''Cochliobolus carbonum'' (anamorph: ''Helminthosporium carbonum'') is one of more than 40 species of filamentous
ascomycetes
Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defi ...
belonging to the genus ''Cochliobolus'' (anamorph: ''Bipolaris/Curvularia''). This pathogen has a worldwide distribution, with reports from Australia, Brazil, Cambodia, Canada, China, Congo, Denmark, Egypt, India, Kenya, New Zealand, Nigeria, Solomon Islands, and the United States. ''Cochliobolus carbonum'' is one of the most aggressive members of this genus infecting sorghum (''Sorghum'' spp.
oaceae, corn (''Zea mays''
oaceae and apple (''Malus domestica''
osaceae.
[Manamgoda, D., Cai, L., Bahkali, A., Chukeatirote, E., and Hyde, K. (2011). Cochliobolus: an overview and current status of species. Fungal Diversity 51, 3-42.] As one of the most devastating pathogens of sweet corn, ''C. carbonum'' causes Northern leaf spot and ear rot disease
while the asexual stage causes ''Helminthosporium'' corn leaf spot.
[Jones, M.J. and L.D. Dunkle, Analysis of Cochliobolus carbonum races by PCR amplification with arbitrary and gene specific primers. Phytopathology, 1993. 83(4): p. 366-370.] ''Cochliobolus carbonum'' is pathogenic to all organs of the corn plant including root, stalk, ear, kernel, and sheath.
[Sindhu, A., et al., A guardian of grasses: Specific origin and conservation of a unique disease-resistance gene in the grass lineage. Proceedings of the National Academy of Sciences, 2008. 105(5): p. 1762-1767.] However, symptoms of infection show distinct manifestations in different plant parts: whole plant - seedling blight affects the whole plant, leaf discoloration and mycelial growth, black fungal spores and lesions appear on inflorescences and glumes, and grain covered with very dark brown to black mycelium which gives a characteristic charcoal appearance due to the production of conidia.
Morphology and keys to identification
''Cochliobolus carbonum'' is divided into at least five different races based on pathogenicity, particularly lesion types on corn leaves.
[Lu, C., et al., Identification of races and mating types of Cochliobolus carbonum from corn in the yunnan province in China. Journal of Phytopathology, 2013: p. n/a-n/a.] ''Cochliobolus carbonum'' race 0 (CCR0) is essentially non-pathogenic.
[Welz, H.G. and K.J. Leonard, Phenotypic variation and parasitic fitness of races of Cochliobolus carbonum on corn in North Carolina. Phytopathology, 1993. 83(6): p. 593-601.] ''Cochliobolus carbonum'' race 1 (CCR1) produces host specific toxins and oval to circular spots on leaves.
[Tsukiboshi, T., T. Kimigafukuro, and T. Sato, Identification of races of Bipolaris zeicola, the casual fungus of Helminthosporium leaf spot on corn in Japan. Annals of the Phytopathological Society of Japan, 1987. 53(5): p. 647-649.] While infection by ''C. carbonum'' race 2 (CCR2) is rare, it is characterized by the production of oblong, chocolate-colored spots.
[Welz, G. and K.J. Leonard, Genetic variation in field population of race 0, 2 and 3 of Bipolaris zeicola in 1987. Phytopathology, 1988. 78: p. 1574.] ''C. carbonum'' race 3 (CCR3) produces linear and narrow lesions
while ''C. carbonum'' race 4 (CCR4) is weakly pathogenic, forming oval to concentric circular spots.
[Dodd, J.L. and A.L. Hooker, Previously undescribed pathotype of Bipolaris zeicola on corn. Plant Disease, 1990. 74(7): p. 530.]
The genus ''Cochliobolus'' is distinguished by the presence of dark to black ascomata with a unilocular, globose pseudothecium and a short, cylindrical neck.
Ascomata also bear hyphae and conidiophores and asci are bitunicate, 2-8 spored, and cylindrical to obclavate.
[Sivanesan, A., Graminicolous species of Bipolaris, Curvularia, Drechslera, Exserohilum and their teleomorphs. Mycologia, 1987. 158: p. 1-261.] Ascomata are black, globose to ellipsoidal, and setae are present over the upper half of the wall mixed with conidiophores.