Stem rot of clematis
Clematis is subject to infection by several diseases including powdery mildew (Erysiphe ploygoni), crown gall (Agrobacterium tumefaciens), and various leaf spots (Cercospora squalidula, Septoria clematidis). However, the most serious fungal disease of clematis is Ascochyta clematidina, which causes leaf spot and stem rot .
Leaf infections in the home landscape are rare, when they do occur they appear as small, water-soaked spots. Later, these spots become large, tan lesions, with red margins. More often, infection occurs on individual stems near the soil line. Red lesions near the base of the stem eventually enlarge and girdle the stem, causing the upper portions of the plant to wilt and die. Later in the season, stem lesions turn gray and produce black spore-containing structures called pycnidia. The fungus survives winter and releases spores in spring and early summer, which infect the new growth.
Diagnosis and control of this disease may be difficult since the fungus infects stems near the soil line and survives on old stem debris. To minimize disease, increase ventilation within the plant by supporting vines on trellises and planting them farther apart in well-drained soil. Remove and properly dispose of all infected stems. Fungicide sprays may aid in controlling this disease. Apply wettable sulfur before new growth starts in the spring and early summer. Repeat fungicide applications may be necessary. Carefully read and follow directions on the fungicide label.
Pirone, P.P. 1978. Diseases and Pests of Ornamental Plants. John Wiley and Sons, New York. p.205.
Chad Behrendt, Crystal Floyd