I recently attended the Horticulture Field Days at the Texas A&M AgriLife Research and Extension Center at Overton. As always, I was amazed at the new color range in periwinkles. They are a great bedding plant, if and only if you know how to grow them and avoid periwinkle blight.
Periwinkle blight (aerial phytophthora) is caused by the fungus Phytophthora parasitica. It’s the No. 1 disease problem for annual vinca or periwinkle (Catharanthus roseus) and can persist in the soil for several years.
Under conditions of frequent overhead irrigation or rainfall, this disease can spread rapidly. The fungus is often accidentally introduced into the landscape by infected plant material. Aerial phytophthora can be an annual problem for periwinkles once the disease organism has been introduced into residential or commercial plantings.
The initial symptoms of disease onset 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 tissues, dark brown lesions develop on the stems. These lesions result in 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. When the foliage remains wet, the disease progresses very rapidly. Plant may be killed within one to two weeks after symptoms appear.
There are a number of simple rules for growing periwinkles. Ignore them at your own peril.
Do not plant periwinkles until after Mother’s Day each year. They are essentially from Africa and are a summer bedding plant, not a spring bedding plant. They like it hot and dry.
Choose only pristine transplants with no dead shoots or brown lesions on the stems.
Do not use overhead irrigation on periwinkles. 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.
Always plant periwinkles in full sun in well drained soils. In addition to more blooms, this insures that the plants and soil do not stay wet.
Do not over-fertilize periwinkles, as new succulent shoots are more prone to the disease.
Remove dead shoots or dying plants immediately and discard in plastic trash bags with your garbage to minimize the disease in the future.
Do not plant periwinkles in beds with a history of aerial phytophthora.
Plant periwinkles in the Cora series if you can find them, as they have some genetic resistance to the disease.
Chlorothalonil (Daconil) fungicide can be used to protect healthy plants from the disease but will not cure infected plants.
Successfully growing periwinkles is all about sanitation and minimizing water contact with the plants. Keeping them hot, sunny and on the dry side will let you enjoy all the new colors, too.
Greg Grant is the Smith County horticulturist for the Texas A&M AgriLife Extension Service. He is author of “Texas Fruit and Vegetable Gardening” and co-author of “Heirloom Gardening in the South” and “The Rose Rustlers.” You can read his “Greg’s Ramblings” blog at arborgate.com, his “In Greg’s Garden” in each issue of Texas Gardener magazine (texasgardener.com), or follow him on Facebook at “Greg Grant Gardens.” More research-based gardening information from the Texas A&M AgriLife Extension Service can be found at aggie-horticulture.tamu.edu and plantanswers.com.