Answer: Your vines are most likely dying because of clematis wilt (Ascochyta clematidina), says Margery Daughtrey, plant pathologist at Cornell University's Long Island Horticultural Research Station in Riverhead, New York. Clematis wilt fungus is common on large-flowering clematis varieties. The symptoms begin as small, water-soaked leaf spots with red margins. The fungus enters the stem through the leaf petiole and eventually forms a canker near the soil line on the stem, choking it and causing all growth above the canker to die, says Daughtrey. The roots, however, usually stay healthy and, if the plant is more than two years old, it'll have enough reserve energy to send up new stems. The best defence against clematis wilt is to plant in a site with good air circulation, move mulch away from the stems and pinch off and destroy any leaf-spot-infected leaves, says Daughtrey. You can also spray the upper and lower surfaces of leaves with a fungicide such as sulfur or benomyl when you begin to noticeleaf spot damage, she adds. Keeping the plants well watered, especially during hot spells in summer, and cleaning up plant debris where the fungus overwinters will help lessen the chances of getting wilt next year.
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