Nematodes are the most frequently overlooked cause of corn disease. These tiny animals cause aboveground symptoms that could be ascribed to any stress (stunting, chlorosis; Figure 1) and can intensify expression of specific symptoms from nutrient deficiency, herbicide injury (Figure 2), and other causes. You might say that the mist shrouding nematodes as corn pathogens is deserved—that the injury they cause is rare and confined to sandy soils anyway, and that they are not worth the effort it takes to find and diagnose them—and you would be wrong. Nematode injury to corn is not rare—it is simply difficult to identify. Likewise, it is simply human nature to discount problems that are hard to see and hard to diagnose.
Figure 1. Nematode damage to corn. (Photo courtesy of G. L. Tylka.)
Figure 2. Nematode injury to corn roots can easily be mistaken for herbicide injury. (Photo courtesy of G. L. Tylka.)
Adding to the difficulty of diagnosing problems from nematode feeding is the probability that few corn nematode species cause direct injury on their own. As previously stated, they interact with other problems to intensify symptoms. They also occur in polyspecific communities, and it is the opinion of corn nematologists that corn injury from nematodes frequently is not a one-nematode-one-disease situation (5,7). The practical implication of corn injury as an “interaction disease” is that it requires highly trained people to diagnose and supply management recommendations. No easy fix exists for the problem of diagnosing corn nematode problems.
The last true corn nematologist, Don Norton of Iowa State University, retired around 1985. Many nematologists, especially Dale Edwards and Greg Noel at the University of Illinois and those at other land-grant institutions in the Midwest, have worked on corn nematode, but for Don Norton, corn nematodes represented the focus of his scientific life. Much of what follows in this article can be found in detail in his final review of nematode parasites of corn (5). Another review of corn nematodes, by Windham (7), provides additional information, with an emphasis on corn nematodes found in the southern states.
Losses from Corn Nematodes
According to Dale Edwards (in 3), the nematodes responsible for suppression of corn yields in Illinois are Hoplolaimus galeatus (lance nematode) (Figure 3), Longidorus breviannulatus (needle nematode), Pratylenchus spp. (lesion nematodes), and Xiphinema americanum (dagger nematode). They will be covered here in the order of their importance. In 1994, the last year for which estimates are available, these nematodes were responsible for corn yield losses valued at $81 million dollars in Illinois alone (3).
Figure 3. Head of a lance nematode. (Photo courtesy of E. C. Bernard.)
It is interesting to note that, despite this impressive loss estimate, few or no funding agencies are willing to support research on corn nematodes. We expect losses to increase for two reasons. First, an observation that is unsupported by research evidence is that the use of insecticides (many also described as “insecticides/nematicides”) to control corn insects had the unintended beneficial side effect of reducing nematode populations. Reduction in the use of such chemicals, or changes to different chemistries with less impact on nematodes, has resulted in an increase in the incidence and population densities of nematodes on corn. Second, many farmers are considering switching from a yearly corn–soybean rotation to a corn–corn–soybean plan for various reasons. Corn–parasitic nematodes will reap a great benefit from this change, and farmers without good information on their corn nematode populations will reap the converse reward: reduced corn yields.
Pratylenchus spp., Lesion Nematodes.
Lesion nematodes (Figure 4), sometimes called root-lesion nematodes or meadow nematodes, are probably the most economically important of the corn–pathogenic nematodes. There are at least 15 species that parasitize corn, of which three are well-documented pathogens of corn: P. brachyurus, P. hexincisus, and P. zeae. Eight species are known or potential pathogens of corn in Illinois. When lesion nematodes were controlled (usually through nematicides) in research plots, yield increases have been measured ranging from 10% to 54%.
Figure 4. Pratylenchus spp. (Photo from .)
Lesion nematodes are small (300 to 750 ?m) migratory endoparasites that damage corn roots by migrating through root tissue and feeding on cells. Typical damage from lesion nematodes ranges from water-soaked areas to extensive necrosis of the root cortical tissue. The root lesions for which they are named are sunken, necrotic areas that are easily identified only from greenhouse-grown corn roots where there are no other pathogens present. Resistance to lesion nematodes has been investigated very little, but it is known that some hybrids are less suitable hosts than others.
