- Periwinkle vines can't tolerate extended periods of cold, wet weather. The plants require warm summer temperatures to actively put on new growth and flower. Overly cool temperatures, especially during the moist spring months, make the plants more susceptible to disease or frost damage. Both kill back the vines, and the periwinkles might not recover even after temperatures begin to rise. Plant periwinkles outdoors only after all danger of spring frost is past to prevent cold-weather die back.
- Even soil moisture is necessary for the best periwinkle growth. The plants quickly begin to wilt and die during warm weather if the soil begins to dry out completely. If the periwinkles aren't supplied with the necessary water, both the leaves and stems begin to wither and the flowers drop off. Watering promptly usually brings the plants back to their former health quickly. Repeated drying can stress the plants and make them recover more slowly or die off. Keep the soil evenly moist and provide 1 to 2 inches of weekly irrigation to prevent drought stress.
- The primary disease affecting periwinkles is Phytophthora, a fungal disease. Plants are most commonly affected during cool, moist weather. Symptoms begin with the stems, followed by the branches wilting and dying. Eventually the whole plant declines. The disease might affect the roots first, which causes stunted growth and leaf drop. Prevent Phytophthora by watering less frequently during cool weather and by planting later in the season. Avoid overfertilization as the excess soil nutrients feed this fungal disease so it spreads more quickly.
- A variety of other fungal diseases also can affect periwinkles and cause them to die off. Root rots affect young Vinca plants grown in overly wet soil. The roots rot and die, causing the entire plant to fall over. Cankers, usually caused by fungal pathogens, begin as sunken lesions on leaf and stem surfaces. These lesions cause the periwinkle to lose its leaves as the disease increases in severity. Mold and mildews also might form as a powdery growth on leaf surfaces. The mildew coating blocks the plant from absorbing sunlight and eventually leads to plant decline. Warm temperatures and dry foliage prevent most fungal issues.