Date of Award

1982

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

A disease herein referred to as Xanthomonas leaf spot (XLS) of crucifer caused by Xanthomonas campestris v. armoraciae (Xca) has been observed on Market Prize cabbage grown in southern Louisiana since the winter of 1977-78. XLS differs from black rot of crucifers caused by X. campestris pv. campestris (Xcc) in that no chlorosis is associated with the lesions, the pathogen invades stomata but fails to move systemically, and the disease develops at temperatures well below those favorable for black rot development. Inoculation of field-grown cabbage, cauliflower, and broccoli with Xca showed that these crops are susceptible and that disease severity varies among cabbage cultivars. Xca was shown to be seedborne in broccoli by isolation of the bacterium from seeds and observation of infected seedlings grown from infested seed lots. Two leaf spotting xanthomonads of crucifers have been reported. They are Xca, the cause of a bacterial spot of horse-radish, and X. campestris pv. raphani, the cause of a bacterial spot of radish and turnip. Plant inoculation studies comparing the XLS bacterium with Xcc, Xca, and Xcr showed that the XLS bacterium, Xca, and Xcr have identical host ranges which are wider than the host range of Xcc. Serological tests showed a close relationship among Xcc, Xca, Xcr, and the XLS bacterium. Phage tests showed that the XLS bacterium, Xca, and Xcr were of a single lysotype which is related to but different from the lysotype of Xcc. Based on the results of this study, it is concluded that Xca, Xcr and the XLS bacterium are indistinguishable and should be considered as a single pathovar. X. campestris pv. armoraciae (McCulloch 1929) Dye 1978 was named first giving it priority over X. campestris pv. raphani (White 1930) Dye 1978 which now becomes a synonym of X. campestris pv. armoraciae. Xanthomonas leaf spot of crucifers is suggested as the common name for the disease incited by X. campestris pv. armoraciae.

Pages

73

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