Influence of tillage and crop rotation on the epidemiology of Pythium infections of wheat in a red-brown earth of South Australia

Propagules of Pythium spp were concentrated in the top 10 cm and associated with soil aggregates >250 μm and <500 μm in a typical wheat-growing soil (red-brown earth) in South Australia. Propagule numbers were significantly affected by tillage and crop rotation. Over a 4-yr sampling period (1987–1990) propagule numbers were significantly higher in soil subjected to direct-drilling (no cultivation before crops were sown) than in soil subjected to conventional cultivation (2–3 cultivations before sowing). This effect was most consistent with a continuous-wheat rotation but was present in the wheat phase of a pasture-wheat and lupins-wheat rotation following a wet autumn in 1988. In 1988, Pythium propagule numbers were higher in soil that had been under pasture the previous year than in soil previously under wheat or lupins. This build up of Pythium inoculum was related to a significantly larger amount of particulate organic material (mostly fine roots from regenerating pasture plants) in this soil during the autumn before wheat was sown. The roots of these pasture plants were heavily infested with Pythium spp. The amount of wheat seed and wheat root that became infected with Pythium was related to the numbers of Pythium propagules in the soil and thus was highest in direct-drilled soil and in soil previously under pasture. Pythium spp could be isolated from different parts of the wheat root system throughout the entire growing cycle of the wheat plant. Two Pythium spp, P. irregulare and P. echinulatum were identified. P. irregulare was dominant and accounted for >80% of isolates recovered from wheat seed and wheat roots.