Plant Science & Landscape Architecture Theses and Dissertations
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Item A Study of Resistance to the Sweet Potato Wilt Pathogen, Fusarium oxysporum Schlecht f. batatis (Wr.) Snyder & Hansen, and of Histological Aspects of the Host-pathogen Complex(1963) Wells, John Milton; Kantzes, James G.; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Research on Fusarium wilt of sweet potato, a vascular disease caused by Fusarium oxysporum Schlecht f. batatis (Wr.) Snyder & Hansen, was undertaken to determine the susceptibility of various sweet potato lines to Maryland isolates of the pathogen under field and greenhouse conditions. Highly resistant lines would be useful as sources of resistance to Fusarium wilt in sweet potato breeding programs. In 2 years of field and greenhouse trials, 94 different lines of sweet potato were inoculated with a composite spore and mycelial suspension of 5 Maryland isolates of F. oxysporum f. batatis. Results indicated that the following lines were highly resistant to the pathogen: the foreign plant introductions P.I. 153655 ("Tinian"), P.I. 153906, P.I. 153907, and P.I. 251602; the variety Pelican Processor; and the breeding selections B-6842 from the United States Department of Agriculture Plant Industry Station at Beltsville, Maryland, and T-7 from the Georgia Coastal Plain Agricultural Experiment Station at Tifton, Georgia. Greenhouse experiments showed that the host range of Fusarium oxysporum f. batatis should include an additional species of Morning Glory, lpomoea pandurata (L.) G. F. w. Mey. Furthermore, no symptoms of infection were obtained on various crop plants commonly grown in rotation on land used for sweet potato culture. Physiological studies in the laboratory and greenhouse indicated that no significant levels of fungitoxic substances were present in either uninoculated or inoculated ''Tinian" plants. Nor could a fungal metabolite be detected, under the existing experimental conditions, which was toxic to a susceptible variety of sweet potato (Porto Rico) but not to the resistant ''Tinian". A study was made of the basis for resistance of the foreign plant introduction "Tinian" (P. I. 153655). Histological examinations of serial stem sections of the susceptible sweet potato variety Porto Rico and of the resistant foreign plant introduction ''Tinian" were made from plants collected at 3-day intervals following inoculation with spores of the pathogen. It was found that "Tinian" responded to infection by the production of tyloses in advance of the fungus. Twelve days after inoculation, 75 - 88% of the vessels which were 22 - 32 mn above the invasion site at the base of the plant were completely filled with tyloses. This compared to only 0 - 3% in the uninoculated control plants. Furthermore, no mycelia or spores could be detected in tnis region but were present in 25 - 50% of tne vessels within 11 mm of tne invasion site. In the variety Porto Rico the occurrence of tyloses in the inoculated plants was not significantly greater than in the uninoculated controls, except near the invasion site where after 12 days 3 - 6% of the vessels contained small 1 tyloses. The pathogen was not limited, as in ''Tinian", to the immediate invasion site. Tnis suggests that tne production of tyloses in "Tinian may represent an important defense mechanism against Fusarium wilt.Item GROWTH REGULATORS AND THE FLOWERING OF EVERGREEN AZALEAS (RHODODENDRON CV.)(1960) Ballantyne, David J.; Link, Conrad B.; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Spraying experiments were conducted in 1958 and 1959 to find the effectiveness of certain growth regulators upon multiple flower bud formation and rate of flower bud development of evergreen azaleas. Paper chromatograms of extracts of vegetative buds and of flower buds treated with 37°F. storage and potassium gibberellate (GAk) sprays, were tested with a wheat coleoptile bioassay in 1959. Foliar sprays of 2, 3, 5-triiodobenzoic acid (TIBA), an antiauxin, showed evidence of inhibiting multiple flower bud formation, and a foliar spray of 2,200 ppm indoleacetic acid (IAA) tended to promote multiple flower bud formation. The time of spraying in relation to the time of floral initiation apparently is important if growth regulators are to influence multiple flower bud formation. The rate of flower bud development was increased by two weeks of 37°F. storage and either two sprays of 200 ppm TIBA or single sprays of 160, 400 or 1,000 ppm TIBA, or by three weeks of 45°F. storage and a single spray of 1,000 ppm IAA. Rate of flower bud development was increased by two sprays of 200 ppm TIBA and one spray of 1,000 ppm gibberellic acid (GA). Flower bud dormancy was removed by foliar sprays of 900 ppm GAk with no cold storage or two weeks of 37°F. Four weeks of 37°F. storage was effective without GAk and six weeks of 37°F. storage gave no increase over four weeks of storage. Concentrations of GAk lower than 900 ppm were ineffective. GAk was effective whether