Abstracts of Papers Relating to Cereal Cyst Nematodes

Smiley, R. W., A. A. Dababat, S. Iqbal, M. G. K. Jones, Z. Tanha Maafi, D. L. Peng, S. A. Subbotin, and L. Waeyenberge. 2017. Cereal cyst nematodes: A complex and destructive group of Heterodera species. Plant Disease 101:1692-1720.

This paper is a 'Feature Article' that was requested and funded by the journal. The 29-page paper reviews current global knowledge regarding cereal cyst nematodes, including pathogen taxonomy and diversity, life cycle, disease process, disease symptoms, pathogen detection and identification, impacts on grain yield and quality, disease management, and future research directions with molecular approaches. The paper includes 318 references. The authors have expertise in a wide range of research interests and approaches, and have local knowledge across most regions of the world where these nematodes cause damage to cereal crops. The authors represent current or previous affiliations in Australia, Belgium, China, Iran, Pakistan, Palestine, Russia, Turkey, United Kingdom, and USA. 

Smiley, R. W., and Marshall, J. M. 2016. Detection of dual Heterodera avenae resistance plus tolerance traits in spring wheat. Plant Disease 100:1677-1685.

 The cereal cyst nematode Heterodera avenae reduces wheat yield in the Pacific Northwest. Resistance and tolerance traits among spring wheat cultivars were poorly defined. Screening trials were conducted with 39 cultivars over a 2-yr period in irrigated commercial fields that were infested by H. avenae. Comparisons were made between drill strips treated or untreated with aldicarb at the time of planting. Root sampling at the time of plant anthesis indicated that cultivars differed greatly in susceptibility to H. avenae, with numbers of newly produced white H. avenae females ranging from <5 to 70/plant. Aldicarb reduced mean numbers of white females as much as 99% on the most susceptible cultivar (Glee) and increased mean grain yield as much as 77% for the least tolerant cultivar (Cataldo). Density of H. avenae eggs in untreated soil following harvest was significantly lower than  the density in aldicarb-treated plots. Agronomically acceptable traits of resistance plus tolerance were identified in one cultivar of hard red spring wheat (WB-Rockland) and two cultivars of hard white spring wheat (Klasic and LCS Star) but in none of the soft white spring wheat cultivars. This is the first report of spring wheat cultivars expressing the dual traits of resistance plus tolerance to H. avenae.

Marshall, J. M., and Smiley, R. W. 2016. Resistance and tolerance of spring barley to Heterodera avenae. Plant Disease 100:396-407.

Heterodera avenae is a cereal cyst nematode that reduces wheat yields in the Pacific Northwest USA. Barley is also susceptible but there were no previous reports of resistance or tolerance to H. avenae in the USA. Spring barley cultivars were assayed in H. avenae-infested fields over two years. Cultivars were planted in plots treated or not treated with aldicarb. Forty-five cultivars were evaluated for the market classes of 2- and 6-row feed barleys and 2- and 6-row malt barleys. One 2-row feed barley (Lenetah) was ranked as resistant and four were tolerant or very tolerant. One 2-row malt barley (Odyssey) was very resistant and 10 were tolerant or very tolerant. Two 6-row feed and two 6-row malt barleys were tolerant or very tolerant but none were resistant. Seven feed barleys were ranked as having a balance of at least moderate resistance plus moderate tolerance; Champion, Lenetah, Xena, Idagold II, Transit, Millenium and Goldeneye. This is the first report of resistance and tolerance of barley in H. avenae-infested fields in the Pacific Northwest. Barley productivity can be improved by planting resistant plus tolerant cultivars or by using highly resistant and highly tolerant cultivars as parents in barley improvement programs.

Smiley, R.W., and G.P. Yan. 2015. Discovery of Heterodera filipjevi in Washington and comparative virulence with H. avenae on wheat. Plant Disease 99:376-386.

