| Summary: | A comparative study was made of physiological and biochemical properties in vitro and of pathogenicity to hop of fluctuating (M) and progressive (V) isolates of Verticillium albo-atrum (Vaa). Symptoms were expressed on leaves 1-3 nodes behind the invasion front in bines. However, symptom expression and reduction in vascular flow in petioles occurred prior to petiolar colonization. Rates of colonization by M and V isolates in susceptible hops were similar. Assessment of numerous enzyme activities, and growth rates on a wide range of carbohydrates indicated considerable variation but did not enable differentiation of M and V isolates. Extracellular polygalacturonase activity was detected from the start of germination and differences in production between M and V isolates were apparent during the first 24 h. Iso-electric focusing of polygalacturonidases revealed no bands unique to any isolate but > 14 separate isozymes were common to Vaa isolates from hop, tomato and V. dahliae (ex cotton). No evidence was found of a Vaa toxin able to alter host cell permeability. Culture fluids contained high molecular weight polysaccharide(s) and caused a rapid decrease in vascular flow rates through excised petioles. There were apparently no host enzymes capable of degrading the polysaccharide(s). Resistance to Vaa of some cultivars was not related to vessel- lengths in bine, root or petiole. A high proportion of conidia were rapidly bound within xylem vessels; also proteins from hop agglutinated conidia, but neither phenomenon was influenced by isolate virulence or host resistance. 2-D electrophoresis of proteins from mycelium also failed to reveal differences between M and V isolates, but did distinguish Vaa from V. dahliae. Vascular occlusion was artificially induced in hop shoots and roots by infiltration with auxin (NAA), Vaa conidia or both. The extent and speed of occlusion was not determined by isolate virulence. Gels formed in 2 days, tyloses in 4-8 days and phenolic infusion of the vessels and occluding structures followed soon after tylosis. Antifungal compounds, induced similarly by inoculation with M or V isolates, were extracted from hop tissue. They caused slight inhibition of Vaa germ-tube growth. No pre-formed inhibitors were detected. Hop isolates of Vaa colonised and produced symptoms on four non-host species but M and V isolates displayed similar virulence to these species. The relative pathogenicity of M and V isolates is discussed and it is suggested that these isolates do not exist as two distinct strains, but exhibit a continuum of pathogenicity.
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