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  • Author or Editor: M.N. Schroth x
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The purpose of this symposium paper is to give an overview and progress report of developments on root-colonizing bacteria to promote plant health. This topic includes such subjects as factors affecting the dynamics of root colonization, mechanisms of microbial antagonism, use of bacteria to control diseases, and commercial development of microbiological inoculants. Papers cited below and reviews (3, 5, 27–30, 34, 39) provide a more detailed account of specific investigations on root-colonizing bacteria and plant health and microbial interactions in the rhizosphere. The rhizosphere is defined as the sphere about roots that is influenced by root activities, such as the exudation of chemicals.

Open Access

Walnut blight of English walnut (Juglans regia L.), incited by Xanthomonas campestris pv. juglandis (Pierce) Dowson, causes significant crop loss in California. To assess levels of resistance in walnut germplasm, leaves and nuts of mature walnut genotypes were inoculated with X. campestris pv. juglandis. Significant differences were found among cultivars in size and frequency of lesions on leaves and in frequency of abscission of diseased leaves. Cultivars also varied in frequency of abscission of nuts following infection and in marketability of infected nuts. Afthough there was considerable variation in disease levels over 2 years, leaves of PI 159568 consistently received significantly higher disease ratings than leaves of `Chandler' or `Adams'. Nuts of `Adams', `Payne', PI 18256, and `Sinensis 5' abscised less frequently following inoculation than nuts of other cultivars. In addition, the quality of infected nuts that did not abscise was consistently better for PI 18256 and `Sinensis 5'. The rank of cultivars for levels of disease in inoculated leaves was not significantly correlated with the rank of cultivars for frequency of infestation of dormant buds associated with infected foliage. The apparent resistance of walnut germplasm may be affected by the abscission or necrosis of infected tissues.

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