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Title: Comparative studies on mycoparasitic Pythium species
Author: Laing, Steven Alexander Kirk
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1989
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Pythium mycoparasiticum is described as a new species. Its mycoparasitic behaviour was compared with those of P.oligandrum and P.nunn against nine species of potential host fungi- P.graminicola, P.vexans, Rhizoctonia solani (2 isolates), Fusarium culmorum, F.oxysporum f sp lycopersici, Botrytis cinerea, Botryotrichum piluliferum, Trichoderma aureoviride and Phialophora sp. In interactions on filter paper or cellulose film, the degree of aggressiveness of each mycoparasite and the degrees of resistance of host fungi were assessed by the difference in cellulose breakdown caused by the hosts alone or in dual inoculations with mycoparasites. P.oligandrum was the most aggressive mycoparasite, active against most hosts; P.nunn was least aggressive, with a narrow host range and P.mycoparasiticum was intermediate in these respects. Of these hosts, Pythium spp and R.solani were least susceptible to parasitism, and T.aureoviride and Phialophora sp were most susceptible. B.cinerea, some other hosts to a lesser degree, was susceptible to P.oligandrum but not to P.nunn. Some of these findings were confirmed by the abilities of mycoparasites to grow on potato-dextrose agar precolonised by the host fungi; this assay method was not as sensitive as others, but it could be used to compare potential host ranges of mycoparasitic Pythium spp and the cellulolytic mycoparasites, Trichoderma harzianum and Gliocladium roseum. Interactions between individual hyphae were studied by videomicroscopy on films of water agar. The incidence and timing of different events were assessed from replicate interactions on videotapes and statistically analysed. The results generally confirmed those of interactions on cellulosic substrates, but several new details of interactions were observed. All three mycoparasitic Pythium spp had similar modes of parasitism, different from those of T.harzianum and G.roseum, used in a small comparative study. They did not affect host hyphae before contact, and none showed tropism to host hyphae. Soon after contacts involving parasite or host tips, the susceptible host hyphae stopped growing and either lysed at the point of contact or exhibited vacuolation/coagulation of their cytoplasm, starting at this point; this was often followed by penetration by mycoparasite. In some cases the parasites penetrated directly, somtimes after proliferating on the host surface. The parasites branched at points of contact and penetrated from these branches. Lysis and cytoplasmic coagulation typically preceded penetration or even the origin of a penetrating branch from the mycoparasites. Based on the timing of these events and the number of hosts affected, P.nunn was significantly the least aggressive mycoparasite, whereas P.oligandrum and P.mycoparasiticum were equally aggressive. The hosts differed in resistance as on cellulosic substrates. Mycoparasitism by P.oligandrum was investigated by inactivating the parasite or host hyphae with fine beams of intense light prior to contact on water agar. The results demonstrated an essential role of mycoparasite activity in host stoppage, lysis and cytoplasmic coagulation. However, lysis also required activity by the host. The findings are discussed in relation to postulated surface recognition events and the respective roles of host- and mycoparasite-derived wall-lytic enzymes. In limited tests, P.nunn was found to be unique among mycoparasitic Pythium spp in its ability to utilise inorganic (nitrate) nitrogen. Germination of oospores of P.oligandrum was found to depend on length of storage in culture and on conditions of aeration and illumination. Germination was stimulated by peptone and, to a lesser degree, malt extract, and also acetaldehyde among the volatile metabolites that were tested. The results of all these studies are discussed in relation to the roles or potential roles of the mycoparasites in biocontrol of plant pathogens.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available