The cereal cyst nematode Heterodera avenae suppresses wheat production in the western USA. A second species of cereal cyst nematode, H. filipjevi, was identified in eastern Oregon during 2008. This paper reports the discovery of H. filipjevi-infested fields in eastern Washington, thereby extending the known distribution of H. filipjevi in the USA. The identity of H. filipjevi was determined and confirmed by species-specific PCR, PCR-RFLP, sequencing and cyst morphology. Soils that were collected from naturally-infested fields in Washington were used to compare the virulence of H. avenae and H. filipjevi on six spring wheat cultivars under controlled-environment conditions. Non-infested soils from nearby fields were used as controls. Cultivars Ouyen and WB Rockland were resistant to H. avenae and susceptible to H. filipjevi. Cultivars Sönmez and SY Steelhead were resistant to H. filipjevi and susceptible to H. avenae. Cultivars Louise and WB 936 were susceptible to both species. The resistance of SY Steelhead to ‘H. avenae’ reported in a previous paper should be corrected as resistance to H. filipjevi due to an earlier erroneous identification of H. filipjevi. Management guidelines that include crop rotations and resistant cultivars are presented. Discovery of additional infestations of H. filipjevi are anticipated when DNA-based tests become used routinely in commercial diagnostic laboratories.

Yan, G.P., R.W. Smiley, P.A. Okubara, and A.M. Skantar. 2013. Species-specific PCR assays for differentiating Heterodera filipjevi and H. avenae. Plant Disease 97:1611-1619.

Heterodera avenae and H. filipjevi are economically important cyst nematodes that restrict production of cereal crops in the Pacific Northwest USA and elsewhere in the world. Identification of these two species is critical for recommending and implementing effective management practices. Primers were designed from the internal transcribed spacer (ITS) regions of H. avenae and H.  filipjevi ribosomal DNA. The primers were highly specific when tested on target isolates but did not amplify DNA from non-target Heterodera, Globodera, Meloidogyne, Pratylenchus, and other plant-parasitic nematode species. PCR reaction and amplification conditions were established, and H. avenae and H. filipjevi were clearly distinguished by PCR fragments of 242 bp and 170 bp, respectively. Robust PCR amplification was achieved with DNA extracted from a single egg or second-stage juvenile (J2) using a laboratory-made worm lysis buffer, and DNA from 0.5 egg or 0.5 J2 using a commercial kit. The PCR assays were successfully employed for the differentiation of H. filipjevi and H. avenae populations collected from eight locations in three Pacific Northwest states. This is the first report of a species-specific PCR assay to detect and identify H. filipjevi. Together, both assays will enhance the diagnosis of cereal cyst nematode species in infested fields.

Haddadi, F., A. Mokabli, and R.W. Smiley. 2013. Characterisation of virulence reactions for Heterodera avenae populations from two localities in Algeria. Phytoparasitica 41:449-456.

Heterodera avenae is widely distributed in areas where most cereal crops are produced in Algeria. However, the virulence of the Algerian populations of this nematode on individual cereal species and cultivars has not been well documented. The virulence of H. avenae populations from Tiaret and from Oued Smar were tested under natural outdoor conditions and in an in vitro test to determine reactions of nine barley, oat and wheat cultivars selected from the International differential assortment for identifying pathotypes of H. avenae. All nine cereal differentials expressed the same reactions to both populations. The nematodes reproduced well on the barley cultivar Emir and the wheat cultivar Capa. Resistant entries included the barley cultivars Siri, Ortolan and Morocco, the oat cultivars Nidar II and A. sterilis I.376, and the wheat cultivars Loros and AUS10894. This matrix of reactions indicated that H. avenae populations from both locations were characterized as H. avenae Group 1 pathotypes but did not conclusively distinguish among pathotypes Ha21, Ha31 or Ha81. The Cre1 gene was identified as a potentially valuable source of resistance when developing wheat cultivars intended for release into these localities.

Smiley, R. W., Marshall, J. M., Gourlie, J. A., Paulitz, T. C., Kandel, S. L., Pumphrey, M. O., Garland-Campbell, K., Yan, G. P., Anderson, M. D., Flowers, M. D., and Jackson, C. A. 2013. Spring wheat tolerance and resistance to Heterodera avenae in the Pacific Northwest. Plant Disease 97:590-600.

