Blumeria hordei is a species of powdery mildew in the family Erysiphaceae. It is found across the world on every continent bar Antarctica, where it infects plants in the genus Hordeum (barley). It has also been recorded on Agrostis exarata, Alopecurus aequalis, and Bromus.
Description
The fungus forms thick white mycelial growth on the leaves of its hosts, which becomes pigmented greyish-brown with age. The
mycelium is formed in spring and summer. Secondary mycelium is said to be dingy greyish-white to grey. When present, the
Ascocarp are often densely packed. As with most
Erysiphaceae,
Blumeria hordei is highly host-specific, almost exclusively occurring on
Hordeum species.
Hordeum is also a host for multiple other species of
Blumeria, including
B. americana,
B. graminis and
B. dactylidis. Other hosts of
B. hordei include
Agrostis exarata,
Alopecurus aequalis, and
Bromus species. These often rather host other species of
Blumeria, such as
Blumeria graminicola on
Alopecurus and
Blumeria bulbigera and
Blumeria bromi-cathartici on
Bromus. Other genera of grasses host other
Blumeria species, many of which are likely undescribed.
Blumeria hordei can be found worldwide, wherever its host species are found.
Taxonomy
Powdery mildew on
Hordeum was first described by E. Marchal in 1902 as
Erysiphe graminis f.sp.
hordei, although
Forma specialis have no standing in the
Code. M. Liu and Hambleton described the species
Blumeria hordei in 2021 during a taxonomic review of
Blumeria. Prior to this, all powdery mildews on grass hosts were considered to belong to
Blumeria graminis. The variation in this species had been long observed, and many
formae and
Forma specialis were created throughout the twentieth century.
was just one of seven new species described when
Blumeria graminis was split up. The type specimen was collected on
Hordeum vulgare in
Quebec, Canada. The specific epithet refers to its host genus.
Pathology
Blumeria hordei affects a commercially vital crop,
barley, and has been reported as one of the most costly diseases of the world's most produced crops.
Consequently,
Blumeria hordei, especially in its former state as a
forma specialis of
Blumeria graminis, has (like
B. graminis sensu stricto) studied at great length for both host resistance genes (to the mildew) and mildew resistance genes (to
Fungicide).
The species is regarded as a useful route to identifying resistances in cultivated
barley.
Currently, most conventional treatments involve the application of
, but work has been done to investigate breeding infection-resistant varieties of barley.
Due to its prevalence globally and its host specificity to a vital crop species, management of Blumeria hordei has been a high priority for millennia of barley producers. In the modern day, the most common management technique is the application of fungicides. As well as conventional fungicides, another chemical treatment for species of Blumeria involves treating barley with a silicon solution or calcium silicate slag. Silicon helps the plant cells defend against fungal attack by degrading haustoria and by producing callose and Plant cuticle. With silicon treatment, epidermal cells are less susceptible to powdery mildew.
Another way to control wheat powdery mildew is breeding in genetic resistance, using resistance genes to prevent infection.
Micromorphology
Description
The primary
mycelium is not inhibited by the secondary mycelium. It is effuse or typically in patches, at first white, becoming pigmented, greyish-yellow to greyish-brown. The secondary mycelium is dense, appearing woolly to felt-like. It occurs on the leaf in patches, often around chasmothecia. It is coloured dingy greyish white to grey. The hyphal
Appressorium are described as nipple-shaped, occurring in opposite pairs.
Conidium are broad and ellipsoid-ovoid. Conidiophores are single or in pairs, with foot cells with bulbous swelling around the middle. They can be branched or unbranched, with a basal septum at the junction with the mother cell or elevated up to 10 µm high. The
Ascus are typically broad ellipsoid-ovoid with short stalks. Ascospores were not observed by M. Liu et al.
Measurements
Primary hyphal cells measure 3–6 µm wide. Hyphal appressoria are 3–6 µm wide. Conidiophores are 60–120 × 5–7 µm with foot cells measuring 25–45 × 5–7 µm and varying between 10–15 µm wide in the middle and 5–7 µm wide at the basal septum. Conidia are 23–38 × 12–18 µm. The
Ascocarp are 170–285 µm in diameter when mature.
