Brown Spot of Rice: Worldwide Disease Impact, Phenotypic and Genetic Diversity of the Causal Pathogen <i>Bipolaris oryzae</i> , and Management of the Disease

Abstract

ABSTRACT Rice brown spot caused by Bipolaris oryzae (syn. Cochliobolus miyabeanus ) is a re‐emerging disease worldwide. Under natural conditions, the disease causes approximately 4% in grain yield losses, ranging from 1% to 34% in countries of Africa and Asia. Rice seeds can be infected from relatively low (0.5%) to high (76%) rates. B. oryzae also infects wild rice ( Oryza australiensis , Oryza latifolia and Oryza rufipogon ) and other plant species, some of which are found in rice fields as alternative hosts. Characterisations of the pathogen's morphology, pathology and genetic diversity have been performed in several studies. B. oryzae colonies showed a wide range of colours varying from black to white and olive when grown on standard culture media. Strains isolated from rice are generally virulent with diverse aggressiveness, even within populations of the same geographic area. Clonal reproduction is predominant during epidemics. However, a low clonal fraction and balanced mating types suggest that sexual reproduction could take place in some areas. Most field studies reported high levels of pathogen genetic diversity and low population structure, suggesting that gene flow occurs between and among populations. Of the different methods used to control brown spot of rice, integrated management based on the use of healthy seed, resistant/tolerant varieties, balanced nitrogen fertilisation and water supply is preferred. This review reveals that a more precise estimation of the losses that this disease inflicts on rice production is needed. It also points out that knowledge of the population biology of the pathogen and epidemiological studies are required.