Petal Blight of Callistephus chinensis Caused by Itersonilia perplexans
Authors: McGovern R. J. , Seijo T. E. 
Published: May 2007

From November 1997 to March 1998, a cut-flower grower in southwest Florida reported post-harvest losses due to a petal blight of 10 and 50%, respectively, in China aster (Callistephus chinensis (L.) Nees) cultivars Rainbow Mix and Matsumoto Lavender. Initial symptoms observed included pinpoint, bleached lesions on petals. Enlargement of lesions resulted in bleached streaks on ray flowers and eventually necrosis of entire blossoms. Symptomatic petals were surface disinfested in 0.5% NaOCl and suspended on the lids of plastic culture plates over acidified potato dextrose agar (APDA). Itersonilia perplexans Derx was isolated consistently from diseased petals but not from symptomless controls. Confirmation of the pathogenicity of I. perplexans used three detached, mature flowers of China aster cv. Matsumoto Rose and seven mature flowers of Bidens bipinnata L., a common composite found near the production site. The pedicels of flowers were placed in sterile, deionized water, and 1 ml of a spore suspension (1.2 × 104 spores per ml) of I. per-plexans was sprayed on aster flowers, and 0.5 ml was applied to flowers of B. bipinnata. An equal number of noninoculated control flowers were sprayed with sterile, deionized water only. The flowers were incubated in a controlled environment chamber at near 100% relative humidity, a constant temperature of 18°C, and a 12-h photoperiod. Bleached spots were detected on the petals of Matsumoto Rose flowers 10 days after inoculation, and the fungus was reisolated on APDA. Control asters remained symptomless, and were negative for I. perplexans. Brown blotches appeared on the petals of B. bipinnata flowers 12 days after inoculation, and infection by I. perplexans was confirmed by reisolation. The fungus was not detected in noninoculated B. bipinnata flowers, including some that exhibited brown discoloration. A survey was conducted in early March 1998 to examine the incidence of Itersonilia petal blight in China aster and the occurrence of the fungus in B. bipinnata. Fifty flowers of each of four China aster cultivars were collected at random without regard to symptoms. Approximately 20 flowers of B. bipinnata exhibiting brown spots were also collected near this site. The fungus was isolated at incidences of 18, 6, 8, and 6% in flowers from aster cultivars Rainbow Mix, Matsumoto Blue, Matsumoto Pink, and Matsumoto Scarlet, respectively. I. perplexans was not detected in B. bipinnata flowers collected from the field during this period. I. perplexans previously was detected in florists' chrysanthemum (Chrysanthemum × morifolium Ramat.) at the same cut-flower production site in January 1973 during a period of uncharacteristically cool weather (2). The weather during the recent outbreak of Itersonilia petal blight in aster was also unusually cool and rainy and resulted from the “El Niño” weather pattern prevalent during the period. I. perplexans has been found to over-season on many weeds in Great Britain, particularly on those in the Compositae (1). Experimental transmission of I. perplexans to B. bipinnata suggests that it and other wild composites may serve as reservoirs of the fungus in Florida.

References: (1) D. G. Gandy. Trans. Br. Mycol. Soc. 49:499, 1966. (2) J. J. McRitchie et al. Plant Dis. Rep. 57:181, 1973.