Pathogenic Nematodes of Wheat and Barley

Root lesion and cereal cyst nematodes are microscopic roundworms that parasitize agricultural crops in every part of the world. Take the steps outlined here to recognize infestation, assess potential damage and protect crops.

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ROOT LESION NEMATODES ARE MICROSCOPIC

roundworms that parasitize agricultural crops in every part of the world. Two species of root lesion nematode, Pratylenchus thornei and Pratylenchus neglectus, are damaging to wheat. Our neighboring states of Washington, Oregon and Idaho experience annual yield losses in spring wheat due to infestations of both nematode species. Studies in Oregon and Washington have attributed up to 36 percent yield reduction in intolerant cultivars due to P. neglectus. A survey of small grain fields in Montana for root lesion nematode was conducted in 2006 and 2007. The assessment showed damaging populations of P. neglectus predominantly occurring in north central counties of Montana in fields of winter wheat. No P. thornei has been detected in the state. Trials conducted in summer 2007 reveal that McNeal and Outlook spring wheat display good tolerance (by maintaining yield) in the presence of damaging P. neglectus populations.

Root Lesion Nematode Biology

Root lesion nematodes are migratory endoparasites. They are mobile in water films among soil particles but move into host root tissue to feed and reproduce. The parasitic nematode penetrates and moves into plant root cells using its sharp, hollow stylet (Figure 1). Cell-wall degrading enzymes secreted from the nematode’s stylet facilitate entry and feeding. As these nematodes feed and mature, they become reproductive and are able to lay an egg in root tissue every three days. There may be up to five generations of root lesion nematodes within one growing season. As eggs hatch, young nematodes can move back out into soil in search of new host plants. Since the nematodes usually live protected inside host roots, they are not dependent on soil moisture to survive. They can also enter a resting stage when field conditions are dry and hosts are not available. Subsequently, they revive under favorable conditions. This allows them to survive through fallow periods. Root lesion nematodes are dispersed from field to field by anything that moves soil (e.g., on farm equipment, shoes, wildlife, domestic animals, and by wind).

Damage and Yield Loss

Damage from root lesion nematodes can be easily mistaken for nutrient deficiencies, root rot diseases and drought. Nematode-affected plants display stunting, yellowing of older leaves, reduced tillering and loss in kernel weight. Damage is affected by a variety’s tolerance and/or resistance and environmental factors including soil moisture and nutrient availability. In a low-precipitation environment, winter wheat losses can be up to 37 percent at a level of 10,000 P. neglectus/kg soil. Spring wheat losses are less in high precipitation environments, but are still 14 percent at a level of 4,000 P. neglectus/kg soil.

FIGURE 1. A root lesion nematode and a close look at the darkened stylet, diagnostic to this group of nematodes. This nematode is 0.5mm long, or half the thickness of a dime.

TABLE 1. Increasing population densities of P. neglectus decreased yields of spring and winter wheat varieties in Montana.

TABLE 2. Montana spring wheat varieties tested for yields in the presence of high populations of P. neglectus (3879 nematodes/ kg of soil).

Tolerence Index is used as a predictive measure of performance in areas of high infestation (higher number = less yield loss).

FIGURE 2. A map of wheat growing districts in Montana. Nematodes were detected in light grey counties. Dark grey counties contained average P. neglectus populations exceeding 2500 P. neglectus/kg soil. Counties listed as ND are counties where root lesion nematode was not detected.

FIGURE 3. Spring populations of root lesion nematodes for fields previously cropped to spring and winter wheat in 2006 and 2007.

