Cochliobolus miyabeanus strain WK-1C inoculated on rice cultivars Nipponbare and CO-39. Note differences in lesion phenotype but that both have necrotic centers surrounded by chlorotic halos. Image taken by Nur Ain Izzati Mohd Zainudin and Kent Loeffler, Dept. of Plant Pathology & Plant-Microbe Biology, Cornell University

The filamentous ascomycete genus Cochliobolus (anamorph Bipolaris/Curvularia) is comprised of more than forty closely related species, some of which are highly aggressive, superpathogens with particular specificity to their host plants. All members of the genus known to cause serious crop diseases fall in a tight phylogenetic group suggesting that a progenitor within the genus gave rise, over a relatively short period of time to the series of distinct biotypes (1), each distinguished by unique pathogenic capability to individual types of cereal.  Aggressive members include the necrotrophic corn pathogens Cochliobolus heterostrophus and Cochliobolus carbonum, the oat pathogen, Cochliobolus victoriae, the rice pathogen, Cochliobolus miyabeanus, the sorghum pathogen, Bipolaris sorghicola, the sugarcane pathogen, Bipolaris sacchari and the hemibiotrophic generalized cereal and grass pathogen, Cochliobolus sativus. 
The interaction between rice and C. miyabeanus is inadequately understood from the perspective of genetic and molecular mechanisms, although it has been reported that, like other Cochliobolus species, the fungus utilizes phytotoxins to trigger host cell death (2). Other necrotrophic Cochliobolus spp. and related taxa (e.g., Pyrenophora tritici repentis, Stagonospora nodorum, Alternaria alternata), are notorious for their ability to evolve novel, highly virulent, races producing Host Selective Toxins (HSTs). In contrast, no HST has been correlated with the ability of C. miyabeanus (anamorph: Bipolaris oryzae) to cause brown spot disease of rice. C. miyabeanus does produce several secondary metabolites that act as non-specific phytotoxins, including Ophiobolins A and B and rapid cell death results when rice cultivar Nakdong is inoculated with conidial germination fluid, suggesting production of a phytotoxin (2). Nowadays, brown spot is widespread and the disease along with sheath blights (Rhizoctonia solani), account for the highest yield loss in south and Southeast Asia (3, 4). Disease symptoms develop gradually and emerge as small purple brown spots that later enlarge into oval lesions with brown necrotic centers that are frequently surrounded by chlorotic halos (5, see image). Under conducive circumstances in the field, the disease can become polycyclic because C. miyabeanus has the potential to complete its asexual cycle in 10–14 days (6). The sexual stage of C. miyabeanus is similar to that of heterothallic C. heterostrophus. Both mating types can be found in the field (7). 

1.    Berbee ML, Pirseyedi M, Hubbard S (1999) Cochliobolus phylogenetics and the origin of known, highly virulent pathogens, inferred from ITS and glyceraldehyde-3-phosphate dehydrogenase gene sequences. Mycologia 91: 964-977.
2.    Ahn I-P, Kim S, Kang S, Suh S-C, Lee Y-H (2005) Rice defense mechanisms against Cochliobolus miyabeanus and Magnaporthe grisea are distinct. Phytopathology 95: 1248-1255.
3.    Xiao JZ, Tsuda M, Doke N, Nishimura S  (1991) Phytotoxins produced by germinating spores of Bipolaris oryzae. Phytopathology 81: 58–64.
4.    Savary S, Willocquet L, Elazegui FA, Castilla NP, Teng PS (2000a) Rice pest constraints in tropical Asia: quantification of yield losses due to rice pests in a range of production situations. Plant Disease 84: 357–369.
5.    Savary S, Willocquet L, Elazegui FA, Teng PS, Du PV, Zhu D, Tang Q, Huang S, Lin X, Singh HM, (2000b) Rice pest constraints in tropical Asia: characterization of injury profiles in relation to production situations. Plant Dis 84: 341–356.
6.    Johnson DR, Percich JA, (1992). Wildrice domestication, fungal brown spot disease and the future of commercial production in Minnesota. Plant Disease 76: 1193–8.
7.    Castell-Miller CV, Samac DA (2012). Population genetic structure, gene flow and recombination of Cochliobolus miyabeanus on cultivated wildrice (Zizania palustris). Plant Pathology DOI: 10.1111/j.1365-3059.2011.02581.x