend that you go first to a "continuous corn field". Then go to the margins of less productive areas of that field as determined at harvest or visually if the needle or lance nematode is responsible. And then we would urge you to sample in the row and in the root zone at several different locations of the proposed plot area or a localized perhaps 10' by 10' portion of the field. Please note that the procedure for sampling for plant nematodes is distinctly different (In The Row!) from procedures advocated for collecting samples to be used to determine soil fertility! We recommend collecting 8 to 10 one inch diameter soil cores that are each 8-10 inches in length and combining those cores as one sample. Multiple samples are preferred, in our opinion, because plant nematodes (like other living causes of plant disease) typically are not evenly distributed across an area. Although the small "soil sample" boxes that are available from many laboratories will hold enough soil for most "nematode tests", we would urge you to provide the lab of your choice with 2 to 4 times that amount of soil. Sturdy, closeable food storage or other plastic bags work well for containing and protecting "nematode samples". Although plant nematodes have been resident on the planet for many, many millions of years, they really are quite delicate organisms that need to be protected from drying and excessive heat. A temperature of 120F or above that can be reached in a closed vehicle on a bright sunny summer day will inactivate most plant nematodes. We would recommend that you have the samples processed by a lab such as the U of M's where the nematologist from experience is in a position to be able to share information about the potential significance of each of the components of the plant nematode population that was detected. And although it is human nature to often want to seek a second opinion, please recognize that it may be impossible for the nematologist of a second lab to comment on the significance of the nematode population data generated by the first lab unless that second nematologist can determine how the first lab actually processed your soil samples. There are several different methods for processing soil samples for nematodes. And those methods do differ in terms of their effectiveness for separating the various kinds of plant nematodes from soil.
We definitely would not recommend that you bother making a nematicide efficacy experiment at a given location unless a sufficient (500 or more per 100 cc of soil) at-planting population of lesion nematodes was present. Although significant over-wintering mortality of plant nematodes can occur during cold and "snow-less" winters, collection of soil samples in autumn after harvest is highly desirable in terms of providing all concerned with sufficient time to collect and process the samples and time to be able to make rational decisions about the significance and usefulness of the nematode data that will be generated over the winter by the nematology laboratory. It is really important to sample after the crop plant has died because the majority of the lesion nematodes that are present will be in the plant roots while the plant is living. As the plant roots die many, but certainly not all, of the lesion nematodes will migrate out into the soil where they can be detected with a soil test.
If a "nematode problem" does seem to have the potential to exist in a given farm field, then what options are available? Rotation to soybeans or some other crop may in some situations be an economically and nematologically feasible procedure. Nematicides are available and their application may sometimes be appropriate. We would also recommend that you evaluate more than one corn hybrid since hybrids may differ in their ability to regenerate roots. At least in the past there seemed to be some hybrids "out there" that were not particularly affected by lesion nematodes because they had an "adequate" root regeneration potential. Although in the "real world" it probably is unlikely to occur, it certainly could be helpful and potentially informative if the at- planting "plant nematode status" of the soil where demonstration variety trial plots are planted were determined and recorded along with other environmental criteria pertaining to the plots. Other options utilizing "precision planting" capabilities may exist in some situations if the plant nematodes that developed on the previous season's crop are truly concentrated in last year's rows and are largely absent from "row middles". This may be feasible since plant nematodes are small (essentially microscopic) and do not move very far very fast. Our control procedures in this and similar cases really just attempt to separate and thus protect the young and vulnerable plant from the plant nematodes!
Several years ago North Central nematologists made a special attempt to call the attention of soybean producers to the existence of the problem caused by the Soybean Cyst Nematode. They did that by frequently repeating the catchy and easy to remember little saying: "Take the (soil) test, Beat the pest!" Although the effects of most of the corn nematodes are not as dramatic as those of the SCN, we have tried through this Fact Sheet to emphasize that the native and omnipresent corn nematodes are sometimes abundant enough to adversely affect grain yields. And because plant nematodes are microscopic for all practical purposes, the only way to determine if they are of potential economic significance is to collect soil samples that are then appropriately processed to reveal, identify, and quantify their presence. Although plant nematodes can be separated from soil samples collected at any time of the year, autumn is by far the preferred time to sample!
We hope that you have found the information contained in this "Fact Sheet" to be interesting and useful. Please call or write if you have additional questions or concerns about our appraisal of the "Corn Nematode Problem" and how it may be addressed.
David H. MacDonald Dimitre Mollov
Room 214 Stakman Hall Plant Disease Clinic
Department of Plant Pathology, Room 495, 1991 Upper Buford Circle, St. Paul, MN 55108