Two fungal diseases of this groundcover have been fairly common in the past few years in Illinois. Because of the similarity of symptoms, it is likely that many cases have been misdiagnosed. Phoma blight (Phomopsis blight) is probably the most common of the two. Rhizoctonia root rot can produce some very similar symptoms but requires different management.
Phoma blight is caused by the fungus Phoma exigua var exigua. Shoots turn brown or black, wilt, and die. Black lesions can be found on the stems, girdling and killing all tissue beyond the infection. Within the black lesions, the fungus forms black fruiting bodies the size of a pinhead. The fungus remains on the plant stems under the plant canopy, making this disease very difficult to control.
Rhizoctonia root rot causes brown, rotted areas on the roots. Poor root growth results in poor top growth, so dying shoots are prevalent with this disease as well. Black lesions may even appear on the stems. Fruiting bodies will not be found in the lesions on plants infected with Rhizoctonia. Closely examine the roots to distinguish between these two disease problems.
Both of these diseases are very difficult to control. Try to avoid overhead watering or excessive watering 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. This task may seem to be impossible—to remove all the dead material and still have live plants. 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 the plastic mulch suggestion, but this may be the only way to establish a healthy bed of vinca.
Fungicides that may provide some protection against Phoma blight include iprodione (Chipco 26019), azoxystrobin (Heritage), copper hydroxide (Kocide), thiophanate-methyl (Bonide Bonomyl, Dragon 3336, or Ferti-lome Halt), potassium bicarbonate (Bonide Remedy), copper hydroxide (Ferti-lome Blackspot), and mancozeb (Pentathalon or Protect T/O). Azoxystrobin and thiophanate-methyl are systemic products; iprodione is locally systemic. The other chemicals have a protective-contact mode of action and do not provide the same degree of control of the pathogen without multiple applications.
Fungicides that may slow progress of 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.
These diseases are most prevalent in cool, wet conditions; but infection can occur any time from June to August following periods of cool, wet weather. Rhizoctonia can occur even in dry conditions. These diseases are very persistent in vinca plantings, and their presence is one of the main reasons that growers often seek an alternative groundcover. Stem blight of vinca is discussed in RPD no. 640, available at Extension offices or on the Web at the VISTA site, http://www.ag.uiuc.edu/~vista/horticul.htm.