160 Journal of Nematology, Volume 10, No. 2, April 1978 nematode, Ditylenchus destructor. Ann. Appl. abilit~it und Mutationen bei Nematoden. Zesz. Biol. 39:468-474. Prob. Post. Nauk Roln. 92 (Proc. 1X Int. 6. LOOF, P. A. A. 1960. Taxonomic studies on the Nem. Symp. (Warsaw, 1967)):209-220. genus Pratylenchus (Nematoda). Tijdschr. 12. TARJAN, A. C. 1967. Variability of diagnostic Plantenziekten. 66:29-90. characters among some plant and soil nema- 7. O'BANNON, J. H., A. C. TARJAN, attd F. W. todes. [Polish text]. Prace Nauk. Inst. Ochrony B1STLINE. 1974. Control of Pratylenchus Roslin 9(1):105-116. brachyurus on citrus and tree response to 13. TARJAN, A. C., and J. H. O'BANNON. chemical treatment. Proe. Soil Crop Sci. Fla. 1969. Observations on meadow nematodes 33:65-67. (Pratylenchus spp.) and their relation to 8. O'BANNON, J. H., and A. T. TOMERLIN. declines of citrus in Florida. Plant Dis. Rep. 1973. Citrus tree decline caused by 53(9) :683-686. Pratylenchus coffeae. J, Nematol. 5:311-316. 14. TARTt~, R., and W. F. MAI. 1976. Morpho- 9. ROMAN, J., and H, HIRSCHMANN. 1969. logical variation in Pratylenchus penetrans. Morphology and morphometrics of six spe- J. Nematol. 8:185-195. cies of Pratylenchus. J. Nematol. 1:363-386. 15. THORNE, G., and M. W. ALLEN. 1959. Varia- 10. SHER, S, A., and M. W. ALLEN. 1953. Revision tion in nematodes, pgs. 412-418. In Holton, of the genus Pratylenchus (Nematoda: C. S. ed., Plant pathology problems and Tylenchidae). Univ. Calif. Publ. Zool. 57: 441- progress. 1908-1958. Univ. Wisconsin Press, 470. 588 pp. 11. STURHAN, D. 1970. Intraspezifische Vari- Corn Yield Increases Relative to Nonfumigant Chemical Control of Nematodes 1 DON C. NORTON", JON TOLLEFSON 3, PAUL HINZ 4, and STEPHEN H. THOMAS 2 Abstract: Corn yields were measured after application of nematicides in 16 experiments, mostly in medium-to-heavily textured soil, at 12 locations in Iowa during 1973-1976. The average maximum yield increase in plots treated with nematicides was 21% over yields in untreated plots. Yields were correlated negatively with nematode numbers or nematode biomass in nearly all comparisons. Correlations of nematode numbers in the soil with yield averaged --0.56 for Helicotylenchus pseudorobustus, --0.45 for Hoplolaimus galeatus, --0.51 for Pratylenchus spp., and --0.64 for Xiphinema americanum. Correlation coefficients for numbers of nematodes in the roots and yield averaged --0.63 for Pratylenchus spp. and --0.56 H. galeatus. Correlation co- efficients for yield and total number of nematodes averaged --0.65 in roots and --0.55 in soils. Negative correlations also were greater for comparisons of yield with total parasitic-nematode biomass than with numbers of individual nematodes of a species or total numbers of parasitic nematodes. Key Words: Helicotylenehus pseudorobustus, Hoplolaimus galeatus, Pratylenchus hexincisus, Xiphinema americanum, rootworms. Nematodes have been associated with production over wide areas of the upper corn, Zea mays L., from the upper mid- Midwest, where medium-to-heavily textured western United States (4, 8, 14, 18). soils predominate, [lave not been evaluated Nematicides have reduced the density of fully. Smolik (16) found a negative correla- nematode populations around corn and tion of yield with parasitic nematodes at have been associated with increased yields harvest in South Dakota. Hoplolaimus (7, 10). The effects of nematodes on corn galeatus (Cobb) Thorne and Pratylenchus hexincisus Taylor 8c Jenkins were impli- Received for publication 14 March 1977. cated in corn yield losses in sandy soils in 1Journal Paper No. J-8770 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa 50011. Iowa (13). Rootworms, wireworms, ear- Projects 1991 and 2119. This study was supported, in part, worms, or stalk rot were not present or by Grant No. R-802547 of the Environmental Protection Agency. Acknowledgment is made to John Owens for use were not affected by the treatments in these o~ his plots in 1974. tests. This research was done to determine =,3,*Departments of Botany and Plant Pathology, Entomol- ogy, and Statistics. Iowa State University, Ames. the role of nematodes as a factor in in- Nematodes on Corn: Norton et al. 161 creased corn yields in plots treated with extraction, the roots were dried at 90 C for nematicides in medium-to-heavily textured 48 h. soils. Correlations: Correlations were tested between corn yields and numbers of soil nematodes/100 cm a, total nematode bio- MATERIALS AND METHODS mass in soil, total numbers of nematodes in General procedures: The results re- roots, and total nematode biomass in roots. ported are from 16 experiments at 12 The higher of the two nematode counts was locations during 1973-1976. The experi- used for calculating correlation coefficients ments were in all sections of iowa except (r). the southwestern part. Two