Aspects of the Biology and Control of Benzimidazole Resistant Isolates ofPhoma clematidina, cause of Leaf Spot and Wilt in Clematis

Isolates ofPhoma clematidina, the cause of clematis wilt, were recovered from diseased clematis plants throughout England and testedin vitroandin vivofor their sensitivity to benzimidazole fungicides. More than one third of 14 isolates tested on fungicide‐amended agar plates were found to be highly resistant to products containing carbendazim, benomyl or thiophanate‐methyl. On unamended agar, the growth and morphology of benzimidazole resistant and sensitive isolates ofP. clematidinawere clearly distinct; on attached clematis leaves not treated with fungicide, the resistant isolates were markedly less virulent than the sensitive ones. In a 3‐month trial onClematiscv. Henryi, high volume sprays of a fungicide containing carbendazim reduced the incidence of leaf spotting caused by a benzimidazole sensitive isolate ofP. clematidinaby almost 80% but gave no significant reduction in disease caused by a resistant isolate. Fungicides found to be very effective against both benzimidazole sensitive and resistant isolates were products containing difenoconazole, azoxystrobin or kresoxim‐methyl. This is the first report of benzimidazole resistance inP. clematidinain the UK. Our findings indicate that growers should consider alternative fungicides and non‐chemical methods for the prevention and control of clematis wilt.