Vinca ground cover has been a popular landscape plant for many years. It has also developed two fungal diseases that are now fairly common in Illinois. Because of the similar symptoms, many cases likely have been misdiagnosed. Phoma blight (Phomopsis blight) is known by most landscapers. Be aware, however, that Rhizoctonia root rot can produce very similar symptoms and 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 fruiting bodies that are black and pinhead-sized. The fungus remains on the plant stems protected under the plant canopy, making this a very difficult disease 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. One difference is that fruiting bodies are not found in the lesions on Rhizoctonia-infected plants.
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 may seem to be an impossible task 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 landscape cloth or black plastic perforated every 4 to 6 inches and then 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 healthy vinca.
The fungicides that may provide some protection against Phoma blight differ from those that protect against Rhizoctonia root rot. Refer to the Commercial Landscape & Turfgrass Pest Management Handbook or the Home, Yard, and Garden Pest Guide for specific products. A chart at the back of each appropriate chapter lists the trade name, active ingredient, mobility, and company for each product listed in the chapter. This way, you can choose a protective-contact type of chemical or possibly something with systemic activity.
These diseases are most prevalent in cool, wet conditions; but infection can occur anytime from June to August following periods of cool, wet weather as we have recently experienced. 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 ground cover. Stem blight of vinca is discussed in Report on Plant Disease (RPD), no. 640, which is available at Extension offices or on Extension’s VISTA Web site.
A few readers have described how devastating Phoma blight has been in their established vinca plantings. They want to know what to do. Only work with plants when they are dry. If possible, use a systemic fungicide from the list of options. A product containing thiophanate–methyl (such as Bonide Bonomyl, Dragon 3336, Ferti-lome Halt) or azoxystrobin (Heritage) provides systemic protection. The products do not eradicate the fungus, so it is important to physically remove infected plants or plant parts. It is very important to remove plant debris. When replanting, consider using a landscape cloth in those areas to minimize plant–soil contact.