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P h y t o p h t h o r a ....D i s e a s e s.... i n .....H o r t i c u l t u r a l ....C r o p s

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PHYTONET---A Network Project on Phytophthora Diseases in Horticultural Crops

The genus Phytophthora possesses some of the most sophisticated weaponry known among plant pathogens (Gregory 1983). “Phytophthora has been seriously recognized as a notoriously difficult genus- especially for those who wished only to identify an isolate and were not concerned with taxonomic problems and / or minute details”. (Waterhouse et al. 1983). The taxonomic key produced by Waterhouse (1963), a mycologist at The Commonwealth Mycological Institute at Kew, United Kingdom, was the first to categorise Phtophthora species into morphological groups. Waterhouse’s separation of Phytophthora species into six groups based on a series of morphological and physiological parameters. These parameters include branching patterns of the sporangiophores, the sporangium apex, abundance of sporangia on solid media, the non caducous or caducous nature of sporangia, internal proliferation of sporangia, production of oogonia and oospores in single culture (homothallic), production of oogonia and oospores only when mating types are paired in a culture. (heterothallic)., nature of antheridium, abundance or absence of oospores on host tissue or in culture, and for certain species, sporangium shape and dimensions, e.g. length- breadth ratios, sporangiophores, oogonium size, ornamentation of oogonial wall, host specificity, and cardinal temperatures. A revision of Waterhouse (1963) key included six new species and was based on the same parameters as the 1963 key, but was presented in a tabular key containing 67 species, 19 of which have been described since1978 (Stamps et al-1990). It incorporates the original groups 1-6 of the previous key with the addition of group 7 for the marine species.

Despite the large number of available keys, the identification of a species is still considered to be difficult, principally because there are relatively few morphological features by which the species are distinguished and great variability and overlapping features exist within species (Leonian 1934, Erwin 1983). Many Phytophthora species have not been identified into species (E. M. Hansen et al. 1979).

Broad variability occurs within phytophthora capsici. P.capsici has been described to include isolates previously designated as P.palmivora MF4 from cacao (Brasier and Griffin 1979) and black pepper (Piper nigrum). As well as isolates from Pepper (Capsicum annum).( Tsao and Alizadeh 1988, Tsao 1991, Mchau and Coffey 1994). This study attempts to identify and classify Phytophthora isolated from black pepper tissue and soil, betel vine tissue and soil as well as from other hosts.