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

Isolates of Phoma clematidina, the cause of clematis wilt, were recovered from diseased clematis plants throughout England and tested in vitro and in vivo for 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 of P. clematidina were 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 on Clematis cv. Henryi, high volume sprays of a fungicide containing carbendazim reduced the incidence of leaf spotting caused by a benzimidazole sensitive isolate of P. clematidina by 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 in P. clematidina in the UK. Our findings indicate that growers should consider alternative fungicides and non‐chemical methods for the prevention and control of clematis wilt.