Leaf and pseudobulb diseases on Bifrenaria harrisoniae (Orchidaceae) caused by Phyllosticta capitalensis in Brazil

Abstract

New leaf spot and pseudobulb diseases caused by Phyllosticta capitalensis are reported for the first time on the orchid Bifrenaria harrisoniae. Bifrenaria harrisoniae (Hook.) Rchb. f. is an orchid species with terrestrial habitat, characteristic of the Brazilian 'campos rupestres'. Due to its high ornamental potential, this species has been collected from its natural habitat and in conjunction with the vulnerability of its habitat, in some Brazilian states it is considered to be a threatened species. Recently, this species has been propagated in vitro by some Brazilian orchid growers for commercial purposes. In June 2006, an exploratory project surveying and describing the phytopathogenic and endophytic mycodiversity associated with the family Orchidaceae in the state of Minas Gerais was initiated. In a survey of the campos rupestres of Serra de Ouro Branco (Fig. 1), at Ouro Branco city in Minas Gerais State, Brazil, samples of the orchid B. harrisoniae with severe leaf spotting symptoms were collected (Fig. 2). Spots were initially chlorotic and circular in shape, became necrotic and black with a chlorotic halo, and coalesced to cover the entire width of the leaves, leading to leaf death. Pseudobulbs were also diseased in severely infected plants (Fig. 3). Morphology of the fungus on host tissue included, pycnidia immersed, solitary or aggregated, subepidermal, variable in shape, black, glabrous, with an apical ostiole (Fig. 4); wall stromatic, composed of several layers of dark brown, thick walled cells; conidiogenous cells aseptate, discrete, hyaline, pyriform to cylindrical, invested in mucus; conidia 1-celled, hyaline, obovate to pyriform, 9-14 Â 5-7 mm, smooth-walled, guttulate, surrounded by a thick mucilaginous coat, with a hyaline apical appendage, 6-12 mm long, in all the isolates. Teleomorph not observed. Material examined: VIC 30556, on leaves of Bifrenaria harrisoniae, Gerdau Açominas RPPN, 1186 m, Serra de Ouro Branco, Ouro Branco, State of Minas Gerais, Brazil, S.M. Lelis & I.F. Braga, 06 Nov 2007. Seventy isolates were obtained by surface sterilising selected leaf fragments in 2% sodium hypochlorite. Thirty isolates were obtained from faint chlorotic lesions, 30 from the periphery of necrotic lesions and 10 from pseudobulbs. The isolates were obtained from lesions of different leaves and pseudobulbs from different plants and presented very similar colonies. To describe the colony morphology of the fungus, pure cultures were grown in Petri dishes containing potato dextrose agar (PDA) and 2% malt extract agar (MEA) for 8 days at 27 C in the dark and under near UV light with a 12-h photoperiod (Fig. 5). In culture: 7.0-7.5 cm after 8 days. On PDA, colonies grayish to dark grey with white aerial mycelium border, showing concentric haloes when grown in (a) (b) Fig. 1. (a) Collection site in Serra do Ouro Branco, Minas Gerais, Brazil and (b) Montane grassland characteristic of the 'Brazilian campos rupestres' where Bifrenaria harrisoniae occurs.