Periwinkle blight (aerial phytophthora) is caused by the fungus Phytophthora parasitica. It’s the number one disease problem for annual vinca or periwinkle (Catharanthus roseus) and can persist in the soil for several years. Sadly, it’s almost always caused by folks not understanding something known as the disease triangle which includes a host, a pathogen, and environmental conditions.
Under conditions of frequent overhead irrigation or rainfall, this disease spreads rapidly. The fungus is often introduced into the landscape by infected plant material. Aerial phytophthora can be a recurring problem for periwinkles once the disease has been established in residential or commercial plantings.
The initial symptoms of this fungus are the presence of water-soaked, gray-green, “greasy” areas on the shoots and leaves. This symptom is quickly followed by a sudden wilting of shoots. As the disease advances within the plant tissue, dark brown lesions develop on the stems. These lesions result in the death of the stem or entire plants. Under wet conditions, the microorganism can move from one plant to another merely by leaf-to-leaf contact or splashing water. When the foliage remains wet, the disease progresses very rapidly. Plants may be killed within one to two weeks after symptoms appear.
There are some simple rules for growing periwinkles. Ignore them at your own peril.
1. Do not plant periwinkles until after Mother’s Day (or even Memorial Day) each year. They are essentially from Africa and are a summer bedding plants, not a spring bedding plants. They like it hot and dry.
2. Choose only pristine transplants with no dead shoots or brown lesions on the stems.
3. Do not use overhead irrigation on periwinkles, always keep the stems and foliage dry, and do not water them at night. Aerial Phytophthora is spread by water. If you repeatedly water periwinkles, they will get this disease and die. After being initially watered in, periwinkles rarely need supplemental irrigation in East Texas, certainly no more than once every few weeks during June, July, and August only.
4. Always plant periwinkles in full sun in well-drained soils. In addition to more blooms, this ensures that the plants and soil do not stay too moist.
5. Do not over-fertilize periwinkles, as new succulent shoots are more prone to the disease.
6. Remove dead shoots or dying plants immediately and discard them in plastic trash bags with your garbage to minimize the disease in the future. Sanitation is critical in plant disease prevention.
7. Do not plant periwinkles in beds with a history of aerial phytophthora.
8. Plant periwinkles in the ‘Cora’ series, if you can find them, as they have some genetic resistance to the disease.
9. Chlorothalonil (Daconil) fungicide can be used to protect healthy plants from the disease but will not cure infected plants.
10. Plant periwinkles in raised beds and containers which provide extra aeration and drainage.
Successfully growing periwinkles is all about sanitation and minimizing water contact with the plants. Keeping them hot, sunny, and on the dry side is the key to success.
Greg Grant is the Smith County horticulturist for the Texas A&M AgriLife Extension Service. He is the author of Texas Fruit and Vegetable Gardening, Heirloom Gardening in the South, and The Rose Rustlers. You can read his “Greg’s Ramblings” blog at arborgate.com and his “In Greg’s Garden” in each issue of Texas Gardener magazine (texasgardener.com). More science-based lawn and gardening information from the Texas A&M AgriLife Extension Service can be found at aggieturf.tamu.edu and aggie-horticulture.tamu.edu.