Erysiphe heraclei
Species of fungus
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Description
The fungus forms mycelium on both sides of the leaves of its hosts, as well as on stems and inflorescences. As with most Erysiphaceae, Erysiphe heraclei is fairly host-specific, infecting almost solely plants in the family Apiaceae (with the exception of one genus in the Pittosporaceae). However, it infects a comparatively large range of genera for a powdery mildew species, which could suggest that it is in fact a complex of multiple cryptic species. Erysiphe heraclei can be found in any habitats where its host species occur, both wild and in urban and suburban gardens and parks.
Its reported host range includes Aegopodium, Aethusa, Ammi, Anethum, Angelica, Anthriscus, Apium, Arracacia, Astrodaucus, Athamantha, Aulacospermum, Berula, Bifora, Bowlesia, Bunium, Bupleurum, Cachrys, Carum, Caucalis, Centella, Chaerophyllum, Chaetosciadium, Cicuta, Cnidium, Conioselinum, Conium, Coriandrum, Cuminum, Daucus, Drusa, Dystaenia, Eryngium, Falcaria, Ferula, Foeniculum, Heracleum, Laser, Laserpitium, Levisticum, Ligusticum, Myrrhis, Oenanthe, Orlaya, Ostericum, Osmorhiza, Pastinaca, Petroselinum, Peucedanum, Phlojodicarpus, Physospermum, Pimpinella, Pleurospermum, Peucedanum, Rhabdosciadium, Ridolfia, Saposhnikovia, Scaligeria, Scandix, Schrenkia, Selinum, Seseli, Silaum, Siler, Sium, Smyrnium, Sphallerocarpus, Thapsia, Todaroa, Tordylium, Torilis, Trachymene, Trinia, Turgenia and Zizia in the Apiaceae and Billardiera in the Pittosporaceae. Some of these species also host other species of powdery mildew.
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Taxonomy
Erysiphe heraclei was formally described by de Candolle in 1815. The lectotype specimen was collected on Heracleum sphondylium. The specific epithet derives from the type host genus. Bradshaw et al. (2024) suggested that Erysiphe betae may be synonymous with Erysiphe heraclei.
Pathology
Erysiphe heraclei affects carrots, a widely-grown vegetable, and consequently research has been conducted into preventing or reducing the severity of infection on this host. Currently, most conventional treatments involve the application of fungicides. The most successful fungicides are said to be sulphur and azoxystrobin. Work has also been conducted to investigate creating and breeding infection-resistant cultivars of carrots.
Micromorphology
Description
The mycelium is hypophyllous (found on leaf undersides) and sometimes covers the entire surface. The hyphal appressoria are lobed and occur both solitarily and in opposite pairs. Conidophores arise from the top of their mother cell. They have cylindrical, straight foot cells. The conidiophores produce single conidia. Conidia are cylindrical; germ tubes arise an end and are variable in length. Conidia appressoria are vary variable and can have up to nine lobes. The chasmothecia (fruiting bodies) have a variable number of appendages in the lower half, which are myceloid and simple or irregularly branched. Appendages are said to often be interwoven with the mycelium. Appendages are pigmented yellowish-brown when mature. The peridium of the chasmothecia has inconspicuous cells of irregular shape. Erysiphe heraclei typically has three to five spores per ascus which are large, ellipsoid-ovoid and colourless. The asci are typically clavate or saccate and are sessile or short-stalked.
Measurements
Hyphal cells measure 40–90 × (2.5–)4–6(–10) μm. Conidiophores are up to 160 μm long, with foot cells that measure 20–70(–90) × 8–10(–12) μm. Conidia are 25–45(–55) × 12–20(–23) μm. The chasmothecia are 80–140(–170) μm in diameter with peridium cells 8–20(–25) μm in diameter. Appendages are (0.25–)0.5–1.5(–2)× the diameter of the chasmothecia and (3–)4–8(–10) μm wide. Asci number (2–)3–7(–10) and are 40–85(–90) × 30–45(–65) μm with ascospores measuring (16–)18–30(–34) × 10–18(–22) μm.