Cause Phoma clematidina (formerly Ascochyta clematidina), a fungus that overwinters in leaf and stem debris. Spores shed from pycnidia that form on old infected stems. Leaf infections can continue down the petiole and into the stem. The disease is favored by wet weather. Many of the large flowering hybrids are susceptible while the small flowered species are resistant.
Symptoms Leaves in the greenhouse may show small, water-soaked spots that turn buff-color with reddish margins. The infection can extend into stems, girdling them and causing shoots to wilt and die. Outdoor plants often are attacked near the ground.
Cultural control
- Remove and destroy infected leaves and stems.
- Space plants for good air circulation. Keep base of plants open so that it can dry out quickly after rain events.
- Grow in well-drained media with pH of 5.7 to 6.4.
- Time irrigation to minimize the amount of time leaves are wet.
Chemical control
- Although not specifically registered, according to IR-4 data, products such as Heritage are safe on this crop. These products may be of some benefit. Group 11 fungicide.
- Products that contain chlorothalonil such as Daconil have been used in other countries to control this disease. Test on a few plants first for phytotoxicity before wide-scale use.
- OHP 6672 4.5 F at 10 to 14.5 fl oz/100 gal water plus another fungicide. Group 1 fungicide. 12-hr reentry.
Note Fungicides that contain etridiazole such as Banrot, Terrazole or Truban are phytotoxic to clematis.
References van de Graff, P., O'Neill, T.M., Chartier-Hollis, J.M., and Joseph, M. E. 2001. Susceptibility of clematis varieties and species to stem infection by Phoma clematidina as an indicator for resistance to wilt. European J. of Plant Pathology 107:607-614.
van de Graff, P., O'Neill, T.M., Chartier-Hollis, J.M. and Joseph, M.E. 2003. Aspects of the biology and control of benzimidazole resistant isolates of Phoma clematidina, cause of leaf spot and wilt in clematis. J. of Phytopathology 151:442-450.