Wheat leaf showing symptoms of tan spot, also known as yellow leaf spot (Pyrenophora tritici-repentis, anamorph Drechslera tritici-repentis). Lesions first appear as tan to brown flecks, which expand into irregular, oval- or lens-shaped tan lesions up to 12mm long, with a yellow or chlorotic margin. Eventually, these lesions will develop a characteristic dark brown to black spot in the center due to the appearance of conidia. Lesions may coalesce, causing large necrotic areas in older leaves. As the disease progresses, entire leaves, spikes, and even whole plants may be killed.
For more information, see CIMMYT’s Wheat Doctor: http://wheatdoctor.cimmyt.org/index.php?option=com_content&task=view&id=119&Itemid=43&lang=en.
Photo credit: CIMMYT.