Survey and Distribution for Root Lesion Nematode in Montana

In the spring of 2006 and 2007, populations of root lesion nematodes were surveyed with the assistance of Extension agents in 17 counties, representing 82 percent of the total wheat acreage in Montana (Figure 2). Agents sampled 148 fields in 2006 and 116 fields in 2007. In 2006, P. neglectus was found in 12 of 17 counties, and in 41 percent of all fields surveyed. In 2007, P. neglectus was found in 11 of 15 counties, and in 37 percent of all fields surveyed. Losses are proportional to the population present in the spring. In MSU trials, 2000 P. neglectus/kg soil was equivalent to 7.3 percent yield loss ( Table 1). Significantly higher populations of root lesion nematodes were found in fields following a crop of winter wheat than in fields following crops of spring wheat (Figure 3). No trend was observed in fields that were annually cropped versus wheat-fallowed.

Variety Tolerance and Resistance to Root Lesion Nematodes

Nematode tolerance refers to the amount of injury or yield loss caused by the nematode that a plant can withstand or recover from. Trials conducted in the summer of 2007 reveal that the spring wheat varieties McNeal and Outlook displayed nematode tolerance by maintaining their yields in the presence of high P. neglectus populations (Table 2). A tolerance index value (higher number = better yield) is used as a predictive measure of performance in areas of high infestation. In contrast, nematode resistance refers to the ability of a plant to prevent reproduction of the nematode. Currently, there are no known Montana-adapted varieties that display resistance to root lesion nematode. Where resistance is lacking, tolerant varieties are a desirable management method. In Montana, spring wheat rotations appear to reduce nematode populations relative to winter wheat. However, since root lesion nematodes are parasitic to most small grains, rotation to non-host crops is a recommended control strategy. Non-host crops including safflower, flax, triticale and field pea do not allow reproduction of P. neglectus and therefore help manage nematode populations.

Rotation studies conducted by the MSU cereal pathology group have shown substantial reductions in P. neglectus populations following crops of field pea, canola, barley, camelina and lentil (Figure 4, page 4). For example, nematode counts that were initially recorded post-emergence in spring 2008 were reduced by nearly half in field pea plots by spring 2009. Lentil proved to be the most effective rotation option in these trials, reducing P. neglectus populations by 96 percent in the following year. Intermediate effects were observed for fallow, canola, barley and camelina. Conversely, root lesion nematode populations averaged close to a 900 percent increase in winter wheat plots over the course of a single rotation.

Currently, the MSU wheat breeding programs, in collaboration with the cereal disease laboratory, are evaluating germplasm for resistance and tolerance to root lesion nematodes. Since high numbers of nematodes were found following winter wheat, studies are focused on the development of resistant winter wheat varieties. At this time, there are no commercially available lines with complete resistance to root lesion nematodes. However, resistant and tolerant cultivars will ultimately be the most economically important management practice for Pratylenchus due to a lack of chemical control and limited rotational schemes.

Barley is already a well-established, profitable rotation on Montana grain farms, and cultivars that exhibit strong resistance to P. neglectus would therefore be of great benefit to growers. Results obtained from greenhouse evaluations show significant variability among barley cultivars for resistance to root lesion nematodes (p < 0.05). In particular, 2-row feed variety Baronesse and 2-row malting variety Harrington both exhibit significantly lower levels of nematode reproduction in their root systems than the moderately resistant Australian wheat variety Excalibur (Table 3, page 5). Both Harrington and Baronesse could be sources of resistance to P. neglectus and/or valuable in crosses. Xena (WestBred), a 2-row spring feed barley, is derived from a cross with Baronesse, and exhibited a level of resistance statistically indistinguishable from those of Harrington and Baronesse. Hayes and Lavina are popular 2-row hooded Montana spring barley cultivars which share a common parent in Baronesse. Although their resistance reactions to root lesion nematodes have not been investigated, they may also be effective in the reduction of this pathogen.

Resistant lines of hard red winter wheat have been identified in MSU breeding studies. Field trials conducted in the absence of root lesion nematodes have indicated that resistance to P. neglectus is not associated with negative effects on grain yield, test weight or grain protein. Work is currently underway to identify genomic regions associated with resistance to P. neglectus for use in marker-assisted breeding. These efforts will accelerate the development of root lesion nematode resistant wheat cultivars suitable for Montana production environments.

