Peronosclerospora noblei (Weston) CG Shaw

Synonyms

Sclerospora noblei Weston

Description

Conidiophores arising from stomata on the abaxial surfaces of chlorotic leaf blades, evanescent, 300-450 µm long in total, main axis 8-13 µm in diameter up to a septum wide at a septum 68-110 µm from the base then increasing to 20-34 µm wide at the start of the branches; branch system 70-130 µm high and 100-130 µm wide, dichotomous, with 2-4 primary branches then secondary, tertiary and quaternary branches, each terminating in a tapering pedicel 10-15 µm long.

Conidia borne singly at the tips of pedicels, obovoid, (21-)25-31(-39) x (13-)17-23(-31) µm, hyaline, aseptate, without operculum or pore, germinating by one or two germ tubes.

Oogonia formed in the leaf blades which split into a tangled mass of vascular fibres, ovioid-eliptical-subglobose, 28-44 µm in diameter, golden-brown, with smooth occasionally scabrid, flattened sides bordered by inconspicuous ridges ; wall (exosporium) 5-17 µm thick, reddish brown.

Oospores one per oogonium, globose, (19-)24-29(-35) µm in diameter, hyaline; wall (endosporium) 3-4 µm thick, hyaline, even, smooth.

Hosts
   
Host family: Poaceae
   
Host species:

Sorghum leiocladum - FH ex. Herb. WH Weston (holotype), BRIP , , , , , , , , , , , ,


Distribution

NSW, QLD

Comments

The American mycologist William Henry Weston Jr (1929) described the sexual state of this downy mildew fungus on a grass originally identified as Sorghum plumosum, but which is now known as Sorghum leiocladum (wild sorghum), a perennial, indigenous grass found along the Great Dividing Range from central Queensland to southern New South Wales. Later, Weston (1942) provided a detailed description of the asexual state, confirming that it belonged in the group of Sclerospora species whose spores germinate by the production of germ tubes. Specimens of leaves with the sexual and sexual states of the species noblei had been sent to Weston by the Australian agricultural scientist Robert Jackson Noble. On the basis of Weston’s observations of the mode of conidium germination, Shaw (1980) transferred the species noblei to Peronosclerospora. Using molecular phylogenetic analyses Telle et al. (2010) showed that P. noblei was a valid species.

The morphology of the asexual state of P. noblei and other Peronosclerospora species is influenced by environmental conditions. Although most conidiophores of P. noblei conform to the description provided above, abnormal conidiophores which (i) have a main axis with or without primary and secondary branches, (ii) are much shorter than normal conidiophores and (iii) bear only 3-8 conidia are occasionally found.

Hyphae of P. noblei perennate in the tiller bases of S. leiocladum and invade tiller buds and developing tillers in a similar way to that of P. eriochloae on early spring grass. Asexual and sexual sporulation is confined to the vegetative, infected tillers which are thicker and have more nodes than normal flowering tillers. The chlorotic leaves bearing the asexual state and later the sexual state on downy mildew-infected tillers are wider than leaves on the flowering tillers and are held in a bunched, upright manner. In the winter months, when S. leiocladum is dormant, tall dead tillers bearing frayed leaves with adherent oogonia are the dominant feature at sites where the downy mildew is common (Ryley 2001, 2002; Ryley and Langdon 2001).

Sorghum leiocladum is found only in Australia and its downy mildew P. noblei has not been recorded elsewhere, so the pathogen can be considered to be indigenous to the continent.

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Tiller abnormality of Peronosclerospora noblei infected Sorghum leiocladum.
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Frayed leaves of Sorghum leiocladum infected with Peronosclerospora noblei.
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Asexual sporulation of Peronosclerospora noblei on Sorghum leiocladum.
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Oospores of Peronosclerospora noblei on Sorghum leiocladum - BRIP 64656.
Scale bar = 10 µm.