Blumeria graminis

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Blumeria graminis infection on barley
Source: Clemson University - USDA Cooperative Extension Slide Series - IPM Images

Blumeria graminis (DC. 1815) Speer 1975

The fungus causes powdery mildew of cereals. The disease is serious and widespread and is characterized by the white and powdery appearance of the leaf surface. Particularly the upper surface of young leaves is colonized. It develops most frequently under moist, relatively cool conditions and medium light intensity. Yield losses can reach 25%. Planting resistant cultivars is the most common control method.

The fungal spores disperse through the wind. After germination, the fungus enters the leaf tissue but continues to grow also on the leaf surface, causing the typical white powdery leafspots. Elliptic conidia, about 30 x 15 µm in size, are produce on the leaves in long chains. Near the end of its life cycle, the fungus forms small black dots on the mycelium. These are spherical chasmothecia, about 200 µm large, with filamentous appendages and 10-20 asci, each containing 8 elliptic ascospores (about 25 x 12 µm). The ascospores are the sexual stage of the fungus.

Vernacular names
• Deutsch: Getreidemehltau
Echter Mehltau der Gräser
• English: powdery mildew of wheat
barley powdery mildew
powdery mildew of cereals
powdery mildew of barley
• Español: oídio de los cereales
• Français: blanc des graminées
oïdium des cereals
oïdium des graminées

Eight different pathotypes (formae speciales) for different cereal species and grasses have been described. These specifically infect those species and cannot change hosts. The most important ones are:
Blumeria graminis f.sp. avenae (infects oat)
Blumeria graminis f.sp. hordei (infects barley)
Blumeria graminis f.sp. secalis (infects rye)
Blumeria graminis f.sp. tritici (infects wheat)

Synonyms:
Erysiphe graminis
Oidium monilioides (anamorph synonym)

For details see the respective page in Wikipedia.