The cereal cyst nematode Heterodera avenae reduces wheat yields in the Pacific Northwest. Previous evaluations of cultivar resistance had been in controlled environments. Cultivar tolerance had not been evaluated. Seven spring wheat trials were conducted in naturally-infested fields in three states over two years. A split-plot design was used for all trials. Five trials evaluated both tolerance and resistance in 1.8 × 9-m plots treated or not treated with nematicides. Two trials evaluated resistance in 1-m head rows where each wheat entry was paired with an adjacent row of a susceptible cultivar. Cultivars with the Cre1 resistance gene (Ouyen and Chara) reduced the post-harvest density of H. avenae under field conditions, confirming Cre1-parents as useful for germplasm development. While Ouyen was resistant it was also intolerant because it produced significantly lower grain yield in controls than in plots treated with nematicides. Susceptible cultivars varied in tolerance. Undefined resistance was identified in one commercial cultivar (WB Rockland) and four breeding lines; UC1711, SO900163, SY-B041418, and SY-97621-05. This research was the first systematic field demonstration of potential benefits to be derived through development and deployment of cultivars with resistance plus tolerance to cereal cyst nematode in North America.

Smiley, R.W., J.A. Gourlie, K.E.L. Rhinhart, J.M. Marshall, M.D. Anderson, and G.P. Yan. 2012. Influence of nematicides and fungicides on spring wheat in fields infested with soilborne pathogens. Plant Disease 96:1537-1547.

A complex of fungal soilborne pathogens and plant-parasitic nematodes reduce wheat yields in the Pacific Northwest. On several other crops in nematode-infested soils, seed treatment with abamectin (Avicta) or Bacillus firmus (Votivo), or foliar application of spirotetramat (Movento) reduced root injury and improved yield. These products along with fungicide seed treatments and aldicarb (Temik) were evaluated in 13 spring wheat trials over three years. During 2011 the mean wheat yield at four locations was 419 kg/ha greater (valued at $122/ha) from seed treated with fungicides and insecticide than from untreated seed, due to protection against soilborne fungal pathogens. Aldicarb increased the mean grain yield over the fungicide plus insecticide treatment by another 798 kg/ha (valued at $254/ha) and also reduced the density of Heterodera avenae but is not registered for use on wheat. Abamectin and B. firmus had negligible effects on grain yield and post-harvest density of Pratylenchus sp. and H. avenae. Spirotetramat reduced density of H. avenae but did not improve grain yield. We conclude that management of fungal pathogens by seed protectants remains essential and that management of nematodes can be achieved through crop rotations and genetic resistance.

Smiley, R.W. 2012. A fluidizing column for extracting cysts of Heterodera avenae from soil. Plant Dis. 96:820-826.

The cereal cyst nematode Heterodera avenae can be extracted from soil using several different floatation or elutriation methods. Automated methods are prohibitively expensive for use in small labs and, for optimal efficiency, floatation methods require that the soil be air-dried for an extended period. A method which suspends soil particles in a water column above a fluidizing plate was reported as being most efficient with wet and dry soils. Use of the fluidizing column for extracting H. avenae has not been reported in the USA and materials to construct the column using contemporary components have not been described. Objectives of this research were to construct a column with components available in the USA, and to compare numbers of cysts and eggs plus juveniles (from cysts) extracted by the column and three other floatation methods; Fenwick can, flask, and Cobb sieving. From a soil containing recently-produced (more dense) cysts, the column extracted at least 18% more cysts and 23% more eggs plus juveniles than the Fenwick and flask methods. The fluidizing column was found to be useful for small laboratories because it is inexpensive ($253 for two columns), easily and quickly constructed by non-professional labor, and produces adequately repeatable results.

Smiley, R. W., Marshall, J. M., and Yan, G. P. 2011. Effect of foliarly-applied spirotetramat on reproduction of Heterodera avenae on wheat roots. Plant Disease 95:983-989.

 Abstract

The cereal cyst nematode Heterodera avenae has the potential to reduce yields of cereal crops in the Pacific Northwest. Spirotetramat (Movento) is a foliar-applied insecticide with ambimobile translocation that reduces fecundity of sucking insects which feed on roots as well as foliage. Spirotetramat (88 g/ha) was applied to foliage during 2010 in two wheat fields infested by H. avenae near St. Anthony, Idaho and Palouse, Washington. In Idaho, two applications at 2-week intervals during late spring to plants already exhibiting swollen white females reduced the post-harvest density of H. avenae eggs plus juveniles by 35% (P = 0.03) compared to the nontreated control. In Washington, a single application before white females became apparent reduced the nematode density by 78% (P = 0.01). Grain yields and test weights were not significantly affected by application of spirotetramat at either location. In addition, symptomatic plants from the Idaho field were transplanted into greenhouse pots and treated with spirotetramat. One application (110 g/ha) reduced numbers of eggs plus juveniles/plant by 78% (P = 0.02). Spirotetramat effectively reduced H. avenae populations and warrants further evaluation as a substitute for crop rotations or long fallow periods that reduce nematode population densities in infested fields. 

