Branch and stem rot can be a major disease problem for annual vinca (Catharanthus roseus) once the disease organism has been introduced into the residential or commercial landscape environment. This disease is caused by a soil-borne fungus called Phytophthora parasitica that can persist in the soil for several years. Under conditions of overhead watering or heavy rainfall, this disease can spread rapidly in a vinca planting. The fungus is often accidentally introduced into the landscape by infected plant material. This disease is often a major problem in greenhouse production systems, where conditions may be ideal for disease development and spread. Disease incidence and severity are favored by extended periods of wet, hot weather. Heavy fertilization also tends to contribute to the disease.
The initial symptom of disease onset is the presence of water-soaked, gray-green, greasy areas on the shoots and leaves, which is quickly followed by a sudden wilting or flagging of one or more shoots. As the fungus advances within the plant tissues, reddish brown lesions develop on plant stems. These lesions result in death of the stem or entire plants. The fungus can move from one plant to another merely by leaf-to-leaf contact. Although the disease is most prevalent on the aboveground plant parts, root rot may also develop. When foliage remains wet, disease progression proceeds very rapidly. Plants may be killed one to two weeks after symptoms appear.
To prevent the accidental introduction of the disease organism into your planting area, always select high-quality plants. Always install these plants in well-drained soils. Raised beds that encourage good soil drainage are recommended. Avoid soils with high clay content. Cultural practices may influence the incidence and severity of branch rot of vinca. By planting annual vinca in late May or June, disease severity may be reduced somewhat. Earlier plantings during March and April tend to suffer more from the disease. Since wet foliage is a major contributing factor for disease development, every effort should be made to minimize leaf wetness. Planting beds should be watered with a soaker hose or drip tubing. If plants are normally watered by overhead sprinkling, they should be watered in the early morning hours to encourage drying of foliage by mid to late morning.
To reduce the risk of introducing the fungus into the growing area, do not install vinca in the same landscape area year after year. Since not all annuals and perennials are susceptible to the same fungus that attacks vinca, growers may wish to consider other bedding plants as replacements for vinca. Begonia, celosia, geranium and ageratum are excellent substitutions in the landscape.
Sanitation practices are helpful in limiting damage to a vinca planting in Phytophthora parasitica-infested soils. Plants showing symptoms of wilt or dying should be removed immediately. Mulching around plants greatly reduces the risk of infection by preventing infested soil from coming in contact with the leaves. Be sure to space plants in a manner that encourages dry leaves and minimizes leaf-to-leaf contact. All vinca plants should be removed at the end of the growing season. Fungicides containing chlorothalonil or aluminum tris are effective when applied on a preventative schedule according to the label information.
(From State-by-State Gardening July/August 2004. Photos by Stephen Vann.)