Vinca, the flowering plants of the family Apocynaceae which are more commonly known as ‘periwinkles,’ are a popular choice of home gardeners for spring to summer annual flowers. Besides the appeal of their beauty, many choose vinca for their relative freedom from insects and mites.
However, in the past few years we have received an increasing number of diseased vinca samples at our laboratory. The three most common pathogens we have identified are Pythium, Rhizoctonia, and Fusarium, all of which are more commonly referred to as “water molds.” These are all soil-borne pathogens, each representing a different class of plant fungi.
Water mold infections can manifest in vinca in several ways. You may see brown or black lesions form on steps, branches and leaves, followed by wilting and ultimately death. In other cases, the roots may be attacked, but the most obvious signs of root rot still manifest above the soil in the form of scorched-looking or yellowing plant tissue, curling and wilting of leaves, and stunted growth.
Thankfully, it’s relatively easy to keep vinca healthy with the right preventative treatments.
An effective treatment for water molds infesting vinca involves a soil drench using Daconil fungicide.
Mix about 3 to 4 tablespoons of Daconil per 2.5 gallons of water in a watering can, and wet the top 3 to 4 inches of soil. Repeat the drenching the next day.
Next, spread 9 pounds each of calcium sulfate (gypsum) and potassium sulfate per 100 square feet (10 feet by 10 feet square). Using a spade, work evenly into the top 3 to 4 inches. You may add compost or manure as well, but be sure it is cured, meaning that it has gone through a composting process where temperatures of 135 degrees Fahrenheit were sustained for at least 5 to 6 hours. Curing reasonably ensures that the compost does not contain viable weed seeds or pathogens.
Be sure to keep an eye on weather forecasts. Prior to any heat waves, lightly spray vinca plants with liquid calcium (about 2 ounces of 6% to 8% liquid calcium per 2.5 gallons of water). If the heat wave continues, repeat this spray at 5 to 7-day intervals.