Journal of Nematology 28(1): 107-114. 1996. {3 The Society of Nematologists 1996. Factors Affecting Population Trends of Plant-Parasitic on Rangeland Grasses 1 G. D. GRIFFIN, K. H. ASAY, AND W. H. HORTON 2 Abstract: The effects of environmental conditions on population trends of plant-parasitic nema- todes were studied in experimental plots of five wheatgrasses in the western Utah desert. In a 3-year (1984-86) field study, soil water and temperature affected the population trends of the ectoparasites, Tylenchorhynchus acutoides and Xiphinema americanum, and the migratory endoparasite, neglectus, on Fairway crested wheatgrass, Agropyron cristatum; 'Hycrest' crested wheatgrass, A. cristatum X A. desertorura; 'Rosana' western wheatgrass, Pascopyrum smithii; 'Oahe' intermediate wheatgrass, Thinopyrum intermedium; and RS-1 hybrid (Elytrig~a repens X Pseudoroegneria spicata). The largest soil populations of these species were collected in 1984 under good plant-growth conditions. A reduction in nematode populations occurred in 1985 and 1986, possibly because of low soil-water conditions. There was a positive relationship between high soil water and maximum population densities of T. acutoides in the spring and fall of 1984, and between low soil water and minimum population densities of the nematode in 1985 and 1986. Pratylenchus neglectus populations were affected by soil water, although to a lesser degree than the ectoparasitic nematodes. Population densities of the three nematode species were significantly lower in the drier years of 1985 and 1986 than in 1984. Nematode populations were greater at the lower soil depths in the fall than in the spring or summer. Key words: Agropyron cristatura, ecology, Elytrigia repens X Pseudoroegneri spicata, nematode, Pascopy- rum smithii, population dynamics, Pratylenchus neglectus, Tylenchorhynchus acutoides, RS-i hybrid, soil temperature, soil water, Thinopyrum intermedium, wheatgrasses, Xiphinema americanum.

There are approximately 153 million ha tors. Genetic differences can be found, for of rangelands in the Intermountain Re- example, in the feeding habits of the dif- gion of the western United States. It is es- ferent nematode genera. Meloidogyne spp., timated that more than 86% of the range- Heterodera spp., and Pratylenchus spp. pro- lands are in poor condition, and produc- duce several generations annually (8,9,13, tivity is less than 60% of their natural 29), whereas X. americanum produces one potential (24). generation per year (12,15,18,19). Plant-parasitic nematodes are associated Wheatgrasses are important forage with rangeland grasses (3,6,31,32). Several range grasses in western North America, genera, including Pratylenchus Filipjev and particularly during the spring and early Xiphinema Cobb, reduce the establishment, summer (2). Minimal data have been ob- growth, and productivity of grasses (7,10, tained regarding environmental effects on 14,28). Nematodes extract significant seasonal population densities of individual amounts of energy from plants (25,26) and species of plant-parasitic nematodes on reduce plant resistance to drought, plant rangeland vegetation (14,20,21,27). stress, and diseases by interfering with This study was initiated to determine cli- plant metabolic activity (7). matic effects on population trends of the Plant-nematode relationships are influ- ectoparasitic nematodes, Tylenchorhynchus enced by genetic and environmental fac- acutoides Thorne and Malek and Xiphinema ame,~canum Cobb, and the migratory en- doparasite, Pratylenchus neglectus (Rensch) Filipjev & Schuurmans Stekhoven on five Received for publication I August 1995. i Cooperative investigation by USDA Agricuhural Re- wheatgrasses, and how nematode popula- search Service and the Utah Agricultural Experiment Sta- tion densities are affected by soil water and tion. Journal Paper No. 4800. Mention of a trademark, pro- prietary product, or