To control it, remove dead stems and apply a drenching spray of Cleary's 3336. Repeat several times at 10-14 day intervals. Once the disease becomes established it is difficult to eradicate. The fungicide drench is helpful, but not always effective. Phytophthora root and stem rot is another fungus disease that has been diagnosed on Vinca.
Stems turn black and patches of the groundcover die.
Correct diagnosis is very important, because if Phytophthora is the culprit, a drench of Subdue would be suggested for control. Select disease resistant cultivars to improve landscape performance and avoid unnecessary fungicide use. Over a two year period, plant pathologists at the Uiniversity of Vermont evaluated 17 cultivars of bee balm for resistance to powdery mildew, caused by the fungus Erysiphe cichoracearum. Mahogany, Prairie Night, Snow White. Keep them off decks, picnic tables, etc. with a strong stream of water.