Richard Bogren | 4/22/2016 7:24:40 PM
Valiant series vinca from PanAmerican Seed for 2016 also shows great promise in disease resistance. Photo by Allen Owings
Excessive irrigation and rainfall can present problems with vinca performance in the landscape. Photo by Allen Owings.
(04/22/16) HAMMOND, La. – Vinca, also known as periwinkle, is one of the most popular annual bedding plants in Louisiana. But vinca had many problems last year because of the extremely wet, cloudy and cool growing conditions through mid-May, said LSU AgCenter horticulturist Allen Owings.
Vinca is a heat- and drought-tolerant plant that normally thrives under most Louisiana growing conditions. “Even with improved varieties, a number of diseases still limit vinca performance in Louisiana and other Southern states,” Owings said.
One of these diseases known as Phytophthora blight is caused by a soil-borne fungal-like microorganism called Phytophthora parasitica. “This pathogen has always been present in our soils and can devastate vinca plantings,” he said.
The disease is favored by warm, wet weather and rapidly spreads during extended periods of rainfall. And compacted soils with poor drainage may predispose roots to Phytophthora infections.
The pathogen is responsible for causing root and crown rot, and the affected plants appear blighted, Owings said. Symptoms on above-ground plant parts appear as water-soaked, greasy lesions on the foliage and on the base of the affected shoots. Individual shoots wilt and weaken. As the disease develops, reddish, dark brown cankers appear on the stems.
The pathogen also causes root rot, resulting in the roots shedding their outer layer. Often, sections of the plant wilt and turn brown while other parts remain healthy.
“A severe attack can essentially wipe out an entire planting,” Owings said.
“There is good news,” he added. “New vinca varieties are resistant to some diseases.”
The Cora and Nirvana series are some of the newer vincas that have genetics in the breeding that makes these varieties resistant to Phytophthora blight, he said. The new Valiant series from PanAmerican Seed for 2016 also shows great promise in disease resistance.
Well-known for its prolific and long blooming season, vinca is a favored flowering plant for home gardeners and professional landscapers around Louisiana for warm season color.
Thirty years ago, periwinkle flower colors were limited to white, white with a red eye and a rosy purple, Owings said. Breeding work in the 1980s involved crossing Catharanthus roseus with other species to improve the color range, increase blooming and flower size and improve the growth habit.
“Vincas now come in a plethora of colors, including pink, deep rose, red, scarlet, white, white with a red eye, lavender blue, peach, apricot, orchid, burgundy and many other shades,” Owings said. They generally grow from 10 to 18 inches tall with a spread of about 1 foot, although trailing types spread to about 2 feet.
Fungicides for home gardens and commercial landscapes are available to manage Phytophthora blight. In addition using to fungicides when the disease is present, good management is essential to combat the disease.
Owings offered the following suggestions to improve vinca performance in the landscape:
– Select a full-sun planting location with eight hours of direct sun daily.
– Prepare the landscape bed to allow for drainage and aeration. Raise the bed at least 6 inches if drainage is questionable.
– Plant in soil with a pH level of 5.5. Vincas need acidic soil.
– Avoid planting earlier than May. Soil and nighttime temperatures need to be warm prior to planting. May 1 is the recommended planting date. Many times, home gardeners and landscapers plant vincas in landscape beds in late March and April.
– Plant so that the top of the root ball is level with or slightly higher than the soil of the bed.
– Space transplants at least 10-12 inches apart. Crowded planting limits air circulation and can create conditions more favorable for disease development.
– Mulch with pine straw to decrease splashing of rainfall and irrigation water from soil to the lower stems and foliage. However, avoid excessive mulching to prevent the plant crowns from being continually wet.
– Manage irrigation properly. Vincas require minimal irrigation. Avoid overhead irrigation, when practical. When plants make it through the spring and early summer without disease, hold back irrigation or Alternaria leaf spotting – another fungal disease – may develop heading into fall.
– Don’t plant vincas in the same bed year after year.
– Don’t plant vincas in beds where Phytophthora blight has been a problem in the past.