July 2015 Plant Disease Advisory
Annual vinca (also called Madagascar periwinkle, Catharanthus roseus) is a popular, colorful, summer annual for landscape beds. Its popularity wanes, however, when it develops Phytophthora aerial blight, a disease that causes severe browning of stems and foliage. Phytophthora species, commonly known as “water molds”, are common causes of root rot disease on plants grown in poorly drained soil.
Although annual vinca may occasionally develop root rot due to Phytophthora, infection of this plant species typically occurs above ground, resulting in a rapid blighting of the foliage. Brown, watersoaked lesions appear on individual stems or at the base of the plant, causing all of the foliage above the lesion to wilt and turn brown. Disease development is favored by hot, wet weather, and plants that receive overhead irrigation and/or heavy fertilization are more prone to the disease. When conditions are conducive to disease, plants may appear to decline overnight.
Annual vinca should be planted in well-drained soil. If plants are overhead irrigated, water early in the morning to avoid prolonged periods of leaf wetness. To reduce the number of spores available for new infections, remove infected plants as they appear. Mulching plants can also help to prevent splashing of spores from the soil surface onto the foliage. Fungicides, such as fosetyl-Al or phosphorous acid, can be used for preventative control, but regular applications will be necessary because infection can occur all summer long. Until recently, few resistant cultivars were available; however, breeders have now developed the cultivar CoraTM, which has good resistance to Phytophthora aerial blight. This cultivar is now available in five different colors.