This popular ground cover seems to have only one disease problem, but one that is a major concern in Illinois. Stem blight is a fungal disease that invades in cool, wet conditions, especially on newly planted beds. Still, I can walk around the Champaign area and find stem blight in established beds without too much searching.
Symptoms first appear as dark brown to black girdling lesions on the stems of overwintered runners. 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 disease is caused by the fungus Phoma; infection usually occurs when temperatures are between 50 degrees F and 65 degrees F. As long as cool, damp conditions remain, the disease spreads in the planting. The spores of the fungus disseminate primarily by splashing and flowing water.
Avoid overwatering or excessive overhead irrigation of vinca beds. It may be helpful 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.
In severely infected areas, you may want to use fungicides to stop further spread of the disease. Chemicals that provide some disease control are mancozeb and the copper compounds (such as Kocide, Champ, and Champion). Other general-purpose fungicides (for example, Fertilome, Ortho, and Hi-Yield) may also be labeled for homeowner use. Always read the label to be certain the chemical is registered for use on the plant in question. Off-label use of fungicides is illegal for commercial operators as well as homeowners.
In new plantings, some growers have success with the following system: Mulch with a landscape cloth that allows air and water flow. Cover this with a ground cover that will dry out quickly. This system will keep the runners and leaves off the soil and should help them dry out quickly so that they are less likely to be infected with the fungus. For more information on stem blight, consult Report on Plant Diseases No. 640.