Our periwinkle has done well with just some occasional weeding. It grows on a hillside with a mixture of sun and shade but mostly shade. For the first time this spring, there are a few dead spots. Most of the area looks healthy. We did have a long and harsh winter but that has never bothered the growth before.
Dead Spots in Periwinkle
Certified GKH Gardening Expert
This could be the onset of a fungal infection. Are the leaves spotted? Oftentimes, spots on leaves come together to form larger patches of dead tissue. Dig up and discard infected plants as well as the soil surrounding their roots. Then treat with a fungicide or sulfur spray.
It could also be attributed to insect pests. Check the plants carefully. These can be treated with neem oil and as an added bonus, it doubles as a fungicide, so treating with neem oil would actually help alleviate both pest and fungal issues. Here is more information: https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm