An-Najah Staff

Population levels of Pythium spp. in soils were determined from 36 fields subjected to different cropping practices. Fungi were present in 32 of the 36 soils, in all 25 irrigated soils and in 7 of the 11 non-irrigated soils; population levels ranged from 0 to 1066 propagules g⁻¹d.w. The mean numbers for irrigated soils under vegetables or under citrus were higher than those for non-irrigated soils under vegetables after cereals or under vines. Twenty species and three ‘species groups’ were isolated, of all these 20 from irrigated soils under vegetables, 14 from citrus orchards, 6 from non-irrigated vineyards, and 5 from non-irrigated soils under vegetables after cereals. Pythium ‘group H.S.’, P. rostratum, P. oligandrum, and P. minor were the only taxa of Pythium common to all categories of cultivated soils.Pythium ‘group H.S.’ was the most common ‘species’; it was present in 81% of all soils and in all soils under citrus. Its highest population levels were detected in irrigated soils under vegetables. Pythium rostratum was the next commonly isolated species; mean numbers of this species were nearly similar in irrigated and non-irrigated soils. Pythium ultimum was found in 39% of all soils; it was most common (61%) in irrigated soils under vegetables, but was absent in non-irrigated soils under vegetables. Pythium oligandrum was present in 31% of all soils but was more common in non-irrigated soils than irrigated soils. Several other species were only found in irrigated soils.