THE GENUS PYTHIUM - An-Najah National University

Little is known about the distribution of population of Pythium species in soil
either on a global or on a local basis, particularly in quantitative terms. The
introduction of selective media has enabled estimates of propagule numbers to
be made. Pvthium population under pine in Georgia (U.S.A.) were determined
by Otrosina and Marx (1975) who found means for numbers of propagules from
all sites to range from 0-5.2 propagules.g dry wt-I over all seasons; isolates
corresponding to the Pythium irregulare-P. debaryanum complex were most
frequently isolated. Lumsden et al. (1976) and Pieczarka & Abawi (1978) studied
Pythium species population in bean fields in Maryland and New York
respectiverly, and found Pythium isolates with spherical sporangia and which
did not produce oogonia and P. uftimum to be most frequent.
Pythium population in forest nursery soils in Australia and Canada and in the
south eastern United States were studied by Vaartaja (1964,1968) and Hendrix
& Campbell (1968) respectively. They found P. irregulare to be the most
abundant species.
The distribution of Pythium were studied by Hendrix & Campbell (1970) in
different soils from many parts of the USA. Heterothallic forms together with
isolates referable as P. irregulare- P. debaryanum complex, were found to
dominate Pythium flora in the soils studied.
Twenty eight to thirty five years ago,Remy (1950), Warcup (1952) and Barton
(1958)all concluded that Pythium species were abundant in cultivated soils but
infrequent in uncultivated, undisturbed soils. However their conclusions were
based on recovery techniques which are known to be less satisfactory for
quantitative studies, thus making direct comparisons of population levels
impossible. Using a selective medium (VP3) and the surface-soil-dilution-plate
procedure, Ali (1982), Ali-Shtayeh et af (l986a,b), Ali-Shtayeh & Dick (1986);
and Dick & Ali-Shtayeh (1986) studied Pythium populations in a large number
of cultivated and uncultivated soils in Britian and found Pythium species to be
more abundant in cultivated farmland soils than in undisturbed, uncultivated
parkland soils. Species widely distributed in uncultivated soils were P.
sylvaticum, P. rostratum, P. parvum, and P. acanthicum .. The abundant and
widespread species in cultivated farmland soils were P. minor, P. parvum, P.
rostratum and P. vanrerpoolii.
Pythium populations in a wide range of irrigated and nonirrigated fields in the
West Bank and Gaza Strip have been studied by the author. A part of this work
has now been published (Ali, 1985, Ali-Shtayeh, 1985,1986 a,b).
In this monograph a guide to the isolation and identification of Pythium
species is provided. Descriptions are also provided for 31 species. The remaining
species isolated from the West Bank and Gaza Strip are outlined briefly. All the
plants from which Pvthium species have been recovdred are listed.
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