Periwinkle or vinca (Catharanthus roseus (L.) G. Don) is widely planted as a flowering annual in home and business landscapes in hardiness zones 9 and 10 and the southernmost portions of zone 8. Many improved selections, such as those in the Pacifica, Tropicana, and Cooler cultivar series, are currently available. Dark gray flower spots that often coalesced and led to blighting of flowers under conditions of high humidity, were observed on all of 18 periwinkle cultivars in a variety trial at Burden Research Plantation in September 1997. The disease affected only the flowers and was also observed on periwinkle in commercial landscape plantings around Baton Rouge. Fungi were isolated from flower spots by plating necrotic tissue on acidified potato dextrose agar (PDA). Fungal isolates were maintained on PDA and 20% V8 juice agar under continuous fluorescent light. A fungus that produced cottony, white mycelium and black spore masses was consistently isolated from diseased tissue. Pathogenicity tests were conducted by misting a spore suspension (70,000 per ml produced on V8 juice agar) onto detached flowers held in a moist chamber and to flowers on intact plants. Inoculated plants were held for 24 h in a dew chamber at 25°C. Typical irregular dark spots appeared on attached and detached flowers within 18 h after inoculation and flowers held for longer periods under high humidity developed a soft rot. Koch's postulates were fulfilled by reisolating the fungal pathogen, which was identified as Choanephora cucurbitarum (Berk. & Ravenel) Thaxt. (1). This is the first report of the occurrence of C. cucurbitarum on periwinkle.
Reference: (1) P. M. Kirk. Mycol. Pap. 152:1, 1984.