With the aim of finding simple and effective disease-control measures, laboratory experiments described in this thesis tested the effects of various cations (Ca<sup>2+</sup>, Mg<sup>2+</sup>, K<sup>+</sup>) and potentially fungitoxic compounds (saponins, gramicidin S, ethanol) on different stages of the life cycle of two pathogenic zoospore-forming fungi, <i>Pythium aphanidermatum </i>and <i>Phytophthora parasitica.</i> Also, nutrient irrigation solutions from various experimental treatments in a large tomato-cropping glasshouse trial at Horticultural Research International (HRI, Yorkshire) were tested for effects on the two fungi in laboratory conditions. Cations, fungitoxic compounds and experimental irrigation solutions were tested for effects on the following aspects of fungal behaviour: mycelial growth, production of sporangia, release of zoospores from sporangia, zoospore motility, zoospore encystment and cyst germination. When tested on individual stages of the life cycle, high concentrations of Ca<sup>2+</sup> and Mg<sup>2+</sup> in nutrient broth reduced mycelial growth by <i>Py. aphanidermatum </i>but not <i>Ph. parasitica.</i> Sporangial production by <i>Ph. parasitica</i> in mineral salts solution was unaffected by amendments of Ca<sup>2+</sup>, Mg<sup>2+</sup> or K<sup>+</sup>, but these amendments suppress the ability of sporangia to subsequently liberate zoospores into water; the exception was 5 mM Ca<sup>2+</sup> which was markedly enhanced subsequent zoospore release. Increasing concentrations of Ca<sup>2+</sup>, Mg<sup>2+</sup> and K<sup>+</sup> in the solution that bathed pre-formed sporangia of either fungus reduced the number of zoospores that were released. These three cations also suppressed the proportion of zoospores that remained motile, and increased the proportion of vortex-encysted zoospores that would germinate. When tested cumulatively on all stages of the life cycle K<sup>+</sup> was more effective than Ca<sup>2+</sup> in suppressing the infection related behaviour of both fungi. These experiments suggest that increasing the ratio of K<sup>+</sup> to Ca<sup>2+</sup> in bathing solutions should suppress the ability of both fungi to spread and cause disease. The trial undertaken by HRI failed to demonstrate natural suppression in a semi-commercial irrigation system, which was the principal trial objective. Consequently the analyses presented in this thesis were unable to determine at which points of the infection sequence the agents of natural suppression might act.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:657445 |
| Date | January 2000 |
| Creators | Matthews, Paul Wade |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/15287 |
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