The host range of infectious hairy-root

Abstract

According to the literature, the distinctive symptom of the hairy-root disease is the formation of a mass of adventitious roots on the stem of the host plant. Hairy-root was divided into four diseases: infectious hairy-root, non-infectious hairy-root, crown gall and wound overgrowth. Infectious hairy-root, caused by Agrobacterium rhizogenes, seems to have the greatest economic importance. Its host range is not very well known. It has been found under natural conditions on apple trees and on two species of Spiraea. Cross-inoculation results are known for 33 plant species. We extended this information by wounding and inoculating 192 plant species, belonging to 83 families, with A. rhizogenes strain TR7. We also attempted to correlate the susceptibility of plants to hairy-root with known or presumed relationships according to Engler (1954, 1964) and Takhtajan (1969). Three lower plants and the only gymnosperm inoculated are not sensitive to the disease. None of our bacterial inoculations on 16 monocots resulted in hairy-root. Eighteen percent (=37 species) of the 202 inoculated dicotyledonous plants are host plants: they belong to 30 genera and 15 families. Thirty-three of the 37 host plants (=89%) belong to the closely related and highly developed subclasses Asteridae and Rosidae. There are indications that families which are typical phenol accumulators are more susceptible to the disease. The pathogenic specificity of A. rhizogenes was also compared with that of A. tumefaciens. Tumorigenic and rhizogenic activities can be induced by both bacterial strains. Up to now, there is no evidence that crown gall and hairy-root neoplasms are substantially different. The underlying concepts for root and tumor formation are discussed. © 1981 The New York Botanical Garden.