DIDYMELLA-EXITIALIS ON WHEAT IN NEW-ZEALAND

Didymella exitialis (Morini) Muller was identified on wheat (Triticum aestivum L.) leaves collected during a survey of New Zealand wheat crops in the 1990/91 season. This fungus was identified on nine cultivars in the major wheat-growing areas and was present in 19 out of 42 crops surveyed. In the 1992/93 season, D. exitialis pseudothecia first occurred in late November and became increasingly common as leaves and crops senesced. The teleomorph was more common than the Ascochyta anamorph. Pseudothecia of D. exitialis were frequently associated with leaf scorch symptoms, but were also often abundant over the surface of senesced leaves. The elongate lesions were tan to dark brown in the centre with diffuse margins. Thirty-two single ascospore isolates were made. Cultures sporulated sparsely on potato dextrose agar under near-UV light, producing the Ascochyta anamorph. Inoculation of wheat plants using pycnidiospores from culture resulted in disease symptoms, mostly on the margins and tips of leaves, after c. 2 weeks. Pseudothecia developed c. 2 weeks later and D. exitialis was re-isolated from infected leaves. Large differences in the leaf area covered with D. exitialis were found between cultivars in three field trials. 'Otane' and 'Norseman' were severely affected in all trials, whereas 'Monad' and 'Kokako' were almost unaffected. Didymella exitialis has not previously been recorded on wheat in New Zealand.