vendor does not constitute a guarantee temperature. or warranty of the product by the U.S. Department of Agri- culture or Utah State University. MATERIALS AND METHODS 2 Nematologist, Plant Geneticist, and Range Scientist, USDA ARS, Forage and Range Research, Utah State Univer- Perennial grasses used in the study were sity, Logan, UT 84322-6300. E-mail: [email protected] 'Hycrest' crested wheatgrass, Agropyron 107 108 Journal of Nematology, Volume 28, No. 1, March 1996 cristatum (L.) Gaertner X A. desertorum the laboratory in portable coolers and then Fisch ex. Link; 'Fairway' crested wheat- refrigerated at 5 C until processed within grass, A. cristatum; 'Oahe' intermediate 10 days. Samples were thoroughly mixed wheatgrass, Thinopyrum intermedium (Host) and screened, and root tissue was re- Barkworth & D. R. Dewey; 'Rosana' west- moved. Four 250-cm 3 aliquants from each ern wheatgrass, Pascopyrum smithii (Rydb.) sample were processed by elutriation (4) L6ve; and the 'RS-I' hybrid quackgrass, and the nematodes concentrated using Elytrigia repens (L.) Nevski X bluebunch centrifugal flotation (16). Nematodes from wheatgrass, Pseudoroegneria spicata (Pursh) the four aliquants were combined and the L6ve. number of nematodes per liter of soil de- The study site was located in the lower termined by microscopic examination. foothills of the western desert in north- Because the lesion nematode, P. neglec- western Utah at an elevation of 1,390 m. tus, is a migratory endoparasite, counts Average annual precipitation is 30-35 cm, were made in both roots and soil. Root tis- of which approximately 70% falls between sues collected from the soil samples were October and May. The soil was a Thiokol misted with deionized water in 500-ml fine silty loam (mixed mesic Xerollic Cal- glass containers at room temperature (24 ciorthids [19% sand, 65% silt, 16% clay; + 4 C); liquid was decanted daily, nema- pH 7.9, OM 3.2%]). todes were counted over a 10-day period, Each cultivar was planted in a plot con- and this number was added to the soil pop- sisting of 10 drilled rows 30 cm apart and ulation. 40 m long. A firm, plant-free seedbed was The study was conducted over a 3-year prepared with a cultipacker, and seeds (1984-86) period. Data were subjected to were planted 1.5-2.0 cm deep and 1.0 cm standard analyses of variance. Means were apart within a row with a modified John separated on the basis of Duncan's Multi- Deere Flexiplanter minimum-till drill ple-Range Test, and nematode population equipped with double disc openers and trends were determined. depth-band regulators. A 70-kg weight was placed on each timing wheel to im- RESULTS prove consistency of seedling depth. Plots were seeded on 26 April 1982. Approxi- Rainfall differed from year to year, but mately 6 cm of rain was received within a there was little yearly deviation in soil tem- week after planting, which contributed to perature (Fig. 1). In 1984, the site received excellent seedling establishment. Livestock 48 cm of precipitation, of which 23 cm was were not allowed on the site. Soil water and received between May and October. An- soil temperature was monitored with a nual precipitation in 1985 was 22 cm, with Field Unit Recording System (EL824-MS little or no precipitation during the sum- WITH EPRON pack, thermistor probes mer months; 6.3 cm was recorded during with soil moisture blocks - Omnidata In- November and December as rain and ternational, Logan, UT) at a soil depth of snow. Annual precipitation during 1986 20 cm. was 26 cm, with 9.6 cm of the rain re- Beginning in 1984, plots were divided corded from September to December. into five equal subplots, and soil and root Spring and summer soil temperatures samples were collected monthly from were similar during 1984-86. April through October. Fifteen composite Pratylenchus neglectus and T. acutoides soil samples were collected from each sub- were collected in the greatest numbers plot with a 9-cm bucket auger at 25-cm in- from all grass selections in 1984 (Table 1). tervals from the center of each row at There was a reduction in P. neglectus and depths of 0-10, 11-20, and 21-30 cm. Au- T. acutoides population densities (P < 0.05) ger samples from each cultivar and soil and, to a lesser extent, in the X. americanum depth were composited and transported to population density between 1984-86. Den- Population Trends of Nematodes on Grasses: Griffin, Asay 109

