Phoma clematidina (Thum.) Boerema is identified as the causal agent of leaf spot and wilt of large flowered Clematis hybrids in New Zealand. Three distinct strains of the pathogen can be distinguished. Phoma clematidina is a wound pathogen normally forming leafspots on both wilt susceptible and resistant cultivars. The final wilt symptom is usually the result of nodal rotting or "stem girdling" following hyphal extension into the node from the infected leaf, as P. clematidina is not a true vascular pathogen. Resistance to wilt was shown to be manifest by abscission or senescence of infected leaves. Pathogenicity is associated with production of a toxin, ascochitine (C15H1605), which could be isolated from leaf lesions and culture filtrate. A reverse phase High Performance Liquid Chromatography system was developed to confirm purity of the ascochitine preparation before identification by 1H Nuclear Magnetic Resonance (NMR) spectroscopy. Identification was confirmed by 13C NMR spectroscopy and mass spectroscopy. Ascochitine induced endogenous electrolyte leakage from sensitive leaf cells and caused blackening of leaf tissue similar to that observed in leaf lesions. The main organelles affected appear to be chloroplasts and mitochondria although extensive cellular damage is evident. Laboratory trials showed chlorothalonil and fenpropimorph to be the most effective fungicides in inhibiting P. clematidina spore germination and mycelial growth, and a mixture of these fungicides was used to effect control in the glasshouse. Cultural practices will however, remain an important factor in disease prevention and control.