Cochliobolus miyabeanus (teleomorph, formerly known as Helminthosporium oryzae, anamorph Bipolaris oryzae) is a fungus that causes brown spot disease in rice. It was considered for use by the US as a biological weapon against Japan during World War II.

Hosts and symptoms

Brown spot of rice is a plant fungal disease that usually occurs on the host leaves and glume, as well as seedlings, sheaths, stems and grains of adult host plants. Hosts include Oryza (Asian rice), Leersia (Cutgrass), Zizania (Wild rice), and other species as well such as Echinochloa colona (junglerice) and Zea mays (maize). Cochliobolus miyabeanus may cause a wide range of symptoms. General symptoms occurring on the hosts can be observed on many parts of the plant, including leaves, seeds, stems and inflorescences, along with the presence of brown spot. Discoloration of stems is another symptom develops from brown spot of rice disease. Oval-shaped brown spots are the fungal growth sign, which have grey colored center developed on host leaves. The fungus produces a toxin known as ophiobolin which inhibits the growth of roots, coleoptiles, and leaves. This pathogen has also been known to produce non-host specific toxins which suppress plant defenses, causing the characteristic brown spots on rice leaves. Dark coffee-coloured spots appear in the panicle and severe attacks cause spots in the grain and loss of yield and milling quality. Also, lesions on glumes and seeds occur if the pathogen associates with other fungi and insects. Such lesions may develop when favorable condition for sporulation is present.

Importance

Cochliobolus miyabeanus is an important plant pathogen because it causes a common and widespread rice disease that causes high level of crop yield losses. It was a major cause of the Bengal famine of 1943, where the crop yield was dropped by 40% to 90% and the death of 2 million people was recorded. It is a possible agroterrorism weapon. Other known severe crop loss cases caused by Cochliobolus miyabeanus are globally distributed. In the Philippines, rice seedling mortality rate has been recorded up to 60%. In India and Nigeria, it can reduce total crop yield by up to 40%. Similar losses are observed in Suriname and Sumatra.

Environment

There are several factors influencing the disease cycle and epidemics of brown spot of rice disease.

Management

Prevention

The spread of the fungus can be prevented by using certified disease-free seed and using available resistant varieties such as MAC 18. Avoiding dense sowing will can also help prevent the spread of the fungus as it reduces humidity. Maintaining control of weeds and removal of volunteer crops in the field can also prevent fungal spread, as well as burning the stubble of infected plants. Seed treatments can also be used as a preventative measure. Seeds can be treated with fungicides or alternatively soaking seeds in cold water for 8 hours before treating with hot water (53–54 °C) for 10–12 minutes prior to planting. Soil treatments can also be used to prevent the spread of C. miyabeanus. The addition of potassium and calcium if the soil is deficient can help boost disease resistance. However, excessive application of nitrogen fertilisers should be avoided.

Control

Once symptoms are observed the disease may be controlled by burning removal and burning of any plants and maintaining water levels up to 3 inches at grain formation. below grain formation.

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