Graduate Thesis Or Dissertation
 

Survival of Phytophthora lateralis in an organic matter fraction of the soil

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  • 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 months. 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.
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Déclaration de droits
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