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APR MAY JUN JUL AUG SEP OCT NOV DEC FIe. 2. Soil water (mega pascals 'MPa') and soil temperature (C) monitored with a Field Unit Recording System (EL824-MS with EPRON pack, thermistor probes with soil moisture blocks - Omnidata International, Logan, UT) at a soil depth of 20 cm. sities of P. neglectus and T. acutoides, were X. americanum, but the mean P. neglectus positively correlated (r = 0.89) with soil population densities were lower in 1985- water. Soil temperature did not affect 86 than in 1984 (P < 0.05). The greatest nematode population density, and there nematode population density of any of the were no differences between soil tempera- three nematode species was 2,184 T. acu- tures (P > 0.05) during the three-year toides per liter of soil during June 1984; the study (Fig. 1). Although soil water did not greatest numbers of P. neglectus, 1,733 and significantly increase, there was a slight in- 1,675 per liter of soil, were found on Fair- crease in the population density ofX. amer- way during April and October of 1984. icanum during the fall of 1985. This was This corresponded to a maximum of 255 associated with a decrease in soil tempera- X. americanum per liter of soil on Hycrest ture. Soil population densities of T. acu- during May 1984. As observed with X. toides were approximately six-fold greater americanum and T. acutoides, low popula- than that of X. americanum on all grasses, tion densities of P, neglectus were observed but population densities were greater in during the summer months. 1984 than in 1985-86 (P < 0.05). There was no direct relationship be- In most instances, the greatest nematode tween soil depth and nematode population population densities were found in the densities of any of the three nematode spe- spring and in the fall. This was more no- cies on any of the plant species. The nu- ticeable with P. neglectus and T. acutoides merically highest X. americanum population than with X. americanum. Pratylenchus ne- densities were generally found during glectus was not affected as severely by low 1984 at 21-30 cm on Fairway and Hycrest, soil-water conditions as was T. acutoides and whereas T. acutoides population densities TABLE 1. Seasonal population means of Pratylenchus neglectus, Tylenchorhynchus acutoides, and Xiphine~na americanum on Rosana western wheatgrass, Fairway and Hycrest crested wheatgrass, Oahe intermediate wheatgrass, and the RS-1 wheatgrass hybrid during a 3-year period.

P. neglectus T. acutoides X. americanum Cultivar Date 1984 1985 1986 1984 1985 1986 1984 1985 1986

Rosana April 1,210a A 367a B 138a B 209d A 33bc B 84b B lb B 37b A 49aA May 66lab A 313a B 90ab B 255cd A 9c C 88b B 33abA 32bc A 5bA June 1,083a A 63b B 52b B 809a A 149a B 35bcB 43abA 5d A 41aA July 387b A 126b B 37b B 453bc A 136a B 13bcC 51abA 31bc A 0bB August 231b A 15b B 19b B 519b A 59bc B 0c B 12b B 60a A 8bB September 380b A 24b B 22b B 287bcdA 52bc A 16boB 8b A 12cd A 4bA October 725ab A 75b B 34b B 293bcdA 69b B 323a A 68a A 23bcdA 17bA Fairway April 1,733a A 558abB 669a B 224b A 36c B 188b A 19b B 49abc B 209a A May 642b A 469a AB 281bc B 69b A 96c A ll2bcA 27b A 15c A 0bA June 415b A 345bcA 224bc A 2,184a A 65c B 40cdB 84b A 20bc B 43bAB