Two factors make the diagnosis of lesion nematode problems difficult. The first factor is that nematode problems are generally diagnosed from the results of soil sample analysis. The number of lesion nematodes in the soil is not necessarily a good indicator of the number in the roots. High lesion nematode population densities can be masked. The second factor is that lesion nematodes often occur in populations of mixed species. The difficulty of identifying the species present is stultifying. Fortunately, molecular markers for several important species of Pratylenchus are becoming available and will help labs with the chore of identifying them.
Control of lesion nematodes, in the absence of suitable chemical controls, is dependent on species identification. Where polyspecific communities occur, rotational crop recommendations should be based on knowledge of host preference. Sanitation and natural product–based soil amendments have provided lesion nematode control in some cases.
Longidorus breviannulatus, Needle Nematodes
Needle nematodes can cause spectacular losses—up to 62%—in infested fields (Figure 5). These nematodes are ectoparasites that damage corn roots by feeding on root tips, thereby stunting the lateral roots and essentially destroying the fibrous root system (Figure 6). Heavily parasitized seedlings may be killed.
Figure 5. Needle nematode damage to corn. (Photo courtesy of D. C. Norton.)
Figure 6. Corn root damage due to needle nematode feeding. (Photo courtesy of D. C. Norton.)
Two factors make needle nematode damage relatively easy to diagnose. The first factor is that needle nematodes are very large (4 to 5 mm long) and are relatively easy to assay for in a corn soil sample. Second, because they are so large, they are confined to sandier soils (with larger pore spaces) and do not have to be considered as the cause of problems in heavier soils.
In Illinois, very good threshold numbers for needle nematode damage have been established. One to five needle nematodes/100 cm3 soil can cause a moderate level of damage, whereas more than 25 can cause very severe damage. Corn planting should be avoided in fields with high numbers of needle nematodes. High rainfall amounts and cool temperatures in spring encourage the appearance of needle nematode damage. Plants will appear to be under severe drouth stress (Figure 5). Infected corn plants can appear to grow out of early damage, but yield will nonetheless be significantly reduced.
Although relatively easy to diagnose, needle nematodes are not easy to control without nematicides. Management of needle nematodes requires monocotyledonous weed control, because the nematodes have a wide host range and can maintain and even increase their population densities on such weeds. Rotation to a nonhost crop, such as soybean, can reduce needle nematode populations if weed control is good.
Hoplolaimus galeatus, Lance Nematode
Of the four species of lance nematode that can parasitize corn, Hoplolaimus galeatus (Figure 7) is the one that affects corn yields in Illinois. Another large nematode (around 1 mm or more), H. galeatus is more likely to be found in sandier soils than in heavy ones, but it is not unusual to find this nematode in silt loam soils, albeit at lower population densities. Lance nematodes are extremely common, with a very wide host range, including monocots and dicots. As few as 100 lance nematodes/100 cm3 soil will damage young corn plants. They usually feed as migratory endoparasites (Figure 7). Those plants that appear to grow out of early damage will yield significantly less than healthy appearing plants in the same field.
Figure 7. Lance nematodes (stained pink) feeding in root. (Photo courtesy of G. L. Tylka.)
Xiphinema americanum, Dagger Nematode
This dagger nematode (Figure 8) species is another very large (up to 2 mm) nematode, but one with less sensitivity to sand content than the needle nematode in that, like the lance nematode, it may be found in silt loam soils; however, X. americanum populations damaging to corn appear to build up only in sandy soils. It feeds ectoparasitically. Beyond that, very little is known about the X. americanum–corn relationship. Control of the nematode would seem to be best effected by tillage, because this nematode is highly sensitive to soil disturbance. Its long life cycle (perhaps a year) and occurrence in the upper layers of the soil profile make it vulnerable to tillage operations.
Figure 8. Head of a dagger nematode showing the long spear (stylet) it uses to feed on corn roots. (Photo courtesy of University of California–Davis.)