CEREAL CYST NEMATODES ARE A POTENTIAL THREAT

to dryland wheat production in Montana, especially where wheat monocultures are the prevailing cropping system. Two species, Heterodera avenae and Heterodera filipjevi, have been reported parasitizing cereal crops in the western United States. H. avenae has been estimated to reduce annual revenues from wheat by a combined $4.3 million in the Pacific Northwest states of Washington, Oregon and Idaho. Effects of parasitism by H. avenae can reduce yields of winter wheat and barley by as much as 50 percent, and total crop failures for stands of spring wheat have been reported in several highly-infested commercial fields in Oregon. Cereal cyst nematodes have only recently been discovered in Montana, with the first reports of H. avenae occurring in 2006. Specimens of H. filipjevi were confirmed on the roots of heavily stunted winter wheat plants in Chouteau County in 2014. The distribution of H. avenae and H. filipjevi in Montana remains unknown, as statewide surveys of the pests have not yet been undertaken. It is probable that Heterodera species are less widespread in Montana than the root lesion nematode, P. neglectus. Mixtures of Heterodera and Pratylenchus in individual fields have been reported in the Pacific Northwest, although no such instances have yet been reported in Montana.

Cereal Cyst Nematode Biology

Cereal cyst nematodes are sedentary endoparasites. Juvenile stage cereal cyst nematodes are fully mobile, and once emerged from a resting stage, females invade and embed themselves in root tissues of young host plants, forming specialized feeding sites called syncytia. Males fertilize the females, and each female body typically retains several hundred eggs. Unlike root lesion nematodes, cereal cyst nematodes undergo one reproductive cycle per growing season. Upon the death of the host roots, the female body dies, dislodges, and forms a hardened dark-brown cyst which serves as a protective structure for eggs and juveniles during periods between hosts (Figure 5A and 5B, page 6). Eggs within cyst structures can remain viable for several years. Juveniles require a period of dormancy prior to their emergence from the cyst structure. The dormancy period varies, depending on the species of cereal cyst nematode and the environmental conditions. There are reports that eggs from H. filipjevi hatch in fall during winter wheat planting, and therefore may more greatly affect winter wheat than H. avenae, whose eggs hatch only in the spring after a cold period. Cysts may be transported from infested to non-infested fields by way of soil clinging to vehicles, farm equipment, clothing, shoes, or animals, as well as contaminated transplanted materials such as root or tuber crops.

FIGURE 4. Average P. neglectus population changes (as % of initial) between spring 2008 and spring 2009 for seven rotation options.

Damage and Yield Loss

Parasitism of host plant roots by cereal cyst nematodes results in shallower root systems with reduced ability to absorb water and nutrients from the soil (Figure 6, page 7). Plants with heavily damaged roots will appear as patches of stunted yellowed seedlings in the field. Furthermore, root invasion by cereal cyst nematodes can create the potential for secondary infection by root rotting fungi, saprophytic bacteria and fungi, and other nematodes. Studies in the Pacific Northwest have demonstrated that considerably higher populations of H. avenae are required to affect winter wheat crops, as the timing and hatching of eggs and juveniles coincides with the growth of spring cereal crops. The full extent of the economic impact of cereal cyst nematodes on Montana wheat and barley production is currently unknown.

Management Options for Cereal Cyst Nematode

As with root lesion nematodes, crop rotations and the deployment of resistant and tolerant wheat and barley varieties represent the most economical and sustainable control options for cereal cyst nematodes. Rotations including broadleaf crops and fallow can significantly reduce nematode populations in field soils. Resistance to cereal cyst nematode is species specific, and therefore it is important to identify any cysts to the species level prior to the deployment of a resistant cultivar. Resistance to H. avenae is available in both wheat and barley, and in wheat for H. filipjevi, but neither resistance has yet been introduced into adapted Montana wheat and barley cultivars. No effective chemical nematicides are currently registered for use on small grains.

