|Abstract or Summary
- Chamaecyparis lawsoniana, an extremely valuable timber tree
species, is highly susceptible to a serious, soil -borne root disease
caused by Phytophthora lateralis. Disease outbreaks of epidemic
proportions are continually causing high mortality in natural stands
of the host tree. Practical control measures rely on an understanding
of the many factors of disease dynamics. One important factor is
pathogen survival. The primary purpose of this investigation was to
develop a better understanding of the survival capacity of P. lateralis
in soil and organic debris.
Survival studies are dependent on techniques used to detect
organism presence. A technique was developed whereby P. lateralis
could be detected in the soil. The fungus could generally be detected
only in an organic matter fraction of the soil. This organic fraction
was separated from the mineral fraction and used as the substrate
from which the fungus was baited. When baits of cedar foliage were floated in water over the substrate, P, lateralis, if present, would
infect and sporulate on the baits. Baits were considered infected only
when sporangia of P. lateralis were observed.
The optimum temperature range for infection of baits was
between 15 and 20 C; the optimum incubation period was determined
as six days. Pre-flooding the organic substrate with distilled water
or a cedar root extract for three or seven days prior to baiting was
shown to increase the frequency of detection of P. lateralis over substrate
which was baited immediately. Blending the organic substrate
had no apparent effect on detection.
When an organic matter fraction of an infested soil was
collected and stored at temperatures of 5, 10, 15, or 20 C for a
period of five months, no apparent loss of infectivity occurred, indicating
a high survival capacity of the fungus. P. lateralis was able to
survive in organic material stored for four months at -5 C, 14 weeks
at 25 C, or two weeks at 30 C.
Two naturally infested soils were each stored for periods of up
to four months. Samples of each soil were stored at four different
moisture levels at each of two temperatures (5 C and 20 C). Survival
of P. lateralis was poor in soil stored at moisture levels equivalent to
less than -15 bars matric water potential. Survival of the fungus was
also decreased when soils were stored under saturated conditions at
room temperature. P. lateralis stored in soils at moisture contents between these two extremes lost little infectivity after four
Organic matter highly infective with P. lateralis was added to
three soils, and each soil was subjected to various biological treatments.
Soils were steamed, planted to cedar (host species), alder,
and Douglas-fir (non-host species) and left untreated. The fungus was
recovered from the organic matter from all treatments after six
months. Survival of P. lateralis appeared best in soils planted to
cedar seedlings. The higher infectivity was probably attributable to
an increase in inoculum, not to a higher survival capacity of the
fungus. Since six months was the longest period over which survival
of P. lateralis was tested, further studies are needed to determine
the upper limit of its survival capacity.