This disease is most prevalent in cool, wet conditions, but infection can occur any time from June to August following temporary periods of cool, wet weather. We have seen a few new cases of this disease lately in Illinois. The disease is very persistent in vinca plantings and is one of the main reasons that growers often seek an alternative ground cover. The fungus remains in old runners hidden below the apparently healthy foliage that we see in the landscape.
Vinca stem blight is a fungal disease that affects Vinca minor, also known as periwinkle or ground myrtle. The pathogen is a fungus called Phoma exigua var. exigua. Symptoms include dark-brown to black girdling lesions on the stems. When these 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 may spread to stems and leaves causing large sections of the bed to die. This disease can be very persistent because the fungus can remain for long periods in moist soil and plant debris typically found under the foliage in a vinca planting.
Avoid overhead watering or excessive watering of vinca beds. 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. This may seem impossible because you may not be able to remove all of the dead material and still have live plants remaining. Work with plants when they are dry to avoid further spread of the disease. It has been suggested that new plantings be mulched with black plastic perforated every 4 to 6 inches and then covered with pea gravel or ground corncobs. In most cases, we would avoid plastic mulch, but this may be the only way to establish a healthy bed of vinca.
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 dozens of these products. Consult with garden-center staff for 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 Diseases (RPD) No. 640, which is available on the Web at http://www.ag.uiuc.edu/~vista/horticul.htm.