Periwinkles are an extremely popular bedding flower because of their wide range of arresting colors and their heat- and drought-tolerance. However, these beautiful flowers are subject to a number of diseases that can turn your gorgeous flower garden into a wasteland if you do not properly prevent and control the issues. Careful planning and observation of your periwinkle plants should enable you to keep them safe from these problematic infections.
Botrytis is a fungal infection that tends to manifest itself in small, black dots that cluster on the edges of the leaves of the periwinkle, then extend inward. The spores of the fungus splash onto the leaves during wet weather, then grow if the environment continues to be moist. This can be controlled by planting your periwinkles in full or nearly-full sun and by removing any infected leaves as soon as you spot them. If you water your periwinkle plants, do so by drip hose rather than sprinkling to avoid wetting the leaves further.
This infection creates dark brown streaks on the stem of your plants, then extends to leaves and, in some cases, flowers as well. It may attack all of your plants, or remain restricted to just a few plants or even certain sections of the plant. You can prevent this fungus by planting in full sun in soil of pH 5.5. The soil should also be well-drained. If the infection has already occurred, remove the affected plants from the area completely -- do not drop debris on the ground--and next year plant something different in the bed to give the fungus a chance to die off without its periwinkle hosts.
Root and Stem Rot
This occurs when the roots and stems of periwinkles develop any of a variety of fungal infections. You will notice that the stems of your plant are turning black and that the leaves are wilted and brown as if the plant is not getting any water. This is because it is not because the root rot has rendered the roots ineffective. If you have plants with root and stem rot, remove them from the bed immediately and treat the remaining plants with a fungicide to prevent the spread of the disease.