Graduate Thesis Or Dissertation
 

The role of Verticillium albo-atrum Reinke and Berth in seed transmission and induction of a latent disease of Pelargonium

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  • Many initially healthy Pelargonium seedlings developed symptoms of Verticillium wilt in an Oregon State University greenhouse when they attained blooming age. The symptoms first appeared in May as "edge burning," yellowing, and wilting of leaves. In late summer, these seedlings which were the offspring of Verticillium infected plants, became severely diseased and developed terminal die-back and other symptoms similar to those of their parents. Experiments conducted to determine the role of seeds in Verticillium transmission of this apparently latent disease failed to give any evidence of Verticillium transmission. Seed transmission studies of both naturally infected and inoculated plants indicated that geranium seeds did not play an important role in dissemination of the Verticillium fungus. Contrarily, culturing the seeds from naturally diseased plants proved them to be infested with bacteria. Results of isolation from naturally infected seedlings indicated that Verticillium was not responsible for the induction of the disease in these seedlings and that bacteria were probably causal organisms in the disease syndrome. Isolation studies were made also from the parent plants which showed diverse symptoms described in 1961 as due to Verticillium albo-atrum. Verticillium was not isolated consistently from these plants in 1963. The 1963 tests revealed a rather high bacterial infection and indicated that diverse symptoms in these plants in 1961 could have been in part due to bacterial infection. In a series of inoculation tests a pathogenic strain from peppermint and two geranium isolates of Verticillium were used. The symptoms of inoculated plants induced by these strains were different from those of naturally infected plants. Necrosis appeared at the base of stems; "edge burning," a common symptom of naturally infected seedlings, was scarce in inoculated plants. Verticillium was easily recovered in PDA from plants inoculated with the mint strain. Contrarily, difficulties were experienced in isolating Verticillium from naturally infected plants and from the plants inoculated with geranium strains of Verticillium. Seedlings from infected plants which were germinated and grown under sterile conditions showed bacterial symptoms and yielded a high percentage of bacteria in culture. Two of the bacterial isolates, tentatively identified as Xanthomonas and Pseudomonas species, proved to be pathogenic. Other isolates did not show any pathogenicity in preliminary tests.
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