Assessing Root Lesion and Cereal Cyst Nematodes in Fields

Unthrifty plants appearing in patches across a field may indicate a nematode infestation. Early damage signs to look for are yellow lower leaves and loss of secondary branching in the root system. Since populations of nematodes occur in ‘hotspots’, extensive soil sampling is required to accurately evaluate nematode populations. At early grain fill, the presence of small white cysts (around the size of a pin head) on roots of affected plants is diagnostic for cereal cyst nematodes. Scouting for cereal cyst nematodes must be performed just prior to crop maturity, as cysts dislodge from the roots at this time, and are too small to be visible earlier in the season. To determine whether cereal cyst nematodes are causing ‘hotspots’ in a field, carefully remove an affected plant with its entire root system intact, using a garden spade. Gently soak roots in a bucket of water to remove adhering soil, and carefully scan for the presence of pin head-sized white cysts (Figure 7, page 7).

TABLE 3. Montana barley varieties tested for resistance to P. neglectus. Mean reproductive factors followed by the same letter are not significantly different from each other (p > 0.05). Data were log-transformed prior to statistical analysis.

* Barley cultivar planted in 2008-2009 rotation study.

A.                                                                                              B.

FIGURE 5. A) Dislodged cyst of species Heterodera filipjevi found on wheat roots in Chouteau County in 2014. B) Another H. filipjevi cyst from the same location, which has burst, revealing a multitude of eggs within. Photos by R. Al-Khafaji.

For migratory parasites such as root lesion nematodes, it is important to include the periphery of ‘hotspot’ zones in field samples in order to accurately gauge pathogen populations in the field, as these types of nematodes are capable of leaving dead and dying tissues in search of healthy root materials for feeding. Keep in mind that mixtures of root lesion and cereal cyst nematodes can occur within the same field, and that nematodes must be identified to the species level to determine the appropriate control measure. Soil testing and species confirmation for cereal cyst and root lesion nematodes should be performed by a laboratory. Soil testing and sampling instructions for both root lesion and cereal cyst nematodes can be found at the Western Laboratories website, www.westernlaboratories.com.

Acknowledgments

The 2006 and 2007 nematode survey was completed with the help of County Extension Agents including:

- Ben Larson, Richland County

- Bobbie Roos, Daniels County

- Bruce Smith, Dawson County

- Damon Bunting, Glacier County

- Daniel Picard, Pondera County

- Darren Crawford, Fergus County

- Gina Snyder, Roosevelt County

- Joe Broesder, Hill County

- Marko Manoukian, Phillips County

- Jeannie Olmstead, Toole County

- Judee Wargo, Chouteau County

- Ken Nelson, McCone County

- Paul Dixon, Yellowstone County

- Terry Angvick, Sheridan County

- Tom Allen, Liberty County

- Verlin Koenig, Valley County

- Wade Crouch, Cascade County

We also thank Mike Huber and Wade Crouch for their work in the 2007 tolerance trials at Ulm, Montana. Special thanks to Dr. Richard Smiley of Oregon State University and Dr. Juliet Marshall of the University of Idaho, for information regarding cereal cyst nematodes. This work was supported by USDA Hatch project MONB00246 and The Montana Wheat and Barley Committee.

FIGURE 6. Root deformation caused by Heterodera infestation, found on affected spring wheat plants in St. Anthony, Idaho. Symptoms range from moderate (far left) to severe (far right). Swelling and lateral root proliferation occurs at sites where female nematodes are developing. Morphological responses such as these are unique to wheat infected by Heterodera and do not manifest in other cereal species such as oat and barley. Photo by R. Smiley.

FIGURE 7. Small white cysts (around the size of a pin head) are visible on the roots of a stunted Yellowstone wheat plant in Chouteau County in 2014, diagnostic for cereal cyst nematode infestation. Two cysts are circled to aid in identification. Photo by R. Al-Khafaji.