- Verticillium wilt of hops in Oregon, caused by Verticillium
dahliae Kleb., was first reported in 1956. Later, V. albo-atrum
Reinke & Berth. was also isolated from infected hop plants, and thus
both species of the fungus were shown to be the causal agents of the
disease in Oregon.
The purpose of this research was to study temperature relations
and host range of hop isolates of Verticillium existing in Oregon,
and also to determine infectivity and pathogenicity of these isolates to
different varieties of hops.
The temperature relations of two V. dahliae and two V. alboatrum
isolates, recovered from infected hops grown throughout the
Willamette Valley of Oregon, were studied by dry weight measurements.
Two additional V. dahliae isolates, one from peppermint and
one from potato, were included in the experiments, The isolates
were grown in Czapek. Dox broth plus yeast extract at ten temperatures
between 5° and 32°C.
The dry weight was affected by the interactions of isolate, temperature,
and period of incubation. At temperatures up to 17°C, the
dry weights of all isolates were increased by longer incubation periods
with no autolysis of the mycelial mass observed in 17 days.
Above 17°, however, the incubation periods required for maximum
dry weight production and the onset of autolysis varied, depending on
the temperature and the isolate. A faster initial growth rate, caused
by increasing temperature, hastened the onset of autolysis, and thus
did not result in greater amounts of dry weight production in longer
periods. On the other hand, a slow initial growth rate, caused by
too high a temperature, resulted in delay of autolysis and production
of greater dry weights in longer incubation periods.
V. albo-atrum isolates did not grow at 30°C and evidence was
obtained showing that failure to grow was due to thermal death of
germinated and ungerminated spore inoculum rather than to inhibition
of germination. All V. dahliae isolates grew fairly well at 30°C but
failed to grow at 32°C. The latter temperature was lethal to the
spore inocula of V. dahliae isolates after 7 to 12 days of incubation.
These results point out a significant physiological difference between
V. albo-atrum and V. dahliae and thus support the validity of considering V. dahliae as a distinct species.
In host range studies, one V. albo-atrum isolate (No. 138),
originally obtained from Fuggle hops, was found to infect and proliferate
greatly in stems of potato, and to a lesser extent in tomato and
eggplant. Among V. dahliae isolates, one originally recovered from
Bullion hops (No. 148), proliferated poorly in potato, eggplant, and
strawberry. The other (No. 150), obtained from Fuggle hops, proliferated
greatly in stems of peppermint and was highly pathogenic to
The conclusion was reached that the hop isolates of Verticillium
existing in Oregon, are primarily pathogens of other plants, with host
ranges that include hops to some extent.
Verticillium isolates from infected hops, peppermint and potato,
infected a low percentage of plants among 14 hop varieties grown in
infested soil. Differently prepared inoculum (laboratory infested
soils and straw culture) or different levels of inoculum, did not affect
the percentage of infected plants appreciably. All Verticillium isolates,
regardless of their origin or species, proliferated poorly in
stems of infected hop plants and no actual wilting or death of the infected
plants was observed.
It was concluded that the strains of Verticillium affecting hops
in Oregon are not virulent pathogens of this crop and that they are
primarily adapted to plants other than hops.