This study defines the importance of environmental conditions occurring across southern Australian subterranean clover (Trifolium subterraneum) forages on the severity of Rhizoctonia (Rhizoctonia solani) damping-off and root rot and on consequent plant productivity. Environmental factors tested included temperature regimes 14/9; 18/13 and 22/17 °C day/night, high vs. low soil moisture; sand vs. loam soil; high vs. low nutrition, and clover cultivars Riverina, Seaton Park and Woogenellup. Environmental factors significantly affected the severity of damping-off, root disease and root and shoot productivity. Damping-off was at or close to 100% at the two cooler of the three temperature regimes. At the warmest of the three temperature regimes (22/17 °C), significant numbers of subterranean clover plants germinated and survived, and germination, dry root and shoot weights all increased conversely with decreased tap and lateral root disease under higher moisture, better nutrition and under ‘heavier’ soil conditions. Findings demonstrate how variations in environmental factors like temperature in particular, but also soil type, nutrition, moisture, individually and interacting, have profound effects on the expression and severity of Rhizoctonia damping-off and root disease and the consequent productivity of subterranean clover forages. Findings explain the severe devastation to subterranean clover pastures observed in the presence of this pathogen when cool seasonal conditions occur across the critical autumn feed-gap period; especially under relatively drier soil conditions and in nutritionally impoverished sandy soils where there is little competition from other soil microbes.