''Cochliobolus miyabeanus'' (formerly known as ''Helminthosporium oryzae'') is a

fungus A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...

that causes brown spot disease in

rice Rice is the seed of the grass species ''Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima ''Oryza glaberrima'', commonly known as African rice, is one of the two domesticated rice species. It was first domesticated and grown i ...

. It was considered for use by the

USA The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

as a

biological weapon A biological agent (also called bio-agent, biological threat agent, biological warfare agent, biological weapon, or bioweapon) is a bacterium, virus, protozoan, parasite, fungus, or toxin that can be used purposefully as a weapon in bioterroris ...

against Japan during

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

Hosts and symptoms

Brown spot of rice is a plant fungal disease that usually occurs on the

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman * Michel Host ...

leaves and glume, as well as seedlings, sheaths, stems and grains of adult host plants. Hosts include ''

Oryza ''Oryza'' is a genus of plants in the grass family. It includes the major food crop rice (species ''Oryza sativa'' and ''Oryza glaberrima''). Members of the genus grow as tall, wetland grasses, growing to tall; the genus includes both annual an ...

'' (Asian rice), ''

Leersia ''Leersia'' is a genus of plants in the grass family which includes species known generally as cutgrasses. The genus is widespread across many countries on all the inhabited continents. It was named for the German botanist Johann Daniel Leers ...

'' (Cutgrass), ''

Zizania Wild rice, also called manoomin, Canada rice, Indian rice, or water oats, is any of four species of grasses that form the genus ''Zizania'', and the grain that can be harvested from them. The grain was historically gathered and eaten in both ...

'' (Wild rice), and other species as well such as ''

Echinochloa colona ''Echinochloa colona'', commonly known as jungle rice, deccan grass, or awnless barnyard grass, is a type of wild grass originating from tropical Asia. It was formerly classified as a species of ''Panicum''. It is the wild ancestor of the culti ...

'' (junglerice) and ''

Zea mays Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. Th ...

'' (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

glume In botany, a glume is a bract (leaf-like structure) below a spikelet in the inflorescence (flower cluster) of grasses (Poaceae) or the flowers of sedges (Cyperaceae). There are two other types of bracts in the spikelets of grasses: the lemma and ...

s and seeds occur if the

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

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 The Bengal famine of 1943 was a famine in the Bengal province of British India (present-day Bangladesh, West Bengal and eastern India) during World War II. An estimated 0.8 to 3.8 million Bengalis perished, out of a population of 60.3 millio ...

, 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. 1. Rainfall and drought - The first factor affecting ''Cochliobolus miyabeanus'' life cycle is rainfall and drought. It tends to proliferate when there is reduced rainfall and in dewy conditions. In addition to a low level of precipitation, severe epidemics of rice brown spot occur during drought season. Compared to well flooded or irrigated areas, disease occurrence is favored in drier environments where a reduced amount of water is present. 2. Temperature and humidity - Another factor affecting disease development for ''Cochliobolus miyabeanus'' is temperature and humidity. Infection efficiency is influenced by the humidity level of the leaves, and lowered minimum temperature for crop cultivation favors epidemics of this disease. Infection by this pathogen is favored by long durations of leaf wetness, however this disease has even been reported without free water when humidity levels are above 89%.''Cochliobolus miyabeanus'' grows well at lower temperatures during its developmental stages compared to the developed stage, so if high temperatures are maintained in the area it is likely that farmers can restrict the growth of this pathogen. The optimal temperature for the pathogen is between 20 to 30C, however the pathogen can occur anywhere between 16 and 36C. 3. Nutrition level - Nutrition of the host plant may also influence the level of disease development. For example, low soil nutrient content is associated with epidemics of rice brown spot. If soil minerals such as nitrogen, potassium, phosphorus, silicon and manganese are deficient, this will likely favor disease development. In specific, in areas where silicon is present in a high amount in the soil, the host becomes less susceptible to this disease because silicon not only alleviates physiological stresses of the host, but also promotes disease resistance ability in the host. Furthermore, soil moisture level contributes to disease occurrence. Brown spot of rice is favored in areas where water content is low in soil.

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.

Sources

References

World Food Crisis: Meeting the demands of a growing population
by Jeff Batten, APS/CPS Annual Meeting, Monday, August 9, 1999 {{Taxonbar, from=Q3767784 Fungal plant pathogens and diseases Rice diseases Cochliobolus