applied before or after two weeks of 37°F. storage. Naphthalene acetic acid in concentrations of 9 ppm or greater inhibited the rate of flower bud development. Apical dominance was removed by 800 ppm or more of TIBA. The wheat coleoptile bioassay indicated that a growth inhibitor in the flower buds was removed by three or more weeks of 37°F. storage and three sprays of 1,000 ppm GAk. The promoter was not in vegetative buds and could not be considered to be IAA, GA or GAk.Item Nematodes associated with roses and the root injury caused by Meloidogyne hapla Chitwood 1949, Xiphinema diversicaudatum (Micoletzky 1927) Thorne 1939, and Helicotylenchus nannus Steiner 1945(1959) Davis, Ronald Allan; Jenkins, W. R.; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Few papers have been published concerning the cytological and histological effects of plant parasitic nematodes on their hosts. Most of this type of work has been done on the root-knot disease. Christie (2) described the development of root-knot nematode incited galls on tomato seedlings, reporting that these nematodes caused hypertrophy of cortical, pericyclic, and endodermal cells, hyperplasia of the pericycle, formation of xylem elements from parenchyma surrounding giant cells, and retardation of meristematic activity in the root tip. He also reported on the development and morphology of giant cells (large, multinucleate cells resulting from a stimulatory effect of nematode feeding). Krusberg and Neilsen (8) observed similar cytological responses in their work with Meloidogyne incognita acrita Chitwood 1949 infections of Porto Rico variety of sweet potato. Other investigators worked primariJy on the cytology and morphology of giant cells and giant cell nuclei. According to Tischler (from Christie, 2), division of giant cell nuclei was by normal mitosis in early stages of giant cell development, but later divisions occurred by amitosis and by fragmentation. However, Nemec (from Christie, 2) felt that divisions by amitosis and .fragmentation as reported by Tischler were actually stages of nuclear coalescence. Linford (9) described the method by which root-knot nematodes feed on giant cells and noted that substances were extruded from the stylet during feeding. Kostoff and Kendall (7), working with galled roots of Nicotiana hybrids, reported that secretions by the nematode increased cell wall permeability causing exosmosis and resulting in an accumulation of food in the region of invasion. Consequently, growth of plant tissues in these regions was accelerated and was expressed morphologically as swellings or galls on the roots. In 1942, Alstatt (1) tested the susceptibility of several strains and varieties of rose stocks, including Rosa multiflora Thunb. to a root-knot nematode. Of 13 different understocks, only one was found resistant. Lyle (10) and Massey (12) indicate that root-knot nematodes cause a serious disease of rose. Reynolds (15) found that in Meloidogyne incognita (Kofoid and White 1919) Chitwood 1949 infections of R. multiflora seedlings, the nematode entered the root and stimulated giant cell development; but galls occurred only rarely and were sometimes found on the end of long roots as a result of the penetration of many larvae. Martin (11) reported M. hapla as producing small, hard, galls on rose roots in Rhodesia and Nyasaland. M. hapla was reported by Van Der Linde (22) to infest a rose thornless understock. Two genera of ectoparasitic nematodes have been associated with root gall formation. Van Gundy (23) reported that galls induced on rough lemon roots by Hemicycliophora arenaria Raski 1958 were due to a hyperplastic response of the cortical tissue. Schindler (18) demonstrated that galling of rose roots was caused by Xiphinema diversicaudatum (Micoletzky 1927) Thorne 1939, but he did not investigate their cytological effects. In a survey of greenhouse roses, Schindler (17) found Xiphinema and Pratylenchus to be the most widely distributed genera and to occur more frequently than any other nematodes. Other genera found were: Criconemoides, Paratylenchus, Helicotylenchus, Hemicycliophora, Belonolainius, Trichodorus, T,ylenchus, Aphelenchoides, Psilenchus, and Meloidogyne. Sher (21) described the pathogenicity of Pratylenchus vulnus Allen and Jensen 1951 on rose, reporting that rose plants infested with this species were stunted and chlorotic and the root systems were necrotic with few feeder roots. Other nematodes which have been found associated with rose are Pratylenchus pratensis (de Man 1880) Filipjev 1936 (3,14), P. penetrans (14) Sher and Allen 1953 P. scribneri Steiner 1943 (13), and Ditylenchus dipsaci (Kuhn 1857) Filipjev 1936 (5). This present study was initiated to determine the occurrence and distribution of nematodes associated with roses grown outdoors. In addition the cytological and histological effects of Meloidogyne hapla Chitwood 1949, by Xiphinema diversicaudatum (Micoletzky 1927) Thorne 1939, and Helicotylenchus nannus Steiner 1945 on rose roots was determined.