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 chasmothecia are often densely packed. As with most Erysiphaceae, Blumeriahordei 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. Blumeriahordei can be found worldwide, wherever its host species are found. [1]
Blumeriahordei affects a commercially vital crop, barley, and has been reported as one of the most costly diseases of the world's most produced crops. [2] Consequently, much work has been done to investigate potential resistance and controls for the fungus. [3] Most conventional treatments involve the application of fungicides, but work has been done to investigate breeding infection-resistant varieties of barley. [4] [5]
Disease cycle
The fungus overwinters as cleistothecia on straw, and in milder climates, also as mycelium and conidia on stubble and straw or volunteer barley and certain grasses. Windborne ascospores or conidia are the primary inoculum (also known as the propagule ) and can be dispersed over considerable distances. Infection by conidia requires high humidity, but not free water on the leaf surface. Sporulation and spore dispersal are favored by drier conditions. Thus the disease does well under alternating wet and dry conditions. Production of conidia declines markedly as the colony ages. Cleistothecia develop on older leaves as the plant matures. Low temperatures, together with the wetting of the cleistothecia for at least 72 hours, induce the maturation of the ascospores. Ascospores are released following rains, but are relatively sparse in comparison to the conidia. [6]
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