Essex soybean seedling colonization by Fusarium oxysporum and F. solani and disease severity studies were conducted using soil collected from areas in a field (P. Minor field, King and Queen County), which exhibited poor stands and growth of Essex soybean in 1986. Tests were conducted also with soil collected from a field (Holland field, Suffolk), which had no history of poor soybean emergence or growth. Population densities of F. oxysporum and F. solani in the P. Minor soil were significantly higher (P≤0.05) than that in the Holland soil. In P. Minor soil maintained at -0.01 MPa water potential, no significant differences were found in disease severity for 15, 20 and 25 C after 6 days. However, disease severity was significantly (P≤0.05) higher at 20 C than at 15 C or 25 C, 13 days after planting. F. oxysporum and F. so/ani were isolated from lesions on Essex cotyledons and hypocotyls at all three temperatures. Rhizoctonia so/ani was isolated with highest frequency from hypocotyl lesions at 25 C. Colonization of soybean plant parts by F. oxysporum and F. solani in P. Minor soil was studied by plating asymptomatic, sequential, 2-mm long tissue segments on Komada's selective medium. Tissue segments from cotyledons, yielded significantly (P≤0.05) more F. oxysporum than F. solani, 3 and 4 days after planting. Both fungi were recovered at high frequencies from hypocotyl segments although F. solani recovery was significantly (P≤0.05) higher than F. oxysporum after 4 days of planting. From root segments, F. solani isolation frequency was significantly higher (P≤0.05) than F. oxysporum after 4 days. In Holland soil, the colonization patterns of F. oxysporum and F. solani were similar but the frequencies were lower than for P. Minor soil. Of 102 representative F. oxysporum, F. solani and R. solani isolates tested, 40% gave disease severity ratings on Essex soybean that were significantly (P≤0.05) higher than for the control, using artificially infested, pasteurized P. Minor field soil. In greenhouse soil- temperature tanks, all Fusarium and Rhizoctonia isolates tested caused significant (P≤0.05) reductions in stem length and plant fresh weight after 13 days at 20 C in soil maintained at -0.01 MPa water potential. Disease severity ratings for all F. oxysporum and F. solani isolates were significantly (P≤0.05) higher than that for the control. All F. oxysporum, F. solani and R. solani isolates delayed emergence of seedlings, compared to the control, but final stands were affected only slightly. / M.S.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/106180 |
| Date | January 1987 |
| Creators | Farias, Graciela Maria |
| Contributors | Plant Pathology, Physiology and Weed Science |
| Publisher | Virginia Polytechnic Institute and State University |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis, Text |
| Format | xvi, 102 leaves, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 17234956 |
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