China rose (Hibiscus rosa-sinensis L.), also known as Chinese hibiscus and rose-of-China, is a perennial flowering shrub with 8- to 10-cm-diameter, brightly colored flowers. In Argentina, China rose grows to a height of 2 to 5 m and is a popular plant in residential and commercial landscapes. It is among the 30 most widely used shrubs in Buenos Aires.
Since 1999, stem and flower blight has been observed frequently on mature plants of the cultivar ‘Hawaii’ in Buenos Aires (Fig. 1) where the disease occurred primarily on plants growing in shady locations. It has also been observed on cultivars ‘Flamingo’ and ‘Toronto’. Lesions first occurred on the tips of stems and rapidly moved downward when environmental conditions were cool and humid (Fig. 2); no lesions were observed on leaves. Flowers became blighted, turned brown, and fell to the ground prematurely. Diseased stems occasionally were colonized by Colletotrichum gloeosporioides (Penz.) Penz. and Sacc., which was reported previously to be a pathogen of H. rosa-sinensis in Argentina (3). Eventually, blight reduced the horticultural quality of plants in the landscape.
To isolate the causal organism, pieces of diseased stems and flowers were surface-sterilized for 1 min in 2% NaOCl, washed with sterile deionized water, blotted dry, plated on potato-dextrose agar (PDA), and incubated at 20±2ºC in growth chambers with alternating periods of 12 h of near ultraviolet light and 12 h of darkness for 6 days. Fungus colonies were subcultured onto fresh PDA.
Once pure cultures were obtained, morphological characteristics were examined and the pathogen was identified as Botrytis cinerea Pers.:Fr. (1). The teleomorph was not observed. After 48 h at 20±2ºC, dense whitish mycelium developed on PDA; mycelium turned gray in color after 72 h. Brown conidiophores of 2 to 2.1 (2.05) mm x 18 to 28 (22.3) µm and unicellular conidia with protruding hila developed after 5 days. Conidia measured 8 to 12 (10.8) µm x 8 to 10 (8.36) µm and developed in botryose clusters. Abundant sclerotia formed on PDA medium. They were black, irregularly shaped, less than 1 mm in diameter, and aggregated on the agar surface.
Pathogenicity was tested on healthy flowering H. rosa-sinensis cv. Hawaii plants growing in 4-liter containers. Inoculum was prepared from 6-day-old colonies of B. cinerea on PDA. Conidia were dislodged with a glass rod in cultures flooded with sterile deionized water. Conidial concentration was determined with an hemacytometer. Each of 10 plants was sprayed with 25 ml of suspension (2.6 x 104 conidia per ml of water). Ten additional plants were each sprayed with 25 ml of sterile water and served as non-inoculated controls. All plants were maintained at 18 to 20ºC and 100% relative humidity for 2 days. Stem and flower blight developed on all inoculated plants within 6 days. The lesions were covered by grey mold 10 days after inoculation. Controls remained symptomless. B. cinerea consistently was recovered from inoculated plants, thus fulfilling Koch’s Postulates.
Botrytis blight previously has been reported on H. rosa-sinensis in Florida (2) and on H. sabdariffa L. in South Africa (4). This is the first report of B. cinerea causing blight on China rose in Argentina.
1. Ellis, M. V., and Waller, J. M. 1974. Sclerotinia fuckeliana (conidial state: Botrytis cinerea) C.M.I. Descriptions of Pathogenic Fungi and Bacteria No. 431. Commonwealth Mycological Institute. Kew, Surrey, UK.
2. Farr, D. A., Bills, G. F., Chamuris, G. P., and Rossman, A. Y. 1989. Fungi on Plants and Plant Products in the United States. American Phytopathological Society, St. Paul, MN.
3. Rivera, M.C., Wright, E.R., and Carballo, S. 2000. First report of Colletotrichum gloeosporioides on Chinese Rose in Argentina. Plant Dis. 84:1345.
4. Swart, L., and Langenhoven, P. 2001. First report of Botrytis blight, caused by Botrytis cinerea, on Hibiscus in South Africa. Plant Dis. 84:487.