We usually see this disease in cooler, wet conditions, but we have seen it again lately in some parts of the state. Chicagoans may not believe it possible as they struggle through drought (seven weeks now), but some parts of the state do have enough moisture at present. Infection can occur any time from June to August following periods of cool, wet weather, and recent cool spells have triggered a new flush of disease.
Vinca (periwinkle) stem blight is a fungal disease that causes dark brown to black girdling lesions on the stems. When lesions occur at the ground line, the entire runner dies. Where healthy stems touch the soil or infected plant parts, new lesions may develop. Within a few weeks, the disease can spread to stems and leaves, causing large sections of the bed to die. The pathogen, a fungus called Phoma exigua var.exigua can persist for long periods of time in moist soil and plant debris.
Avoid overhead watering or excessive watering of vinca beds. It may help to improve air circulation in the area by pruning surrounding plant material and overhanging branches. Because the fungus can survive in the soil on dead plant material, remove fallen leaves and dead tissue.
Fungicides may help contain this disease, and registered chemicals are listed in the usual pest control handbooks. Copper compounds are listed without trade names because there are literally dozens of these products. Consult with garden center staff for information about the copper compounds; then read the labels carefully to select a product that is cleared for use on vinca. Mancozeb also works well, but you may have trouble locating it. Stem blight of vinca is discussed in Report on Plant Disease No. 640.