COMPARISON OF THE PERFORMANCE OF SUBTERRANEAN CLOVER CULTIVARS IN SOUTHERN NEW-SOUTH-WALES .2. EFFECTS OF PHYTOPHTHORA-CLANDESTINA AND BROMOXYNIL ON SEEDLING SURVIVAL, GROWTH, AND SEED SET

Effects of the root disease caused by Phytophthora clandestina were assessed at Wagga Wagga on 14 cultivars of subterranean clover (Trifolium subterraneum) following application of the herbicide bromoxynil. Seedling death over autumn-winter ranged from 22% in Trikkala, the most resistant cultivar, to 96% in Woogenellup, the most susceptible. Compared with previous years (1984-85), herbage yields and seed set of the most susceptible cultivars were depressed. Cultivars with the most severe disease symptoms (Woogenellup, Nungarin, Northam) did not set seed. Later maturing cultivars (Karridale, Enfield) appeared to recover from the disease and set more seed than earlier maturing cultivars (Dalkeith, Daliak) with similar reactions to P. clandestina. Field susceptibility (determined by leaf damage scores, seedling losses, and dry matter yield depression) was similar to glasshouse phytophthora ratings for most cultivars; however, Trikkala, Karridale, and Enfield were more affected in the field following herbicide application than expected from the glasshouse reaction. Trikkala, Junee, and Dalkeith were the cultivars most tolerant in the field to root rot, while Woogenellup, Northam, and Nungarin were the most severely affected. Leaf damage following herbicide application was greatest in cultivars showing the most severe root lesions and having the highest susceptibility to P. clandestina in a glasshouse test. P. clandestina reduced the productivity of susceptible subterranean clover cultivars and has the potential to reduce long-term persistence by reducing seed set.