Graduate Thesis Or Dissertation
 

Growth and sporulation of Phytophthora lateralis in vitro as influenced by the chemical and physical environment

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  • 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 greater reduction. 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 the light.
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