Brown spot is a fungal disease that infects the coleoptile, leaves, leaf sheath, panicle branches, glumes, and spikelets. Its most observable damage are the numerous brown color big spots on the leaves which can kill the whole leaf.
Causal organism
Earlier the causal fungus of the disease was known as helminthosporium oryzae . The new name of the fungus is dreshlera oryzae dreshler. The perfect stage of the fungus is cochilobolus miyabeanus.
The fungus produce conidia are which are slightly curved which is budge at the middle and tapering at both ends, brown in colour,8-10 celled . The fungus produces a toxin Cochliobolin.
Symptoms
Leaf spot. At first minute brown colored dot to circular spots appears on the leaf which gradually enlarge. The spot normally remain isolated and almost uniformly scattered. Leaf sheaths are also attacked in a similar way and numerous spot may develop on it. As the number of spot increases, it affects normal functioning of leaves.
Grain discoloration. If infection occurs at realy growth stage, plants fail to develop panicle. This fungus also attacks during milky stage. Infection on grains causes them to turn black, and they remains unfilled. The husks of the affected grains are covered with oilvaceous velvety growth of the fungus. Infected grains act as a site for launching secondary infection.
Disease cycle
Although the fungus is soil and seed borne it remains in soil for a very limited period of time and seed borne inoculums fails to cause infection at temperature 280 c and above. The fungus survives in rice stubbles and in wild grass host like sateria, leersa hexendra etc in absence of rice.
Factors favorable for disease development
The disease can develop in areas with high relative humidity (86−100%) and temperature between 16 and 36°C. It is common in unflooded and nutrient-deficient soil, or in soils that accumulate toxic substances. For infection to occur, the leaves must be wet for 8−24 hours.
The fungus can survive in the seed for more than four years. It can spread from plant to plant through air. Major sources of brown spot in the field include:
• infected seed, which give rise to infected seedlings
• volunteer rice
• infected rice debris
• weeds
Brown spot can occur at all crop stages, but infection is most critical during maximum tillering up to the ripening stages of the crop.
Rice Brown Spot
Brown Spot
Rice Brown Spot
Sheath Blight