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Corn grey leaf spot
Grey leaf spot (GLS) is a foliar fungal disease that affects maize, also known as corn. There are two fungal pathogens that cause GLS, which are ''Cercospora zeae-maydis'' and ''Cercospora zeina'' .〔Ward, J. M. J., E. L. Stromberg, D. C. Nowell, and F.W. Nutter, Jr. 1999. Gray leaf spot: A disease of global importance in maize production. ''Plant Disease'' 83: 884-895〕 Symptoms seen on corn include leaf lesions, discoloration (chlorosis), and foliar blight. The fungus survives in debris of topsoil and infects healthy crop via asexual spores called conidia. Environmental conditions that best suit infection and growth include moist, humid, and warm climates. Poor airflow, low sunlight, overcrowding, improper soil nutrient and irrigation management, and poor soil drainage can all contribute to the propagation of the disease.〔http://www.oisat.org/pests/diseases/fungal/cercospora_leaf_spot.html〕 Management techniques include crop resistance, crop rotation, residue management, use of fungicides, and weed control. The purpose of disease management is to prevent the amount of secondary disease cycles as well as to protect leaf area from damage prior to grain formation. Corn grey leaf spot is an important disease of corn production in the United States, economically significant throughout the Midwest and Mid-Atlantic regions. However, it is also prevalent in Africa, Central America, China, Europe, India, Mexico, the Philippines, northern South America, and Southeast Asia.〔Crous PW, Braun U (2003). Mycosphaerella and its anamorphs. 1. Names published in Cercospora and Passalora. CBS Biodiversity Series 1: 1–571.〕 The teleomorph (sexual phase) of ''Cercospora Zeae-Maydis'' is assumed to be ''Mycosphaerella sp.''〔Nyvall, Robert F. (“Field Crop Disease” ). Wiley-Blackwell, 1999, p. 282.〕
==Host and Symptoms==
Corn is the only species that can be affected by ''Cercospora zeae-maydis''. There are two populations of ''Cercospora zeae-maydis'', distinguished by molecular analysis, growth rate, geographic distribution, and cercosporin toxin production. ''Cercospora Zeae-Maydis'' differs from its cousin group ''Cercospera zeina sp. nov'' in that it has faster growth rate in artificial media, the ability to produce the toxin cercosporin, longer conidiophores, and broadly fusiform conidia.〔Wang J, Levy M, Dunkle LD (1998) . Sibling species of Cercospora associated with grey leaf spot of maize. Phytopathology 88: 1269–1275.〕 ''Cercospera zeina sp. nov'' affects corn in the Eastern Corn Belt and Mid-Atlantic States; ''Cercospora Zeae-Maydis'' is found in most corn producing areas of western Kentucky, Illinois, Indiana, Iowa, Wisconsin, Missouri, Ohio, and west Tennessee (Midwest). Both populations share the same symptoms and virulence, the ability of the fungus to invade the host.〔Pedro W. Crous, Johannes Z. Groenewald, Marizeth Groenewald, Pat Caldwell, Uwe Braun, Thomas C. Harrington, "Species of Cercospora associated with grey leaf spot of maize."
Stud Mycol. 2006; 55: 189–197.〕
Major outbreaks of grey leaf spot occur whenever favorable weather conditions are present (see Environment section). The initial symptoms of grey leaf spot emerge as small, dark, moist spots that are encircled by a thin, yellow radiance (lesions forming). The tissue within the “spot" begins to die as spot size increases into longer, narrower leaf lesions. Although initially brownish and yellow, the characteristic grey color that follows is due to the production of grey fungal spores (conidia) on the lesion surface. These symptoms that are similar in shape, size and discoloration, are also prevalent on the corn husks and leaf sheaths. Leaf sheath lesions are not surrounded by a yellow radiance, rather a brown or dark purple radiance.〔Beckman, P. M., and Payne, G. A. 1982. External growth, penetration, and development of Cercospora Zeae-Maydis in Corn Leaves. Phytopathology 72:810-815 ()〕 This dark brown or purple discoloration on leaf sheaths is also characteristic to northern corn leaf blight (''Exserohilum turcicum''), southern corn leaf blight (''Bipolaris maydis''), or northern corn leaf spot (''Bipolaris zeicola''). Corn grey leaf spot mature lesions are easily diagnosed and distinguishable from these other diseases. Mature corn grey leaf spot lesions have brown rectangular and vein limited shape. Secondary and tertiary leaf veins limit the width of the lesion and sometimes individual lesions can combine to blight entire leaves.
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