Abundance of Pythium Species in Nursery Soils in South Australia

Abstract

The abundance of Pythium propagules and frequency of 11 Pythium species were estimated in 16 soil plots in South Australian forest nurseries, in one ornamental nursery. and in one pine forest. A soil-plating technique Was utilized and a medium developed which was highly selective for these species and allowed their identification directly on the plates. The number of Pythium propagules in a unit volume of soil varied greatly. being lowest (24 per cubic centimetre) in the forest and highest (226 per cubic centimetre) in an alkaline loamy nursery soil. Loams had significantly more Pythium propagules than sandy soils. The immediately previous usage and soil amendments usually made no great difference to the total number of Pythium propagules. It is concluded that the abundance of pathogenic Pythium spp. in these nurseries is not the main factor governing the great variation found in the disease incidence of conifer seedlings. P. irregulare Buisman was frequent in all plots except the forest, and P. mamillatum Meurs in most sandy nurseries. The frequencies of P. ultimum Trow, P. paroecandrum Drechsler, P. rostratum Butler, P. iwayamai Ito, P. oligandrum Drechsler, P. acanthicum Drechsler, and P. echinulatum Matthews varied inconsistently between the plots. A new species, coded as P. col, was rare but occurred in eight plots. Another new species (P. dis) occurred only in one plot but frequently in this. This was a nursery freshly established on grassland pasture which differed from the others also in the frequencies of several other species.