First Report of Sclerotinia sclerotiorum on African Daisy in Arizona. M. E. Matheron, Yuma Agricultural Center, University of Arizona, Yuma 85364. J. C. Matejka, Yuma Agricultural Center, University of Arizona, Yuma 85364. Plant Dis. 78:09241. Accepted for publication 19 April 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0924D.
In January, 1992, several African daisy (Gerbera jamesonii H. Bolus ex J.D. Hook) plants were dying in a roadside planting in Yuma, Arizona. Initial symptoms were wilting of leaves and crown necrosis, followed by plant necrosis and death. After the onset of initial symptoms, a mass of white mycelia was apparent on plant stems at the soil surface. Large black, irregularly shaped sclerotia were present within necrotic stem tissue. Isolation from diseased tissue yielded Sclerotinia sclerotiorum (Lib.) de Bary. In the greenhouse, African daisy plants were started from seed and grown in potting mix in 10-cm-diameter plastic pots. The potting mix was infested with an isolate of S. sclerotiorum from African daisy by burying potato-dextrose agar disks containing fungal mycelia just below the surface and adjacent to the crown of each test plant. One month after inoculation, symptoms were similar to those observed on infected plants in the field, with formation of sclerotia within diseased stems and recovery of S. sclerotiorum from diseased plant tissue. Rapid decay of lettuce (Lactuca sativa L.) heads occurred when agar disks containing fungal mycelia of S. sclerotiorum originating from either African daisy or lettuce were placed on the outer leaves of lettuce heads, which were subsequently incubated in a moist chamber at 24 C in the laboratory. Over 20,000 ha of lettuce are grown in Arizona from autumn to spring. Lettuce fields containing plants infected with S. sclerotiorum could be a source of inoculum for plantings of African daisy, which is widely grown as an ornamental during the same period and in the same area as lettuce. This pathogen has been reported on African daisy in Maryland and New York, but this is the first known report of 5. sclerotiorum on this host in the western United Stales.