EMERGING DISEASE PROBLEMS IN INTENSIVE RICE SYSTEMS IN AFRICA.
ENSAM, 2 place Viala, 34060 Montpellier cedex, France
Background and objectives
Rice cultivation methods are very different among African countries where rice has very different stories. Irrigation was developed several centuries ago in Egypt and in Madagascar, the two main rice producers of Africa. In West Africa, upland rice remains dominant, but irrigation systems were developed during the 30 past years. This evolution provoked new epidemics. Research projects are carried on by national and international research institutes to obtain necessary knowledge to control the main pathogenic agents. These researches concern mainly epidemiological aspects, pathogens population analysis, and plant resistance.
Results and discussion
Rice Yellow Mottle Virus is probably the most damaging pathogen in irrigated fields of West Africa. The perennial vaid rice, Oryza longistaminata, is the main source of inoculum in humid valleys of the sahelian and northern part of the savannah areas where it grows on borders fields. In these areas resistant cultivars are necessary to control the disease. In the forest area, there is no Oryza longistaminata, and cultivated rice itself is the main source of inoculum. In this region, change in cultural practices by avoiding overlap between successive crops is probably a very efficient control method. The variability of the virus appears high for serological, molecular and pathogenicity characteristics. The interest for complete resistance identified in few O. sativa and Q. glaberrima cultivars is very high and their transfer in improved cultivars is in progress .
Bacterial blight due to Xanthomonas oryzae pv oryzae is present in the sahelian area where a very susceptible alternative host, the annual wild rice, Oryza brevigulata, is common. The disease is very severe when susceptible cultivars are tested, but the complete resistance of many improved cultivars released about 15 years ago, is still efficient.
In the coldest areas of central Africa and in Madagascar, bacterial brown sheath rot is responsible of severe losses. Comparison of RFLP's of the 16S RDNA of strains isolated from diseased tissues, revealed that, in Madagascar, the epidemics are caused by bacteria belonging to a genetic group corresponding to Pseudomonas fuscovaginae . No resistance to this disease has been identified.
Rice blast disease, Magnaporthe grisea, is still an important problem in different locations. A population analysis of a collection from Ivory coast indicates that, as in most other tropical areas, the population is clonal and consists of a small number of lineages. The virulence diversity between isolates belonging to the same lineage is high and no simple combination of resistance genes efficient against all lineages has been yet identified.
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