Graduate Thesis Or Dissertation


The biology and pathogenicity of a sheath nematode Hemicycliophora similis Thorne, 1955 Public Deposited

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  • A study, using greenhouse and laboratory experiments, was made of Hemicycliophora similis from Bandon, Oregon to determine: identity and morphometrics, time and development of a life cycle, host plant preferences and optimum environmental conditions for reproduction and pathogenicity. Morphometric investigations revealed discrepancies between the measurements of this nematode species and those recorded in nematological literature for H. similis. Personal communications with R. P. Esser, H. J. Jensen and B. M. Zuckerman suggested the variations noted do not necessitate describing a new species. For purposes of this research the nematode is named Hemicycliophora sirnilis. Life history investigations were conducted to determine the number of days required to complete a life cycle and the various stages as the nematode developed from egg to egg. Eggs mature to first-stage larvae, without sheath or spear, in 3-5 days and hatch as second-stage larvae in 8-10 days. Second-stage larvae inoculated onto carrot seedlings matured into third-stage larvae in 12-15 days. Fourth-stage larvae appeared in 25-28 days, and adults were present in 35-38 days. Embryonating eggs were observed lodged within an incompletely shed fourth-stage cuticle attached to adult females and within the normal sheath surrounding the adult. One molting specimen was found with two eggs lodged within the fourth-stage cuticle. Such eggs were seen to develop to second-stage larvae; however, none were observed over a long enough time period to determine if larvae hatched and emerged free of the cuticular confinement. Evaluated as hosts were beet, broccoli, carrot, corn, dill, pea, radish, rutabaga, tomato and turnip. Carrot, tomato and turnip supported the highest increase in population; no substantial increase was observed on beet, broccoli, corn and dill. The nematode formed galls consisting of large multibranched growths and reproduced on many of the plant species tested. Galling of seedling roots was most severe on carrot, but slight galling was observed on beet, rutabaga and turnip. Multibranching was observed with varying severity in all hosts. Germination of carrot seeds was reduced substantially when grown in sandy soil infested with H. similis. Slight germination reductions occurred with all hosts tested except corn and pea. Adults and larvae were observed feeding behind root tips and on nematode induced galls. Root discoloration was noted on several plants. To evaluate temperature and soil optima, rooted cranberry cuttings, germinated cranberry seedlings and carrot seedlings were utilized. Carrot seeds were planted in sandy soils of two different textures and pH, infested with H. similis and grown in three temperature regimes: 30°C and 24°C night, 22°C day and 14°C night and 30°C day 6 °C night. The optimum temperature for reproduction on carrot was 22°C day 14°C night in both soil types. A pH gradient developed in cups with the least acidic pH occurring at the base corresponding to maximum nutrient uptake by plant roots. The soil mix in which bog sand from Bandon was incorporated developed a more acidic condition presumably due to the additional organic N content which supplied ammonical nitrogen. The soil mix with Newport sand had no additional organic N and a less acidic condition developed. Maximum nematode population and root density also occurred at the base of the cup. Plants grown in infested soil had sparse multibranched roots; no galls were observed in this experiment. Cranberry cuttings were grown in sterile bog soil from Bandon, Oregon and inoculated with 225 nematodes/450 cc soil. The optimum temperature for nematode reproduction and plant growth was 30°C and 24°C night. Runner growth and root dry weights were not significantly effected by nematode feeding. Germinated cranberry seedlings were subjected to similar conditions; temperature response was similar to that of cuttings. Root dry weights and runner growth of cranberry seedlings was greater in nematode inoculated treatments than non-inoculated controls: nematode feeding on cranberry roots resulted in an increase of multibranching. This research indicated nematode feeding was detrimental to growth of a variety of plant species. High soil populations of H. similis caused severe reductions to root growth and seed germination. However, since this study was conducted in controlled conditions, it was not substantiated that this nematode caused a reduction of growth to field-grown cranberry plants. Further investigations are needed to correlate these findings with field data to determine pathogenicity on cranberry.
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