THE GENUS PYTHIUM - An-Najah National University

I. INTRODUCTION
The genus Pythium is now regarded as the type genus of the family Pythiaceae
Schroter in the order Peronosporales of the class Ooomycetes (Waterhouse,
1973; Plaats-Niterink, 1981). The genus was introduced by Pringsheim (1858).
His genus was based on two fungi, Pythium monospermum and P. entophytum.
The latter fungus was transferred to Lagenidium (Zopf, 1890), so that P.
monospermum remained as the type species. Pringsheim (1858) placed Pythium
in the Saprolegniaceae, but de Bary (1881) placed the genus in the
Peronosporaceae. Schroter (1897) created the family Pythiaceae for the genera
Pythium and Nematosporangium. Butler (1907) did not accept Nematosporangium
as representing a distinct group of species, so he included all the species under
Pythium.
Monographs on the genus have been published by Butler (1907), Matthews
(1931), Sideris (1931, 1932), Middelton (1943), and Frezzi (1956). Waterhouse
(1967, 1968) compiled the original descriptions and illustrations of all Pythium
taxa described at that time; a total of )79 taxa. She produced a key for eighty
nine taxa which she considered as .valid and rejected ninety taxa. This key
depended heavilly on the original descriptions of the species. Since many of
these were produced from the study of single isolates, without taking into
consideration the possible sources or ranges of variation, original descriptions
were often inadequate for comparative purposes. As a result the key frequently
has to depend upon small differences in reproductive cell-size; it also emphasizes
some nonmorphological characters. Robertson (1980) produced a key to the
twenty seven species of Pythium recorded so far from New Zealand; illustrations
and descriptions were given for sixteen species. Plaats-Niterink (1981) published
the latest monograph of the genus Pythium. She recognised and described eighty
five species, seven of which are heterothallic. Descriptions were again based on
single isolates. Therefore, extent of intraspecific variation was not accounted for
in both Robertson and Plaats-Niterink keys. However, it is difficult to assess
intraspecific variation from published descriptions since the majority of these
were prepared from single isolates.
Pythium species occupy a wide diversity of habitats. They can be found as
saprobes in fresh and salt water and in soil, and as parasites in aquatic and
terrestrial plant material, fresh water and marine algae, fungi, invertebrates and
vertebrates (Middleton, 1943; Plaats-Niterink, 1981).
Species of Pythium have been often referred to as 'sugar fungi' (Garret, 1951;
Burges, 1958), a term which infers their inability to degrade complex
carbohydrate polymers, like cellulose. However, it has been recently found that
several species of Pythium are capable of degrading cellulose (Taylor & Marsh,
1963; Tribe, 1966; Willoughby & Redhead, 1973; Park, 1980 a,b; Deacon, 1979).
It has been therefore suggested (Deacon, 1979) that the cellulolytic activity of
Pythium species removes these fungi from the substrate group 'sugar fungi'.
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