Issue 11, July 2, 2009
Vinca Blights
Vinca (Periwinkle) is grown as an evergreen or semi-evergreen ground cover in Illinois. It is popular because it provides a pleasant, soft, green ground cover in shady locations. Unfortunately, there is a fungal disease problem of periwinkle that can be very persistent once it invades a site. This disease is called Vinca stem blight. The damage from vinca stem blight has been obvious this spring. Pull back some of the foliage in your vinca bed and look for black or brown stems and runners hidden below.
Vinca stem blight is caused by a fungus named Phoma exigua var. exigua. The fungus thrives in moist conditions such as those found in dense plantings of ground covers, especially in the spring when there is plenty of moisture and succulent tissue. Black lesions may develop on the leaves and stems first, often girdling and killing runners. Stems and foliage then become brown as they are killed by this fungus. Those stems infected last year are hosting the fungus now. If you see the black lesions, look for pin-head sized fruiting bodies of the fungus in the black tissue.
A look-a-like disease, Rhizoctonia root rot of vinca, causes top decline by rotting the roots. It may also cause black stem lesions, but you won't find fruiting bodies in the lesions. If you are not sure which disease is present, here are some tips. Look for fruiting bodies in the black stem lesions to confirm Phoma blight. Dig up an infected plant and look at the root system. Phoma does not cause a root rot. Rhizoctonia will cause roots to decline. Additionally, a University of Illinois publication on stem blight of vinca minor is available free of charge on line.
Vinca stem blight is difficult to control, but you can make some headway right now. Work with plants only when they are dry. This helps minimize disease spread. Remove infected plants, including old runners that may be hidden among other plants. Clean out dead leaves to open up air movement in the bed. If irrigation is needed, water the soil rather than the foliage. There are many fungicide options to protect new growth. Those with some systemic activity and listed in the Illinois Commercial Landscape and Turfgrass Pest Management Handbook, include 18 Plus, 26 GT, Banner MAXX, Chipco 26019, Compass, Cygnus, Heritage, Iprodione, ProPensity, Propiconazole, Savvi, Sextant, Spectator, and Zyban. There are also protective contact products available. Fungicide applications should be initiated when the disease first appears.--Nancy Pataky
Author:
Nancy Pataky