By Pierce Paul and Anne Dorrance, Ohio State University Extension
Over the last few years, interest in nematodes and the use of nematicide seed treatments in corn have increased among producers all across the Midwest. When present and in high numbers, nematodes can indeed cause considerable yield loss in corn, and quite often these losses may go undetected or may be attributed to other causes. In corn, nematode problems are usually very difficult to detect because these pathogens usually cause uneven growth, without any clear above-ground symptoms. Uneven growth could be the result of several factors including other soil borne pathogens, poor drainage, soil compaction, and herbicide carry over; nematodes are rarely ever considered the cause of such a problem.
Several different types of nematode can attack corn including spiral, lesion, cyst (this is not the soybean cyst), stubby root, needle, lance, and dagger nematodes, and the level of damage and yield loss depend on the type of nematode present and the population level. Moreover, it is rare to find a single type of nematode causing the damage in any given corn field, these occur as a community which are comprised of a number of different species. Damage is usually caused by a nematode complex made up of several different types of nematodes or a nematode-fungi complex. Initial wounds made by nematodes, especially those that enter and feed inside the roots (endoparasites), may serve as entry points for infection by secondary or opportunistic fungi, adding to the overall level of damage.
It is unclear whether we do indeed have a nematode problem in corn in Ohio and if a nematicide will be beneficial. As stated above, the impact of nematodes on corn is very difficult to discern. In Ohio, we have a very good database on the distribution and effects of soybean cyst nematode (SCN), but SCN does not affect corn. We do not have any information on the distribution or effect of nematodes on corn in Ohio.
A survey of corn fields, focused first on those that are lighter soils and continuous corn is needed to determine which nematode species are present and at what population levels. Such a survey was recently completed in the state of Illinois by Terry Niblack, extension specialist and nematologist at the University of Illinois. More than 550 fields were surveyed and nematodes were found in every field, at populations ranging from 100 to 4000+ nematodes per 100 cc of soil. Most of these were plant parasitic nematodes, belonging to more than nine different genera, of which the “tylenchides,” nematodes with small styles and pointy tails, were the most frequent. These were found in 99% of the fields. However it is unclear what these organisms are doing in corn fields, since members of this group include plant parasites as well as parasites of fungi and algae. Of the known “tylenchides” observed, the spiral nematode (Helicotylenchus) was the most frequent. These were found in 99% of the soils, in most cases at moderate- to high-risk population levels (above 150 nematodes per 100 cc of soil). The lesion nematodes (Pratylenchus) were the second most prevalent nematode in Illinois cornfields survey by Dr. Niblack and her team. These were found in 84% of the fields. These were also found at population level considered above moderate risk thresholds in more than 50% of the fields in which they were present. Among these was Pratylenchus penetrans, a notoriously damaging species of lesion nematode on other crops such as potato. Greenhouse studies have shown that P. penetrans can also cause severe damage in corn; however, the level of damage caused in corn fields is unclear. The third and fourth most frequently observed nematodes were the stunt and lance nematodes, and these were also found at levels considered above moderate damage thresholds. The pin, ring, dagger, stubby-root, needle, root-knot, and sting were found in localized areas in Illinois, with the needle and sting nematodes found predominantly in sandy soils.
So, can we assume that the corn nematode population profile in Ohio would the similar or identical to that observed in Illinois? Not really, soil type, crop rotation, and corn hybrids can all influence the distribution and type of nematode communities that may be present in any given area. Surveys are needed to help us determine what we have in Ohio. Even if we find the same set of nematodes, can we assume that they are causing damage to our corn crop? No, studies are needed to determine population levels and damage thresholds on our modern corn hybrids on our soil types in Ohio. However, even without a survey, several of our current crop management practices favor potential nematode problems. According to Niblack, these include our widely used no-till or conservation tillage and corn-on-corn cropping systems and the abandonment of soil applied insecticides which in the past provided the added benefit of controlling nematodes.
So, while we wait for resources to conduct field surveys across the state of Ohio, we can use our understanding of the biology of these pathogens to make a projection as to where nematodes are most likely to be a concern and management practices for minimizing losses caused by these organisms. Nematodes are most likely to cause problems in no-till, corn-on-corn fields, and as such crop rotation and tillage would be the based approach for minimizing these problems (along with other disease problems). However, again, further research is needed in order to provide other management recommendations, such as seed treatments and hybrid resistance or tolerance. The effect of a seed treatment nematicide on the overall nematode population and effect on nematode life cycle are both unknowns. Trials from other states have shown variable yield responses to nematicides seed treatments among locations and among hybrids.
This article was prepared based on a presentation given by Terry Niblack of the University of Illinois at the 2010 ASTA Meeting in Chicago, IL.