July 505b A 141c B 107bc B 504b A 528a A 7d B 21b B 52ab A 0bB <% August 335b A 68c A 306bc A 273b A 63c B 15d B 16b B 59a A 15bB t,o September 582b A 637a A 36c B 88b A 49c A 8d B 252a A 76a AB 4bB October 1,675a A 113c B 346b B 245b B 251b B 475a A 13b B 73a A 5bB Hycrest April 362b A 169b A 307ab A l15b B 13c C 204b A 36bc B 44b B 247a A May 907a A 721a A 420a A 96b AB 12c B 140bcA 255a A 56b B 20bB June 367b A 189b B 211bc AB 707a A 32c B lllbcB 100b A 35b B 9bB July 316b A 64b B 171cd AB 257b B 441a A 25c C 19bcB 56b A 29bB August 199b A 43b B 93cd B 199b A 149b A 7c B 84bc B 209a A 13bC September 282b A 287b A 37d B 60b B 141b A 9c C lc B 67b A llbB October 980a A 43b B 101cd B 156b B 56c B 535a A 255a A 37b B llbB Oahe April 315b A 448a A 71a B 172a A 35cd B 63bcB 13b A 37cd A 37bA May 632a A 272b B 53ab C 415cd A 4d B lllb B 161a A 36cd B 203aA June 196bc A 116cdA 15c B 636bcdA 63bc B 35c B 23b B ld B 65bA July 95c B 219boA 16c B 269d A 288a A 3c B 33b A 12lab A 25hA August 76c A 30d B 34bc D 409cd A 104b B 9c B lib B 163a A 15bB September 141bc A 26d B 60ab B 707bc A 53bcdB 9c B 104a A 31cd B 13bB October 164bc A 98cdB 31bc C 849ab A 104b B 225a B l17a A 81bc A 8bB RS-I April 967ab A 742a A 700a A 743c A 8c B 172b B 4c B 21c B 293aA May 1,090a A 709a AB 270bc B 325c A 9c B 84bcB lc B 33bc B 160bA June 628abcA 359b B 130c B 2,016a A 40c B 65bcB 88abA 9c B lc B July 725abcA 187bc B 352abcAB 297c A 383a A 21c B 32bcA 32bc A 16c A August 332c AB 41c B 605ab A 1,443b A 112b B 15c B 19bcB 124a A 20c B September 484bc A 60c B Illc B 315c A 105b AB 12c B 136a A 124a A 8c A October 867ab A 180bcB 152c B 407c A l13b B 459a A 0c B 9lab A 3cB

Values are the means of nematodes collected froin 0-30 cm soil depth from four plots each consisting of five subplots (n = 20). Samples were collected monthly with a 9-tin bucket auger at 25-cm intervals from the center of each row. Nematode populations were determined trom roots and soil of four 250-cma aliquants from each sample. Means in columns not followed by a common lowercase letter are different (P < 0.05). Means in rows not followed by a common uppercase letter are difterent (P < 0.05). Population Trends of Nematodes on Grasses: Griffin, Asay 111 were higher in the 0-10 cm of soil (P < which would lead to a X. americanum pop- 0.05) (Table 2). The greatest P. neglectus ulation density peak during the late sum- and X. americanum population densities mer and early fall. Previous studies have were found on Fairway and Hycrest at a shown that X. americanum population den- soil depth of 21-30 cm during 1984, sities decline at high soil temperatures and whereas the greatest T. acutoides popula- low soil water (11,17). Lownsbery and tion densities were found on RS-1 at 0-10 Maggenti (17) reported that the optimum cm during 1984. The smallest density of all temperature for X. americanum was 21 C, nematodes was found on Rosana at 21-30 and the nematode population density de- cm during 1986. clined at 27 C. Summer temperatures at the Thiokol study site exceeded 21 C from DISCUSSION June to September and approached 30 C during July and August. While X. america- Few data have been generated on the hum cannot survive high soil temperatures, root phenology of grasses. Caldwell et al. it can survive winter months at low tem- (5) found that root biomass of A. deserto- peratures (12). A possible carryover of the rum, one of the parents of Hycrest, in- nematode population density from 1985, creased during the spring and fall. Thor- plus the normal hatch in the following geirsson (30) reported that A. desertorum spring, may explain the higher population extracted soil water during the early density levels in April 1986. spring, indicator that roots had initiated Tylenchorhynchus acutoides may not be as growth, and that A. desertorum initiated ad- sensitive to soil water, soil temperature, ventitious roots in early spring and fall and oxygen as has been reported for X. when soil water was more optimal or abun- americanum (17,23,33). The optimum tem- dant. There is a direct relationship be- perature