Pythium Root and Stem Rot (fungus – Pythium spp.): Lesions produced on stems and roots with rapid progression of symptoms. Infected plants die soon after infection. Avoid overwatering and use suggested fungicide.
Other Root and Stem Rots (fungi – Rhizoctonia solani and others): Lesions occur on roots and lower stem. Periwinkle is more resistant to this problem than most annuals, but disease can occur when fungi are present in high populations. Rotate plantings and use recommended soil fungicides if needed.
Aster Yellows (mycoplasma): Infected plants are chlorotic and stunted. Control leafhoppers that transmit the disease-causing entity and rogue infected plants.
Canker and Dieback (fungus – Phomopsis sp.): Shoot tips will turn dark brown-to-black and die back to the soil. Favored by rainy weather. Preventative spray of copper fungicide starting in spring on new growth will control the fungus.
Phytophthora Aerial Blight (fungus – Phytophthora parasitica): Infection is generally characterized by leaf blight and upper stem deterioration. Infection is distinctly an aerial problem, not involving lower stem and root tissue. In most cases, leaves are shriveled and dull-gray in color. An aerial sunken stem canker can also develop. Symptoms develop rapidly in outdoor planted beds, especially during conditions of prolonged, rainy weather. Bark mulches or other mulch materials might help minimize splash dispersal of infested soil particles. No fungicides are currently cleared for use in control.