by R. Smiley and C.M. Ocamb
Cause Several species of lesion nematodes. The most important species found on wheat in the Pacific Northwest are Pratylenchus neglectus and P. thornei. These nematodes have broad host ranges, including spring- and fall-planted cereal crops, many grasses, and numerous broadleaf crops and weeds. High nematode populations can occur even in the very driest of wheat- and barley-producing regions and cause serious damage. Pratylenchus penetrans is common in irrigated, sandy soils but is not known to cause severe damage to small grain cereals. Wheat and barley varieties differ in ability to tolerate invasion by these nematodes. Yields of intolerant varieties decline as the number of nematodes in soil increase. Yields of tolerant varieties remain relatively stable even when populations of root-lesion nematodes are high. Although some wheat varieties are tolerant, all current commercial varieties allow rapid multiplication of these nematodes and therefore may increase the level of risk for a subsequent planting of an intolerant wheat variety or other crop. Barley generally exhibits the highest level of tolerance and also suppresses multiplication and the population of lesion nematodes in soil.
Root-lesion nematodes are migratory endoparasites. Pratylenchus neglectus and P. thornei reproduce without mating (parthenogenesis) and males are rare or absent. Reproduction by P. penetrans requires mating and males and females occur in near-equal numbers. Numerous eggs are laid in roots as well as in soil and may hatch within 4 weeks. All life stages can invade roots but can also migrate back out into the soil. Most lesion nematodes are found in roots late in the growing season and in soil and remnants of old roots following plant maturation.
Symptoms Roots are thinner and less branched than normal. Lower leaves may become prematurely chlorotic, tiller numbers and tiller height may be reduced, and plants may appear unthrifty. An appearance similar to premature drought or inadequate supply of nitrogen or sulfur results from nematode damage that reduces uptake of water and nutrients.
Sampling Root-lesion nematodes are distributed throughout the soil profile. Highest populations occur in the top foot of soil but may occur at depths of 2 to 3 feet particularly in winter wheat-summer fallow rotations. Sampling depth should be at least 12 inches and samples should be taken from moist soil in early autumn or spring. Deeper sampling is required during winter because these nematodes migrate deeply into the soil during winter. Root fragments should not be removed from samples that will be evaluated for lesion nematodes.
- Susceptible crops may increase the level of damage if produced before wheat or other intolerant crops. This has been documented for some cultivars of canola, mustard and chickpea, and for most cultivars of wheat.
- Resistant crops suppress multiplication and may improve yields of subsequent plantings of sensitive wheat or other crops. See Table 'Plant resistance to root lesion nematodes, Pratylenchus neglectus and P. thornei, in the Pacific Northwest.'
- Winter wheat varieties with highest levels of tolerance to P. neglectus include Tubbs, Goetze, Brundage 96, Idaho 587, Masami, Bauermeister, Weatherford, Chukar, Westbred 528, MDM, ORCF-102, and Lambert.
- Winter wheat varieties with highest levels of tolerance to P. thornei include Westbred 528, Goetze, Chukar, Bruehl, ORCF-101, Weatherford, Rod, Simon, Finley, Idaho 587, Malcolm, ORCF-102, Moehler, Eltan, Coda, Tubbs, Lambert, and Gene.
- Spring wheat varieties with highest levels of tolerance to P. neglectus include Buck Pronto, Hollis, Tara 2002, and Jerome.
- Spring wheat varieties with highest levels of tolerance to P. thornei include Buck Pronto, Tara 2002, Jerome, Jefferson, Wawawai, Zak, Otis, Macon, and Vida.
- The most tolerant barley varieties include fall-planted Kold and spring-planted Camas and Bob.
- No chemicals are registered for controlling these nematodes on wheat or barley.
- Fumigation is too expensive for use on cereal crops, particularly in dryland environments where damage causes the greatest economic loss. Fumigation is also ineffective for controlling deep-lying populations.
References Smiley, R.W. 2009. Root-lesion nematodes reduce yield of intolerant wheat and barley. Agronomy Journal 101:1322-1335.
Smiley, R.W. 2015. Root-lesion nematodes: Biology and Management in Pacific Northwest Wheat Cropping Systems. PNW Extension Bulletin 617.
Smiley, R.W., Gourlie, J.A., Yan, G., and Rhinhart, K.E.L. 2014. Resistance and tolerance of landrace wheat in fields infested with Pratylenchus neglectus and P. thornei. Plant Disease 98:797-805.
Smiley, R.W., Yan, G., and Gourlie, J.A. 2014. Selected Pacific Northwest crops as hosts of Pratylenchus neglectus and P. thornei. Plant Disease 98:1341-1348.
Smiley, R.W., Yan, G., and Gourlie, J.A. 2014. Selected Pacific Northwest rangeland and weed plants as hosts of Pratylenchus neglectus and P. thornei. Plant Disease 98:1333-1340.