Wheat leaf showing powdery mildew caused by Blumeria graminis. These fungal growths are initially white to pale gray, while older fungal tissue turns yellowish gray. They consist of fuzzy or powdery colonies of mycelia and conidia, on the upper surfaces of leaves and leaf sheaths (especially on lower leaves), and sometimes on the spikes. This superficial fungal material can be rubbed off easily with the fingers. Host tissue beneath the fungal material becomes chlorotic or necrotic and, with severe infections, the leaves may die.For more information, see CIMMYT's Wheat Doctor: <http://wheatdoctor.cimmyt.org/index.php?option=com_content&task=view&id=115&Itemid=43&lang=en.Photo credit: Thomas Lumpkin/CIMMYT.