Greg Grant

Look around town. You see it everywhere — dying beds of brightly colored periwinkles.

What gives? It’s actually a fungal disease and that has been a problem since I first started my career in horticulture years ago. Unfortunately, most gardeners and landscapers still don’t understand how the disease works. So here we go (again): Periwinkle Aerial Phytopthora 101.

Aerial phytophthora is caused by the fungus Phytophthora parasitica and is also known as periwinkle blight. 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.

1. Do not plant periwinkles until after Mother’s Day. They are from Africa and are a summer bedding plant, not a spring bedding plant. 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. 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.

4. 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.

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 in plastic trash bags with your garbage to minimize the disease in the future.

7. Do not plant periwinkles in beds with a history of aerial phytophthora.

8. Plant periwinkles in the Cora series, 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.

Successfully growing periwinkles is all about sanitation and minimizing water contact with the plants. While driving through Tyler the other day I noticed a man out watering his dying periwinkles. Little did he know, he was killing them with the water hose.

Greg Grant is the Smith County horticulturist for the Texas A&M AgriLife Extension Service and co-author of “Heirloom Gardening in the South.” You can read his “Greg’s Ramblings” blog at arborgate.com or read his “In Greg’s Garden” in each issue of Texas Gardener magazine (texasgardener.com). More science-based gardening information from the Texas A&M AgriLife Extension Service can be found at aggie-horticulture.tamu.edu.