Smiley, R.W., G.P. Yan, and J.N. Pinkerton. 2011. Resistance of wheat, barley and oat to Heterodera avenae in the Pacific Northwest USA. Nematology 13:539-552.

Summary    The cereal cyst nematode Heterodera avenae Woll. occurs in at least seven western states of the USA and reduces grain yield in localised regions and in selected crop management systems. Virulence phenotypes for H. avenae populations in North America had not been reported. Nine individual assays in six experiments were conducted to determine the reactions of barley, oat and wheat cultivars to five H. avenae populations in the Pacific Northwest (PNW) states of Idaho, Oregon and Washington. Three populations were evaluated for virulence to 23 entries of the ‘International Test Assortment for Defining Cereal Cyst Nematode Pathotypes’, plus selected local cultivars and entries representing a greater diversity of resistance genes. The virulence phenotype(s) for populations of H. avenae in the PNW did not correspond to any of the 11 pathotypes defined by the Test Assortment. Five PNW populations exhibited affinities with Group 2 but were not defined by pathotypes Ha12 and Ha22. Reproduction was prevented or greatly inhibited by barley carrying the Rha3 resistance gene and by most carriers of Rha2 resistance, and by selected oat cultivars with multigenic resistance. Wheat cultivars carrying the Cre1 resistance gene were highly effective in suppressing H. avenae reproduction. Current PNW wheat cultivars do not carry the Cre1 resistance gene. Crosses between Ouyen, an Australian bread wheat with Cre1 resistance, and several PNW wheat cultivars were resistant. The CreR gene also prevented H. avenae reproduction in the trial where it was tested. Intermediate levels of reproduction occurred on wheat cultivars carrying the Cre5, Cre7, and Cre8 resistance genes, each of which was considered useful for pyramiding into cultivars with Cre1 resistance. This research identified genetic resources of value in PNW cereal crop breeding programmes.

Yan, G.P. and R.W. Smiley. 2010. Distinguishing Heterodera filipjevi and H. avenae using polymerase chain reaction-restriction fragment length polymorphism and cyst morphologyPhytopathology 100:216-224. 

ABSTRACT 
The cereal cyst nematodes Heterodera filipjevi and H. avenae impede wheat production in the Pacific Northwest (PNW). Accurate identification of cyst nematode species and awareness of high population density in affected fields is essential for designing effective control measures. Morphological methods for differentiating these species are laborious. These species were differentiated using polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) of internal transcribed spacer (ITS)-rDNA with up to six restriction endonucleases  (TaqI, HinfI, PstI, HaeIII, RsaI and AluI). The method was validated by inspecting underbridge structures of cyst vulval cones. Grid soil sampling of an Oregon field infested by both species revealed that H. filipjevi was present at most of the infested grid sites but mixtures of H. avenae and H. filipjevi also occurred. These procedures also detected and differentiated H. filipjevi and H. avenae in soil samples from nearby fields in Oregon and H. avenae in samples from Idaho and Washington. Intraspecific polymorphism was not observed within H. filipjevi or PNW H. avenae populations based on the ITS-rDNA. However, intraspecific variation was observed between H. avenae populations occurring in the PNW and France. Methods described here will improve detection and identification efficiencies for cereal cyst nematodes in wheat fields.

Smiley, R.W. 2009. Occurrence, distribution and control of Heterodera avenae and H. filipjevi in the western USA. p. 35-40 in Cereal Cyst Nematodes: Status, Research and Outlook. I.T. Riley, J.M. Nicol, and A.A. Dababat (eds). CIMMYT, Ankara, Turkey.

SUMMARY 
Heterodera avenae occurs in at least seven states in the western USA. H. filipjevi was first reported in North America during 2008 and is currently reported only in Oregon, where it occurs as a mixture with H. avenae. Juveniles of both species emerge from cysts primarily during the spring. It is estimated that H. avenae reduces wheat yield by 21,000 tons, valued at US$3.4 million, in the Pacific Northwest states of Idaho, Oregon and Washington. Neither H. avenae nor H. filipjevi pathotypes in the Pacific Northwest are adequately characterized by indexing plants of the current International Test Assortment. However, reproduction of H. avenae is consistently absent in wheat containing the Cre1 gene and in barley containing the Rha2 gene. A donor of the Cre1 gene was crossed with locally-adapted wheat cultivars and the crosses are being screened for resistance. Rotation of winter wheat with weed-free broadleaf crops or long fallow (14 months) reduces damage to subsequent wheat but rotations often are not profitable in the driest areas of the region. Nematicides are not registered for managing damage by cereal cyst nematodes in North America. Fungal parasites of cysts and/or eggs have been detected but have not been investigated and do not appear to provide effective control in Oregon.

