Watch now for vinca stem blight, a fungal disease that affects Vinca minor, also known as periwinkle or ground myrtle. This woody-stem vinca is used as a ground cover in landscape beds. The pathogen is a fungus called Phoma exigua var. exigua. Some older literature calls it Phomopsis. Symptoms include dark brown to black girdling lesions on the stems. Within the lesions, the fungus forms black, pinhead-sized fruiting bodies. Diagnosticians look for these fruiting bodies to confirm the disease. When stem 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 survive for long periods in moist soil and plant debris typically found under the foliage in a vinca planting.
Vinca blight is most prevalent in cool, wet conditions; but infection can occur anytime from June to August following temporary periods of cool, wet weather. Illinois weather has been ideal for the development of this disease. It is persistent in vinca plantings, a main reason that growers often seek an alternative ground cover. The fungus remains in old runners, hidden below the apparently healthy foliage.
Because the fungus can survive in the soil on dead plant material, remove fallen leaves and dead tissue. This task may seem impossible: You may not be able to remove all the dead material and still have live plants remaining. Work with plants when they are dry to avoid further spread of the disease. Try to avoid overhead watering or excessive watering of vinca beds. Try to improve air circulation by pruning surrounding plants and overhanging branches. It has been suggested that new plantings be mulched with black plastic perforated every 4 to 6 inches and covered with pea gravel or ground corn cobs. In most cases, we would avoid the 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 pest-control handbooks--Home, Yard, and Garden Pest Guide and Commercial Landscape and Turfgrass Pest Management Handbook. It may be necessary to remove the dead or diseased plants and then apply a fungicide to stop further spread of the fungus. Stem blight of vinca is discussed in Report on Plant Disease (RPD), no. 640, available on the Web at the VISTA site for University of Illinois Extension publications.
Rhizoctonia root rot of vinca can produce above-ground symptoms nearly identical to Phoma blight. The diagnostic clue is that fruiting bodies are not in the lesions on Rhizoctonia-infected plants. The below-ground difference is that Rhizoctonia causes brown, rotted areas on the roots; Phoma blight does not. Both diseases are difficult to control. Cultural methods are the same. Fungicides are options but do not offer complete control. It may be more difficult to find information on fungicides effective against Rhizoctonia. Commercially available fungicides that may slow Rhizoctonia root rot include iprodione (Chipco 26019) and PCNB (Engage, Terraclor). Iprodione has a locally systemic mode of action, and PCNB is a protective-contact fungicide. The thiophanate methyls are all that homeowners have to use, and efficacy is not outstanding.