Chang Lee Hwa. / Thesis (M.Phil.)--Chinese University of Hong Kong. / Bibliography: leaves 141-147.

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Polyclonal antisera were raised against isolates of bean common mosaic virus (BCMV) and bean common mosaic necrosis virus (BCMNV) using conventional serological methods. Infected tissues containing, respectively, 22 recognized BCMV and BCMNV isolates were tested against the two antisera by antigen-coated plate (ACP) ELISA and double antibody sandwich (DAS) ELISA. Results indicated that each immunoglobulin was virus-specific by DAS-ELISA, providing clear distinction between BCMV and BCMNV. A reverse transcription, polymerase chain reaction (RT-PCR)-based assay in combination with restriction endonuclease analyses, was developed for molecular detection of BCMV, BCMNV and their pathogroups. Specific detection of the two viruses was accomplished by constructing two virus-specific primer pairs that amplified a PCR product specific for each virus. Distinction of two BCMNV pathogroups (PG-III and PG-VI) was achieved by restriction enzyme XbaI digestion of BCMNV PCR products. However, none of the tested restriction enzymes clearly differentiated the five recognized BCMV pathogroups. A primer pair Dts/Uny15 specific for BCMV pathogroup V was also developed. By its RT-PCR application, four BCMV-PG-V isolates were differentiated from the other known variants of BCMV pathogroup I, II, IV and VII. Thus, by a combination of RT-PCR and restriction enzyme analyses, it was possible to differentiate both viruses, and two pathogroups of BCMNV, and one pathogroup of BCMV. / Graduation date: 1996

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Indigenous Rhizobium isolates from naturally-formed bean root nodules were antagonistic to some of the root rotting pathogens of snap bean. Rhizobium isolates inhibited the radial growth of Fusarium species in dual culture agar plate tests but were not inhibitory to Rhizoctonia solani or Pythium isolates. With one exception, all indigenous Rhizobium isolates showed some degree of antagonism towards F. solani f. sp. phaseoli in vitro. The level of in vitro inhibition depended upon the agar plate technique utilized. A high level of in vitro inhibitory activity was recorded in 38% of the Rhizobium isolates tested where wide zones of inhibition formed between the test isolates and persisted for more than 1 week. A similar inhibitory effect of 8/17 nodulating Rhizobium isolates to Fusarium root rot of snap bean was observed in growth pouch experiments. Protection of bean plants from severe Fusarium root rot occurred in combinations where the inoculum concentration of Rhizobium (10\ 106 cells/pouch) was equal to or greater than the inoculum concentration of F_. solani (10² , 10⁴ spores/pouch). Ten Rhizobium isolates, which were highly antagonistic in vitro, had no apparent inhibitory effect on Fusarium root rot in vivo. Two Rhizobium isolates, RCC324 and RCC607, inhibitory to Fusarium root rot did not reduce Rhizoctonia root rot of bean. Soil experiments supported the results of growth pouch experiments whereby inoculation of bean seed with a high concentration of Rhizobium (RCC106 at 10⁸ cells/seed) effectively reduced bean root rot incited by a low inoculum potential of the pathogen, F. solani (inoculum: soil, 1:10⁴ or 1:120). These results indicated the potential exists for field control of Fusarium root rot of snap bean by a highly antagonistic nodulating isolate of Rhizobium. / Land and Food Systems, Faculty of / Graduate

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Survey of crops of common bean (Phaseolus vulgaris L.) in Sonora, Mexico revealed the presence of two isometric viruses and one flexuous rod virus on the basis of host reaction, particle morphology, serology and physico-chemical properties. The isometric viruses were identified as Bean Southern Mosaic Virus (BSMV) and Cowpea Chlorotic Mottle Virus (CCMV); the flexuous rod virus was identified as Bean Common Mosaic Virus (BCMV). Using bean cultivar differentials, two strains of the potyvirus BCMV were identified, NY-15 and a previously undescribed strain designated YV-1. Host range, serological tests, and RNA electrophoresis indicated that the Sonoran BSMV cultures are similar to BSMV-strain A. Serology and RNA-electrophoresis indicated that the Sonoran CCMV isolates are identical to CCMV-strain A. BSMV and CCMV were always isolated as a mixture from seed lots and from field collected bean tissue. BCMV occurred alone or in mixed infections with BSMV and CCMV. BCMV was seed transmitted with an average efficiency of 58 percent. The BSMV-CCMV mixture was transmitted with an efficiency of 6 percent. BSMV and CCMV were seed transmitted together, but separate transmission of BSMV or CCMV was not detected. Commercial seed lots from two major bean growing regions of Sonora (Hermosillo Coast, Sonora River) were contaminated with the BSMV-CCMV mixture but not with BCMV. The average contamination level was 13 percent. Two common weeds present in Sonoran agricultural areas were found to be potential alternate hosts of CCMV. Both Sisymbrium irio L. and Melilotus indica L. were infected systemically, although the infection in M. indica was latent. Potential losses due to Sonoran bean viruses were measured in greenhouse experiments with the cultivar Pinto 111. BCMV strains caused a 29.4 to 60.1% reduction, whereas BSMV-CCMV mixtures induced a 22.5 to 74.6% yield reduction. A synergism occurred between the BSMV-CCMV mixture and BCMV resulting in more severe symptoms and a yield reduction of 92.7%. Synergistic effects were also observed between BSMV and CCMV. Actual yield reduction resulted from impaired flower production and, consequently, reduced pod production. Significant effects on plant tissue production, flower fertilization and seed quality were not observed. Cowpea chlorotic mottle virus infected mung bean (Vigna radiata (L.) Wilczek) a previously unreported host. Infection of mung bean by BSMV was only possible when CCMV was present in the inoculum. Both BSMV and CCMV could be isolated from symptomatic plants infected with the BSMV-CCMV mixture, however, symptoms on mung bean were unchanged from infection by CCMV alone.