Yan, G.P., and R.W. Smiley. 2009. Discovery of Heterodera filipjevi on wheat in the USA. p. 94-99 in Cereal Cyst Nematodes: Status, Research and Outlook. I.T. Riley, J.M. Nicol, and A.A. Dababat (eds). CIMMYT, Ankara, Turkey.

SUMMARY
Until recently, cyst nematodes from wheat and barley fields in the Pacific Northwest (PNW) were identified as homogenous populations of Heterodera avenae. During February 2008, while using PCR-RFLP to examine H. avenae collected in the PNW, cysts from a winter wheat field exhibiting patches with up to 90% plant mortality near Imbler (Union County, OR) consistently revealed a restriction pattern matching that of a H. filipjevi DNA standard rather than H. avenae with six endonucleases TaqI, HinfI, PstI, HaeIII, RsaI and AluI. The pattern was different than those of H. latipons and H. schachtii with TaqI, HaeIII and RsaI. Comparisons of cyst vulval cones revealed a characteristic morphological difference between the Imbler cysts and specimens of H. avenae. A distinct underbridge with bifurcated arms was present in vulval cones of the Imbler cysts whereas no underbridge was found in H. avenae. Other morphological and morphometrical characters in cysts and second-stage juveniles were consistent with the description of H. filipjevi. Our molecular tests led to the first discovery and documentation of H. filipjevi in North America. Grid soil sampling revealed that H. filipjevi was present at most of infested grid sites but mixtures of H. avenae and H. filipjevi also occurred in the field where H. filipjevi was discovered. Intraspecific polymorphism was not observed within H. filipjevi populations based on ITS-rDNA. The pathotype and effective resistance genes for introgression into wheat are being identified. 

Smiley, R.W., G.P. Yan, and Z.A. Handoo. 2008. First record of the cyst nematode Heterodera filipjevi on wheat in Oregon. Plant Disease 92:1136.

FULL REPORT
Plant and soil samples from an irrigated winter wheat (Triticum aestivum) field near Imbler (Union County), OR, were evaluated for root diseases during April 2007. The field exhibited patches with up to 90% plant mortality. Previous crops were winter wheat (2004), chickpea (Cicer arietinum, 2005) and spring wheat (cv. ‘Jefferson’, 2006). Stubble was baled and removed and the field was cultivated before replanting to winter wheat cv. ‘Chukar’ in October. Patches of stunted seedlings (3-5 leaf stage) appeared in March 2007. Stunted seedlings exhibited chlorotic or necrotic lower leaves, healthy younger leaves, few or no tillers, rotting of lower culms and crowns, and light brown roots with little or no branching. Signs and symptoms of fungal pathogens (Pythium spp., Gaeumannomyces graminis var. tritici, Rhizoctonia solani AG-8 and Typhula incarnata) were present on affected plants. Most small grain fields in Union County are infested with Heterodera avenae (4) but none of the roots, on either healthy or stunted plants, exhibited the bushy branching pattern typical of sites where H. avenae females penetrate and encyst. Extraction of motile nematodes (Whitehead tray method) from soil revealed high populations of Pratylenchus neglectus (6,560/kg of soil), Tylenchorhynchus spp. (2,369/kg) and a species initially thought to be H. avenae (3,098 juveniles/kg). Cysts were also extracted. During PCR-RFLP identification (1) of H. avenae collected in Oregon, Washington and Idaho, four restriction enzymes applied to amplified DNA of cysts from the Imbler field consistently revealed a pattern identical to that of a H. filipjevi DNA standard and distinct from patterns of H. avenae, H. schachtii, and H. latipons. DNA standards were obtained from Roger Rivoal, INRA, Rennes, France. Morphological evidence confirmed that the specimens were H. filipjevi , a member of the ‘H. avenae Group’ of cereal cyst nematodes (2,3). Measurements of second-stage juveniles (n = 15) included length of body (range = 530-570 μm, mean = 549, st. dev. = 13.0), stylet (22.5-24.5, 23.2, 0.6) with anchor-shaped basal knobs, tail (52.5-62.5, 57.4, 2.7), and hyaline tail terminal (30-38, 33.5, 2.6). The lateral field had four lines of which the inner two were more distinct. Shapes of the tail, tail terminus, and stylet knobs were also consistent with H. filipjevi. Cysts (n = 10) were lemon-shaped, light brown in color. The cyst wall had a zigzag pattern. The vulval cone was bifenestrate with horseshoe-shaped semifenestra. The cysts were characterized by body length including neck (range = 718-940 μm, mean = 809.7, st. dev. = 61.8), body width (395-619, 504, 71.2), L/w ratio = (1.1-2.2, 1.4, 0.3), neck length (75-140, 103.2, 22.1) and width (50-95, 71.4, 10.9), fenestra length (50-65 μm, 56.5, 6.6) and width (27-40, 29.0, 3.8), heavy underbridge (60-80, 69, 8.5), vulval slit (7.5-8.5, 7.8, 0.4), and many bullae. Cysts hatched much more readily and at lower temperatures than populations of H. avenae, as has been described for H. filipjevi. Detection of H. filipjevi in Oregon represents a new record for the occurrence of this species in the U.S.A. and for North America. The pathotype and resistance genes for incorporation into wheat, barley and oat are being identified.
     References: (1) Bekal, S. et al., Genome 40:479. 1997. (2) Handoo, Z.A., J. Nematol. 34:250, 2002. (3) Holgado, R. et al., J. Nemat. Morph. Syst. 7:77-84, 2004. (4) Smiley, R.W. et al., J. Nematol. 37:297, 2005.