experiments Prominence vahte: Used to ascertain were in soil containing more than 90% relative quantitative values among nema- sand, but all others were in loam, silt loam, todes was the prominence value, PV = or clay loam soils. Five locations were density ~/frequency (2). Values were calcu- suspected of having a nematode problem. lated from mean numbers of nematodes in The other 11 test sites were not pre- untreated plots in tile mid-to-late-summer viously assayed. Granular nematicides samplings. applied were aldicarb, carbofuran, 0[5- Biomass: More juveniles than adult chloro-l-(1-methyl)-I H-l,2,4-triazol-3-yl]O,O- nematodes were present in mid-to-late sum- diethyl phosphorothioate (CGA-12223), met in iowa. To estimate biomass, 25-30 ethoprop, fensulfothion, phenamiphos, and nematodes of each species were picked at terbufos. Nematicides were applied in a random from midseason samples and meas- 17.8-cm band unless specified otherwise. ured, and the biomass was calculated by the Granules were applied with tractor- formula of Andrassy (1). The biomass for mounted applicators or calibrated gear- individual nematode species used in this driven, hand-operated applicators and study was: Helicotylenchus pseudorobustus incorporated into the top 5 cm of soil. Row (Steiner) Golden, 0.183 t~g; Hoplolaimus widths were 71-76 cm. Most plots were four galeatus, 0.792/~g; Pratylenchus spp., mostly rows wide and 9.1 m long. The two outer P. hexincisus, 0.072 t~g; Paratylenchus rows were sampled within the row for projectus Jenkins, 0.032 t~g; Longidorus nematodes, and the center 7.6 m of the breviannulalus Norton and Hoffmann, inner rows was harvested by hand for 4.657 ug; Quinisulcius acutus (Allen) yield. A composite of ten 2-cm core Siddiqui, 0.154 t'g; Trichodorus sp., 0.659 samples/15.2 m of row was sampled from ~g; Tylenchorhynchus nudus Allen, 0.125 each treatment about mid-June, mid-to-late #g; and Xiphinema americanum Cobb, July, and late August to early September. 0.995 #g. A pretreatment sample was collected from Rootworms: In an attempt to iden- the untreated plots, but because of small tify the degree of rootworm [Diabrotica nematode numbers, the data are not in- longicornis (Say), 1). virgifera LeConte, and cluded. Each treatment was replicated 4 to 1). undecimpunctata Barber] control by 7 times. All tests were of a randomized tile chemicals, 10 of the 16 experiments block design. The nmnber of treatments were evaluated for rootworm damage. The per experiment ranged from 4 to 10, in- amount of root damage was classified by a cluding an untreated control. Cultivars l-to-6 visual rating, from little to a/most- varied with experiment. complete root damage. Evaluations were Tile soil in each sample was mixed made on five plants from each treatment thoroughly in the laboratory by rolling, and plot in mid-to-late July, when maximum 100 cm a was processed by the centrifugal- feeding has occurred. flotation method (9) within 2 days of sampling. Fibrous roots collected from the RESULTS soil samples were processed for endopara- Prominence values for the parasitic sites by thin-layer water extraction for 7 nematodes/100 cm a soil in all replications days, with a change of water after the third of the untreated plots for all tests were: day, in 1973-1974, and by a shaker metlmd H. pseudorobustus, 698; H. galeatus, 342; (3) for 4 days in 1975-1976. After nematode Pratylenchus spp., 318; X. arnericanurn, 162 Journal of Nematology, Volume 10, No. 2, April 1978 104; T. nudus, 33; L. breviannulatus, 0.2; duction in the Midwest. Measures of the P. projectus, 0.1; Q. acutus, 0.1; and Tri- degree of nematode infection were corre- chodorus sp., 0.1. Prominence values/gin lated negatively with corn yields for 58 of dry root were: Pratylenchus spp., 3,960; and 60 such comparisons made with individual H. galeatus, 833. species. Correlation of nematode numbers The average percentage yield increase in the soil with yield averaged --0.56 for in chemically treated plots compared with Helicotylenchus pseudorobustus, --0.45 no treatment ranged from 2 to 20% (Table for Hoplolaimus galeatus, -0.51 for 1). In comparison with controls, the maxi- Pratylenchus spp., and -0.64 for Xiphinema mum yield increases in the 16 experiments americanum. Correlation coefficients for averaged 21%, with the greatest increase numbers of nematodes in the roots and in any experiment being 72%, for carbo- yield averaged -0.63 for Pratylenchus spp. furan at 1.12 kg/ha. In general, however, and --0.56 for H. galeatus. Numbers of carbofuran generally resulted in greater Pratylenchus spp. and H. galeatus com- yield increase at 2.24 kg/ha than at the bined were more closely correlated with lower rates. yield than were numbers of either con- The nematicides were selective in re- sidered singly in 71% of the instances ducing nematode numbers (Table 2).