Occurrence of Pythium spp. in New Zealand soils, sands, pumices, and peat, and on roots of container‐grown plants

The major components of potting mixtures in New Zealand nurseries (viz, soil, sand, pumice, and peat) were assayed for the occurrence of Pythium spp., using a selective medium. Pythium spp. were recovered from all but 1 of 23 soil samples and from 13 of 20 sand and pumice samples. No Pythium spp. were recovered from the upper 25 cm of peat deposits at Waitakaruru, near Thames, or from the roots of plants growing in the area. Pythium spp. were recovered from a fenced‐off area of cultivated field soil on each of 53 sampling dates over 12 months. The populations ranged from 174 to 2203 viable propagules/g, and were found to be directly influenced by soil moisture but riot by soil temperature. Pythium spp. were found to be frequently associated with diseased or decaying roots of container‐grown plants in 12 North Island nurseries. A list of the hosts from which species of Pythium were isolated is presented. In the course of the survey 16 species of Pythium were recovered. P. acanthicum, P. ajertile, P. anandrum, P. aquatile, P. echinulatum, P. megalacanthum, P. middletonii, P. rostratum, P. splendens , and heterothallic forms of Pythium are recorded for the first time in New Zealand.