Smiley, R.W., R.G. Whittaker, J.A. Gourlie, S.A. Easley, and R.E. Ingham. 2005. Plant-parasitic nematodes associated with reduced wheat yield in Oregon: Heterodera avenae. Journal of Nematology 37:297-307.

ABSTRACT
Heterodera avenae is widely distributed in the western USA. Wheat in Oregon and Washington is grown mostly in non-irrigated winter wheat/summer fallow rotations in low rainfall regions. Economic and social pressures have motivated growers to pursue a transition from winter wheat/summer fallow rotation to no-till annual spring cereals. Some irrigated fields are also planted annually to cereals. The impact of H. avenae on spring wheat yield in Oregon had been observed but not quantified. Spring wheat was planted with or without aldicarb to examine relationships between H. avenae and yield under dryland and irrigated conditions in moderately infested fields in eastern Oregon. Spring wheat yields were negatively correlated (P < 0.05) with initial densities of H. avenae. Aldicarb application improved spring wheat yield by as much as 24%. The yield of irrigated annual winter wheat was also negatively correlated with initial density of H. avenae. Infective juvenile stages of H. avenae reached a peak density during mid-spring. Additional research is necessary to develop management strategies for cereal cyst nematode in the western United States.

Smiley, R.W., R.E. Ingham, W. Uddin, and G.H. Cook. 1994. Crop sequences for winter wheat in soil infested with cereal cyst nematode and fungal pathogens. Plant Disease 78:1142-1149.

ABSTRACT
In the Pacific Northwest, Heterodera avenae is spreading to soils infested with root-infecting fungal pathogens of wheat. A poorly drained, silty clay loam infested with multiple pathogens was used to examine productivity of winter wheat in 11 crop sequences. Breaks between wheat crops included summer fallow or crops of pea, barley, rape, alfalfa, or Kentucky bluegrass. In the fifth year, winter wheat was planted in all sequences, after half of each plot was treated with aldicarb. Yield of annual winter wheat was always 40-60% less than wheat alternated with fallow or any other crop, except alfalfa contaminated with grass weeds. Wheat yielded equally following 1- or 2-yr breaks from wheat. Effective breaks included summer fallow, pea, and weed-free alfalfa. H. avenae was the most important individual constraint to yield. Combined damage from H. avenae and Gaeumannomyces graminis var. tritici caused highest overall yield loss, whereas H. avenae and Pythium spp. had the greatest negative effect on number of roots. During the fifth year, where aldicarb was applied, root damage by H. avenae decreased but damage by Rhizoctonia solani and Pythium spp. increased, resulting in no yield improvement.
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