Abstract
Phytophthora cinnamoni has been shown to occur commonly on a variety of woody plants particularly in the South of England, but also as far North as Aberdeen and to be causing serious losses in nurseries of Castanea sativa seedlings, Chamaecyparia cultivars, Rhododendrons, cultivated heathers and numerous other plants. Phytophthora cinnamomi and other root-infecting Phytophthoras have also been found causing significant losses in mature trees, particularly broadleaf. During this work, difficulties were sometimes encountered in the identification of isolates. Principal components and single link grouping cluster analyses of a limited number of characters and measurements of named species together with unidentified Phytophthora isolates enabled all, but one, of the latter to be related to known species. On the basis of these analyses, it is thought that a numerical taxonomic study might yield useful information regarding the taxonomy of the genus Phytophthora. studies on the behaviour of this pathogen in soil and in plant tissue have given an indication of its spectrum of behaviour in soil with regard to type and rate of spore formation, growth, rate of lysis and some of the factors influencing them. One of the most interesting observations was that of interactions between P. cinnamomi and Trichoderma virido. It was found that T. viride stimulated an A2 isolate of P. cinnamomi to produce germinable ocspores in soil, in the absence of the opposite mating type. It also appeared to be a natural and vigorous antagonist of this pathogen in soil. It has been observed that P. cinnamomi is able to overwinter in this country, as oospores and chlamydospores, in the absence of a host. Experiments have also shown that suction pressure and not absolute water content is one of the main factors controlling sporangial production in soil.