range of T. acutoides also may ex- tween root growth and nematode popula- ceed that of X. americanum. The fecundity tion densities (7). Hence, the increased of the nematodes also is apparently differ- nematode density observed in this study ent since there was a six-fold greater pop- could be associated with increased root ulation density of T. acutoides than X. amer- growth resulting from adequate soil water icanurn. Although there have been no data since nematodes were collected in the generated on the biology and fecundity of greatest numbers from all grass selections T. acutoides, it appears from its large pop- during the spring of 1984, when suppos- ulation density that it may have at least two edly adequate soil water would favor plant life cycles per year. Hence, T. acutoides growth. Since there is a lack of data on root could be an important pathogen of range- phenology, we can only speculate that dif- land vegetation. ferences among the three nematode spe- Soil temperature may" have less effect on cies in relation to maximum populatio,a population densities ofP. neglectus than on densities may be due to differences in the ectoparasitic nematodes. Although re- plant root growth, host preference, and production of P. neglectus on wheatgrass is nematode fecundity. The low reproduc- greatest at 30 C (9), there was less fluctua- tive potential of X. americanum may ac- tion in the nematode population density count for the low population densities. than with the ectoparasitic nematodes. Previous studies have shown that X. ameri- This is possibly due to the nematode being canum produces only one generation per less affected by environmental conditions year on alfalfa, Medicago sativa L.; spruce, since it inhabits both roots and soil. Differ- Picea pungens L.; and peach, Prunus persica ences in nematode counts among cultivars (L.) Batsch; population density peaks usu- at the three soil depths may also be associ- ally occurred during the spring or fall (12, ated with variation in plant phenology and 15,19). The lack of soil water may have depth of root growth. Soil-depth differ- delayed the 1985 spring egg hatching, ences in numbers between the nematode I",D

,< TABLE 2. Population means of Pratylenchus neglectus, Tylenchorhynchus acutoides, and Xiphinema americanum on Rosana western wheatgrass, Fairway and Hycrest crested wheatgrass, Oahe, intermediate wheatgrass, and the RS-1 hybrid wheatgrass at three soil depths during a 3-year period.

P. neglectus T. acutoides X. americanum Depth Cultivar (cm) 1984 1985 1986 1984 1985 1986 1984 1985 1986

Nematodes per liter Rosana 0-10 555aA 170aB 92aB 807aA 125a B 148aB 34aA 46a A 24a A 11-20 869aA 142aB 41bB 217bA 42b B 60bB 24aA 21b A 17a A bo 21-30 580aA ll0aB 34bB 187bA 50b B 31bB 35aA 19b B 13a B Fairway 0-10 347bA 289aAB 228a B 837aA 206a B 177aB 35a A 53abA 52a A 11-20 994aA 347aB 305aB 366bA 147abB 96bB 48aA 60a A 36a A 21-30 1,183aA 363aB 31laB 335bA l13b B 89bB 102aA 34b B 30a B Hycrest 0-10 435aA 305aAB 171aB 503a A 167a B 255a B 51bA 105a B 66a A 11-20 589aA 190aB 22lab 375aA 94b B 133bB ll7aA 57b B 46abB g~ 21-30 439aA 154aB 182aB 231aA 102b B 62bB 153aA 54b B 34b B Oahe 0-10 270aA 161aAB 5laB 1,482aA 150a B 114aB 45aA 60a A 59a A 11-20 299aA 189aAB 30aB 282bA 70b B 57bB 77aA 70a B 52a A ',O 21-30 125bA 168aA 39aB 146bA 59b B 23bB 77aA 71a A 46a A RS-1 0-10 521bA 369aB 356a B 1,678a A 203a B 201a B 30aA 74a A 33a A 11-20 1,121aA 316aB 343aB 423bA 65b B 90bB 45aA 66a A 82a A 21-30 541bA 290aB 295aB 275bA 63b B 63bB 45aA 46a A 101a A