Other Corn–Pathogenic Nematodes
A number of other plant–parasitic nematodes both occur in Illinois and are known or thought to be corn pathogens in other parts of the United States. Because they are of only local interest in Illinois, as far as we know, they will only be listed here. Additional information on their biology and control may be found in Norton (5), Windham (7), and White (8).
Belonolaimus spp., sting nematodes
Criconemella spp., ring nematodes
Dolichodorus heterocephalus, awl nematode
Helicotylenchus spp., spiral nematodes
Hemicycliophora spp., sheath nematodes
Paratrichodorus spp., stubby-root nematodes
Tylenchorhynchus spp., stunt nematodes
Corn–Pathogenic Nematodes We Don’t Have to Worry About (Yet)
Three root-knot nematode species of worldwide importance, Meloidogyne incognita, M. arenaria, and M. javanica, will infect and damage corn; but all of these species are limited in distribution to areas where soil does not typically freeze in the winter. Global warming may affect their distribution. One root-knot nematode species that is well adapted to cooler regions is the northern root-knot nematode, Meloidogyne hapla, which fortunately does not attack corn at all. According to Norton (5), the status of corn as a nonhost probably contributes to control of this nematode on many other crops in the midwestern and north-central states.
The stem nematode, Ditylenchus dipsaci, will infect corn, but corn is such a poor host that nematode populations decline under corn. Corn plants die or fall over before harvest when infected at an early stage. As with H. avenae, resistance is expressed as a necrotic response to invasion, and this can result in extensive damage to internal stem tissues in young plants (Caspary, in 6).
Heterodera avenae, the cereal cyst nematode, can cause losses up to 50% in infested fields in Canada and Europe (6). It is found in the United States, in the Pacific Northwest. Corn is actually resistant to H. avenae, but the resistance-conferring, hypersensitive response itself can result in damage to young plants if infection rates are high.
Heterodera zeae, the corn cyst nematode, is a serious problem in some areas of the world. It was found in the United States in Maryland, where it has been quarantined since 1984 (1). H. zeae has also been found in Virginia. Primarily found in warm temperate to tropical regions, this nematode has not shown signs of adapting well to conditions in cooler climates. Its optimal temperature for reproduction is 30°C (2).
It is time to reassess the impact of corn nematodes on corn production and determine whether our research and development efforts are what they should be, given the current value of corn yield losses and the expected future increases thereof. Corn breeders should be in the vanguard of this reassessment, because resistance or tolerance will be our best weapon to fight losses from nematodes.
- Baldwin, J.G., and M. Mundo-Ocampo. 1991. Heteroderinae, cyst and non-cyst forming nematode. Pp. 275-362 in Manual of Agricultural Nematology. W.R. Nickle, ed. Marcel Dekker, New York.
- Hutzell, P.A., and L.R. Krusberg. 1990. Temperature and the life cycle of Heterodera zeae. J. Nematol. 22:414-417.
- Koenning, S.R., C. Overstreet, J.W. Noling, P.A. Donald, J.O. Becker, and B.A. Fortnum. 1999. Survey of crop losses in response to phytoparasitic nematodes in the United States for 1994. Suppl. to J. Nematol. 31:587-618.
- Mai, W.F., P.G. Mullin, H.H. Lyon, and K. Loeffler. 1996. Plant-Parasitic Nematodes. A Pictorial Key to Genera. Cornell University Press.
- Norton, D.C. 1984. Nematode parasites of corn. Pp. 61-94 in Plant and Insect Nematodes. W.R. Nickle, ed. Marcel Dekker, New York.
- Rivoal, R., and R. Cook. 1993. Nematode pests of cereals. Pp. 259-303 in Plant-Parasitic Nematodes in Temperate Agriculture. K. Evans, D.L. Trudgill, and J.M. Webster, eds. CAB International, Wallingford, UK.
- Windham, G.L. 1998. Corn. Pp. 335-357 in Plant Nematode Interactions. K.R. Barker, G.L. Pedersen, and G.L. Windham, eds. ASA-CSSA Agronomy Monograph 36.
- White, D.G. 1999. Compendium of Corn Diseases. APS Press: St. Paul, MN.