|Abstract or Summary
- Phytophthora lateralis Tucker and Milbrath, causal agent of a
serious fungus root rot of Chamaecyparis lawsoniana, has seriously
damaged natural stands in southwestern Oregon forests and affected
ornamentals throughout the Pacific Northwest. Progress with an effective
control program in the field has been limited by lack of critical
knowledge of fungal biology. Knowledge is needed of reliable means
of producing the various spore stages to be used, for example, in
inoculation studies or survival capacity studies.
Where culture is in liquid media, agitation enhances vegetative
growth. Gentle agitation results in normal growth morphology and in
dry weight production as great as that of vigorously shaken cultures.
Increase in fungus growth of shaken cultures in contrast to still
cultures is independent of oxygen concentration. pH influences
P. lateralis by favoring growth in the more acidic range from 4.5 to
5.5 and inhibiting growth in the range from 6.5 to 7.0. The reaction
of the medium from still cultures becomes slightly more acidic as
the cultures develop, whereas medium from shaken cultures becomes
much less acidic or even slightly basic with growth.
P. lateralis was found to have a partial growth requirement for
calcium. The effect occurs only when p-sitosterol, which is not required
for growth, acts synergistically with the calcium (10⁻³ M).
The fungus does however require sterols ((i-sitosterol, cholesterol)
for production of chlamydospores and sporangia.
Sporangial production, but not growth, is greatly stimulated by
illumination (190 foot-candles, supplied by a combination of cool white
and near-ultraviolet 40 watt fluorescent lamps) but only when colonies
are grown in a medium containing sterols. Between temperatures of
10 and 25 C more sporangia form in cultures illuminated for 12 hrs
each day than form with continuous illumination. Illumination, even
at relatively low light intensity (25 foot-candles) inhibits chlamydospore
production by two-thirds. Higher light intensities cause correspondingly
The optimum temperature for chlamydospore production is
about 24 C; fewer spores form at lower temperatures and these are
notably smaller and have thinner walls. The optimum temperature
for chlamydospore production is the same as the maximum temperature for growth. Best sporangial production occurs at
14 to 15 C. Neither sporangial or chlamydospore production is
stimulated by diurnal fluctuations of temperature.
Detached chlamydospores germinate readily over a wide range
of temperatures, with good germination between 17 and 23 C. A
dormancy period is not required. In nutrient media germ tubes
develop into colonies while in deionized water functional sporangia
form. The optimum temperature for zoospore discharge by sporangia
is 12 to 13 C. More zoospores are released in the dark than in