Values are the means of yearly collections over a 7-month period (April October). Samples were collected from foul plots, each consisting of five subplots. Samples were collected monthly with a 9-cm bucket auger at 25-cm intervals from tile center of each row at depths nf 0-10, 11-20, and 21-30 cm. Nematode populations were determined from roots and soil of four 250-cm~ aliquants Dom each sample. Means in columns nol followed by a common lowercase letter are different (P < 0.05). Means in rows not tollowed by a common uppercase letter are different (P < 0.05). Population Trends of Nematodes on Grasses: Griffin, Asay 113 species may indicate a greater sensitivity of of grassland agriculture. Ames: Iowa State University X. americanum to the increased soil temper- Press. 3. Barker, R. E., J. D. Berdahl, J. M. Krupinsky, ature in the upper soil levels. Nematodes and E. T. Jacobson. 1981. Collections of western have been found to migrate a distance of wheatgrass and blue grama associated nematode gen- more than 1 m (22). Since X. americanum era in the western Dakotas. Pp. 237-240 in Proceed- population densities decline when summer ings of the XIV International Grassland Congress, Lexington, KY. temperatures are high (17), the nematode 4. Byrd, D. W., Jr., K. R. Barker, H. Ferris, C.J. might have migrated to the 41-60-cm Nusbaum, W. E. Griffin, R. H. Small, and Connie A. depth to a more moderate soil tempera- Stone. 1976. Two semi-automatic elutriators for ex- tracting nematodes and certain fungi from soil. Jour- ture. Since nematodes move through a nal of Nematology 8:206--212. film of moisture, lack of soil water could 5. Caldwell, M, M., J. H. Richards, D. A. Johnson, have inhibited downward movement of R.S. Nowak, and R. S. Dzurec. 1981. Coping with the nematode during 1985 and 1986, thus herbivory: Photosynthetic capacity and resource ailo- cation in two semiarid Agropyron bunchgrasses. Oeco- leading to higher numbers of X. america- logia 50:14-24. num in the 0-10 cm soil level. Pratylenchus 6. Freckman, D. W., D. A. Duncan, and J. R. Lar- neglectus population densities were not son. 1979. Nematode density and biomass in an an- consistently associated with particular soil nual grassland ecosystem. Journal of Range Manage- ment 32:418~422. depths, since the nematodes are found 7. Griffin, G. D. 1984. Nematode parasites of al- where root tissue is most dense (7). falfa, cereals, and grasses. Pp. 243-321 in W. R. The pathogenicity of X. americanum, P. Nickle, ed. Plant and insect nematodes. New York: neglectus, and T. acutus to certain plant spe- Marcel Dekker. 8. Griffin, G. D. 1988. Factors affecting the biology cies, including wheatgrasses, has been doc- and pathogenicity of Heterodera schachtii on sugarbeet. umented (7,9,10). The large numbers of T. Journal of Nematology 20:396-404. acutoides found on wheatgrasses suggest 9. Griffin, G. D. 1992. Pathological effects of Pra- t~lenchus neglectus to wheatgrasses. Journal of Nema- that these grasses are good hosts for the tology 24:442-449. nematode. In this study, differences in 10. Griffin, G. D., and K. H. Asay. 1992. Impor- preference of nematodes on Hycrest and tance of soil texture on the pathogenicity of plant- Fairway crested wheatgrass, Oahe inter- parasitic nematodes to range grasses. Phytopathology 82:1096. (Abstr.). mediate wheatgrass, Rosana western 11. Griffin, G. D., and K. R. Barker. 1966. Effect wheatgrass, and RS-1 hybrid, and environ- of soil temperature and moisture on the survival and mental effects on nematode population activity of Xiphinema americanum. Proceedings Hel- trends were observed. A better under- minthological Society of Washington 33:126-130. 12. Griffin, G. D., and H.M. Darling. 1964. An standing of climatic effects on nematode ecological study of Xiphinema americanum Cobb in an population densities is needed. Once data ornamental spruce nursery. Nematologica 10:471- on nematode population plant-growth re- 479. lationships are generated and understood, 13. Griffin, G. D., and F. A. Gray. 1990. Biology and pathogenicity of Pratylenchu~ neglectus on alfalfa. they can be utilized to determine nema- Journal of Nematology 22:546-551. tode species that are the most important 14. Ingham, R. E., and J. K. Detling. 