INFLUENCE OF TILLAGE AND CROP-ROTATION ON THE EPIDEMIOLOGY OF PYTHIUMINFECTIONS OF WHEAT IN A RED-BROWN EARTH OF SOUTH AUSTRALIA

Propagules of Pythium spp were concentrated in the top 10 cm and assoc iated with soil aggregates > 250 mu m and < 500 mu m in a typical whea t-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 high er in soil subjected to direct-drilling (no cultivation before crops w ere sown) than in soil subjected to conventional cultivation (2-3 cult ivations before sowing). This effect was most consistent with a contin uous-wheat rotation but was present in the wheat phase of a pasture-wh eat and lupins-wheat rotation following a wet autumn in 1988. In 1988, Pythium propagule numbers were higher in soil that had been under pas ture the previous year than in soil previously under wheat or lupins. This build up of Pythium inoculum was related to a significantly large r amount of particulate organic material (mostly fine roots from regen erating pasture plants) in this soil during the autumn before wheat wa s sown. The roots of these pasture plants were heavily infested with P ythium spp. The amount of wheat seed and wheat root that became infect ed with Pythium was related to the numbers of Pythium propagules in th e soil and thus was highest in direct-drilled soil and in soil previou sly under pasture. Pythium spp could be isolated from different parts of the wheat root system throughout the entire growing cycle of the wh eat plant. Two Pythium spp, P. irregulare and P. echinulatum were iden tified. P. irregulare was dominant and accounted for > 80% of isolates recovered from wheat seed and wheat roots.