A comparative study of Verticillium albo-atrum isolates causing fluctuating and progressive wilt of hops
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 Vertiaittium atbo-atrum
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
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
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 vessellengths
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
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
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.