1984. Plant- parasites to rangeland grasses and plant herbivore interactions in a North American mixed- breeders in development of resistant or grass prairie. III. Soil nematode populations and root biomass on Cynomys ludovicianus colonies and adjacent tolerant grasses (1). uncolonized areas. Oecologia 63:307-313. 15. Jaffee, B.A., M.B. Harrison, R.L. Shaffer, and M. B. Strang. 1987. Seasonal population fluctu- LITERATURE CITED ations of Xiphinema americanum in New York and Pennsylvania orchards. Journal of Nematology 19: 369-378. I. Adkisson, P. L., and V. A. Dyck. 1980. Resistant 16. Jenkins, W. R. 1964. A rapid centrifugal- varieties in pest management systems. Pp. 233-251 in flotation technique for separating nematodes from F.G. Maxwell and P. R. Jennings, eds. Breeding soil. Plant Disease Reporter 48:692. plant resistance to insects. New York: John Wiley. 17. Lownsbery, B. F., and A. R. Maggenti. 1963. 2. Asay, K. H., and R. P. Knowles. 1985. The Some effects of soil temperature and soil moisture on wheatgrasses. Pp. 166-176 in M.E. Heath, R. F. population levels of Xiphinemc~ americanum. Phytopa- Barnes, and D. S. Metcalfe, eds. Forages, the science thology 53:667~668. 1 14 Journal of Nematology, Volume 28, No. 1, March 1996

18. Malik, Z., and M. S. Jairajpuri. 1983. Popula- 27. Smolik, J. D., and J. K. Lewis. 1982. Effect of tion dynamics and life cycle of Xiphinema americanum range condition on density and biomass of nematodes Cobb, 1913. Indian Journal of Parasitology 7:21-28. in a mixed prairie ecosystem. Journal of Range Man- 19. Norton, D. C. 1963. Population fluctuations of agement 35:657-663. Xiphinema americanum in Iowa. Phytopathology 53: 28. Smolik, J. D., and L. E. Rogers. 1976. Effects 66-68. of cattle grazing and wildfire on soil-dwelling nema- 20. Norton, D. C., and D. P. Schmitt. 1978. Com- todes of the shrub-steppe ecosystem. Journal of munity analysis of plant-parasitic nematodes in the Range Management 29:304-306. Kalsow Prairie, Iowa. Journal of Nematology 10:171- 176. 29. Taylor, A. L., and J. N. Sasser. 1978. Biology, 21. Orr, C. C., and O.J. Dickerson. 1967. Nema- identification, and control of root-knot nematodes todes in true prairie soils of Kansas. Transactions (Meloidogyne species). Raleigh: North Carolina State Kansas Academy of Science 69:317-334. University Graphics. 22. Pinkerton, J. N., and G. S. Santo. 1986. Con- 30. Thorgeirsson, H. 1985. Temporal and spatial trol of Meloidogyne chitwoodi in commercially grown partitioning of the soil water resource between two Russet Burbank potatoes. Plant Disease 70:860-863. Agropyron bunchgrasses and Artemisia tridentata. M.S. 23. Ponchillia, P. E. 1972. Xiphinema americanum as thesis, Utah State University, Logan. affected by soil organic matter and porosity. Journal 31. Thorne, G. 1974. Nematodes of the northern of Nematology 4:189-193. great plains. Part II. Dorylaimoidea in part (Nemata: 24. Schmantz, J. E., M. D. Belinger, and R.W. Adenophorea). Technical Bulletin No. 41. South Da- Harris. 1980. Range. Pp. 246-315 in An assessment kota Agricultural Experiment Station, Brookings. of the forest and range land situation in the United States. USDA-Forest Service. FS-345. Washington, 32. Thorne, G., and R. B. Malek. 1968. Nematodes D.C.: United States Government Printing Office. o£ the northern great plains. Part I. 25. Scott, J. A., N. R. French, and J. w. Leetham. (Nemata: ). Technical Bulletin No. 31. 1979. Patterns of consumption in grasslands. Pp. 89- South Dakota Agricultural Experiment Station, 105 in N. R. French, ed. Perspectives in grassland Brookings. ecology. New York: Springer-Verlag. 33. Van Gundy, S. D., L. H. Stolzy, T. E. Szuszk- 26. Smolik, J.D. 1974. Nematode studies at the iewicz, and R. L. Rackham. 1962. Influence of oxy- cottonwood site. US/IBP Grassland Biome Technical gen supply on survival of plant-parasitic nematodes Report No. 25 t. Boulder: Colorado State University. in soil. Phytopathology 52:628-632.