This disease is caused by two fungi (Phomopsis livella and Phoma exigua) that attack periwinkle in the spring soon after the new growth begins. This disease is most prevalent during very rainy seasons. The fungal spores spread from plant to plant on splashing water and contaminated tools. This disease can be devastating, killing an entire planting in a few weeks.
Apply a fungicide labeled for this disease, following label directions. Remove badly infected plants. Avoid overhead watering. To reduce the chances of new infections, water early in the day, rather than late afternoon or evening, so the leaves have time to dry out before nightfall.
This widespread plant disease is caused by a fungus (Botrytis cinerea) that is found on most dead plant tissue. The fungus initially attacks foliage and flowers that are weak or dead, causing spotting and mold. The fuzzy mold that develops is composed of fungus strands and millions of microscopic spores. Once gray mold has become established on plant debris and weak or dying leaves and flowers, it can invade healthy plant tissue. The fungus is spread by splashing water or by infected pieces of plant tissue contacting healthy tissue. Cool temperatures and high humidity favor gray mold growth. Rain and overhead watering enhance the spread of the fungus. Infection is more of a problem in spring and fall, when temperatures are lower. In mild-winter areas where freezing is rare, gray mold can be a year-round problem.
Remove and discard all fading flowers and diseased leaves. Apply a fungicide labeled for this disease, following label directions.
Several different fungi cause leaf spots. Some of these may eventually kill the plant or weaken it so that it becomes susceptible to attack by other organisms. Others merely cause spotting that is unsightly but not harmful. These fungi are spread by splashing water, wind, insects, and contaminated tools. They generally survive the winter in diseased plant debris. Most of these fungi do their greatest damage in mild weather (50° to 85°F).
Rake out infected fallen leaves.
This plant disease is caused by a fungus (Pellicularia filamentosa). It is the most serious disease affecting periwinkle. It occurs mostly in heavy, poorly drained soil and during periods of wet weather. It can be found throughout the growing season in most periwinkle plantings. The fungus enters the plant through the roots and crown and rots the cells, causing the plant to wilt and die. The fungus persists indefinitely in the soil and spreads from plant to plant on contaminated tools and in splashing water.
Remove and destroy badly infected plants. Allow the soil to dry between waterings until the spread of the disease is halted. If the area is replanted, improve the drainage by amending the soil with organic material. To prevent recurrence of the disease, allow the soil to dry between waterings until it is barely moist.
Periwinkle can suffer from two kinds of scorch: heat scorch and winter burn. Scorch is caused by excessive evaporation from the leaves. In hot weather, water evaporates rapidly from the leaves. If the roots can''t absorb and convey water fast enough to replenish this loss, leaf tissue is killed. Winter injury occurs on plants growing in the full sun of a southern or western exposure. On a very sunny, clear winter day, the sun heats the leaf surface, increasing the plant''s water requirement. If the ground is frozen or dry, the periwinkle roots can''t absorb the needed water. The leaves then turn yellow or brown and dry out.
Keep periwinkle well watered during hot weather. Fertilize the plants to encourage quick regrowth. In areas with hot summers, periwinkle does best in shady locations. To reduce the chance of winter burn, be sure the soil is moist before the ground freezes. Mulch plants or cover them with tree boughs during unusually cold sunny weather.