subterranean clover cv Leura Longer thinner less dense

subterranean clover (cv. Leura) Longer, thinner & less dense roots with long root hairs Phosphorus acquisition root disease interactions 0. 5 mm French serradella (cv. Margurita) • Root rot diseases are ever-present in pasture soils • Root rot fungi ‘prune’ the roots of sub clover in glasshouse studies this interferes with nutrient acquisition Q: Will root disease negate future attempts to breed sub clovers with more P-efficient root systems?

What we did (1) Field bioassays (cv. Woogenellup) in autumn-winter to measure: root damage, yield loss, and response to soil P SCORE 0 SCORE 1 SCORE 2 SCORE 3 Sub clover cv. Woogenellup (W 3, NSW; aut-wint 2009)

What we did (1) Field bioassays in autumn-winter to measure: root damage, yield loss, and response to soil P (2) Fungicide treatments using field microswards in spring to measure yield in response to soil P and fungicide application

What we found Autumn-winter • Soil P fertility did not constrain yield, even in low P soil (growth was more constrained by cool temperatures) • Substantial yield penalty associated with damaged roots (root pruning [bonsai] effect suspected) root damage Score 0 Shoot yield 1 (mg DM/plant) 2 3 critical P

Spring • Highly significant response to P-fertiliser application • No significant interaction or response to fungicide treatment (despite there being clear root damage at the site in autumn-winter) “Sawyer’s Gully” 200 Pasture growth rate (kg/ha/d) LSD: P level (P=0. 05) = 18 150 P 1 100 P 2 50 0 Water Nitram Thiram Apron + Apron Thiram Fungicide treatments

Key messages: Autumn-winter • P uptake does not limit growth by subterranean clover when low temperatures are more limiting for pasture growth. • Root damage directly constrains plant yield. controlling root pathogens or plant resistance to root damage will improve autumn-winter yields, irrespective of soil P fertility conditions. • Application of extra P-fertiliser will not counteract yield constraints due to root damage. Spring • Less evidence of root pathogen constraints to yield in spring. • No evidence that pathogens will restrict benefits of an improved root system.

Economic consequences of root disease on-farm Typical yield impacts due to root damage $300 12 wethers/ha 15 wethers/ha 18 wethers/ha $250 Net farm income ($/ha) $200 $150 $100 $50 $0 0% 10% 20% 30% 40% 50% Reduction in autumn-winter pasture growth • Ausfarm simulation using the GRAZPLAN pasture and animal models. • Merino wether enterprise / fertilised phalaris-annual grass-sub clover pasture at Canberra (1970 -2002). • Reduced autumn-winter production mimicked by restricting fertility scalar for 90 d from break of season.

Unanswered question – have we taken an effective enough approach reducing root disease impacts? 1 2 3 cv. Woogenellup seed-treated with Apron (metylaxyl) prior to sowing cv. Woogenellup untreated cv. Riverina untreated Surviving bioassay plants (subterranean clover) at 2 -3 leaf stage (Sawyer’s Gully, NSW; autumn-winter 2009) 8