It's nearing time to plant summer beds and I wonder if you can help me with a problem with Vincas (periwinkles) that has mystified me for two seasons. Random plants in the same beds suffer with a mysterious black "rot" and wilt that affects both stems and leaves. I thought it might be an over watering problem, but cutting back didn't alleviate the problem. These beds get lots of hot, south Arkansas sun and generally (80 %) of the plants do great. Is there something that can be done prior to planting or a treatment once the disease occurs?
Periwinkle or annual Vinca can suffer from a disease called stem canker. It is usually not a problem in a dry bed, but if you use a sprinkler system or overhead watering on a regular basis, it can hit. The water causes the soil to splash up onto the stem, causing the disease. It causes the plants to start wilting in sections. If you follow one of the wilted plants back to the main stem, you should see a sunken lesion. If you cut it out at that point, the plant can grow back. You may want to plant a different annual in this bed this year, or try the 'Cooler' series of Vinca, which is slightly resistant to the disease. Mulch the plants to keep the soil from splashing on the stems, which is what causes the problem. Then avoid overhead watering as much as possible.
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