Poinsettia powdery mildew : immunolabeling, infection, and internal transcribed spacer regions
Celio, Gail J
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Powdery mildew disease caused by the ascomycete fungus Oidium sp. is a significant problem for growers of poinsettia (Euphorbia pulcherrima). Transmission electron microscopy of samples prepared for study using high pressure freezing (HPF) followed by freeze substitution (FS) was used to examine infection of poinsettia leaves by the fungus and the cytochemistry of host cell wall appositions (papillae). Haustoria of Oidium sp. were globose and compact, with slender lobes coiling around the main haustorial body. The extrahaustorial membrane was thicker than the host cell plasma membrane, with which it is continuous, and appeared more convoluted when encasing a mulit-lobed haustorium. Immunogold labeling using antibodies for callose, xyloglucan, and arabinogalactan proteins revealed exclusive localization regions within the heterogenous zones of the papillae. The localization pattern was somewhat different than those seen in other pathosystems using similar antibodies. Internal transcribed spacer (ITS) regions were sequenced from collected samples of Oidium sp. on cultivated poinsettia and wild poinsettia (Euphorbia heterophylla), and Microsphaera euphorbiae on wild poinsettia. Sequences were identical with the exception of three of the four samples taken from wild poinsettia, which differed by 3-4 base pairs. Phylogenetic analyses were conducted with these sequences and those of 36 other powdery mildew fungi from the genera Erysiphe sect. Erysiphe, Microsphaera, and Uncinula, which have the same anamorphic form as Oidium sp. and M. euphorbiae (Oidium subgenus Pseudoidium). Parsimony and neighbor-joining trees gave similar topologies. Oidium sp. and M. euphorbiae had high support as a group, though there was <50% bootstrap support for many branches overall. Conidial infection structure development of Oidium sp. on cultivated and wild poinsettia was observed at 4, 8, 12, 24, 36, and 48 hours after inoculation (hai) and the presence of primary germ tubes, appressoria, haustoria, and secondary germ tubes from conidia was noted. Germination and appressorium formation did not differ between plant types over time. Haustorium formation and secondary germ tube emergence was significantly different at 24 and 38 hai for run 1, and at 24 hai for run 2.