Rice yellow mottle virus (RYMV) in Madagascar Island provides an opportunity to study the spread of a plant virus disease after a relatively recent introduction in a large and isolated country with a heterogeneous host landscape ecology. Here, we take advantage of field survey data on the occurrence of RYMV disease throughout Madagascar dating back to the 1970s, and of virus genetic data from ninety-four isolates collected since 1989 in most regions of the country to reconstruct the epidemic history. We find that the Malagasy isolates belong to a unique recombinant strain that most likely entered Madagascar through a long-distance introduction from the most eastern part of mainland Africa. We infer the spread of RYMV as a continuous process using a Bayesian statistical framework. In order to calibrate the time scale in calendar time units in this analysis, we pool the information about the RYMV evolutionary rate from several geographical partitions. Whereas the field surveys and the phylogeographic reconstructions both point to a rapid southward invasion across hundreds of kilometers throughout Madagascar within three to four decades, they differ on the inferred origin location and time of the epidemic. The phylogeographic reconstructions suggest a lineage displacement and unveil a re-invasion of the northern regions that may have remained unnoticed otherwise. Despite ecological differences that could affect the transmission potential of RYMV in Madagascar and in mainland Africa, we estimate similar invasion and dispersal rates. We could not identify environmental factors that have a relevant impact on the lineage dispersal velocity of RYMV in Madagascar. This study highlights the value and complementarity of (historical) nongenetic and (more contemporaneous) genetic surveillance data for reconstructing the history of spread of plant viruses.

Salaudeen M.T. (2014): Relative resistance to Rice yellow mottle virus in rice. Plant Protect. Sci., 50: 1-7. We identified sources of Rice yellow mottle virus (RYMV) resistance in rice cultivars. Eight cultivars together with susceptible and resistant controls were evaluated under screenhouse conditions as inoculated and uninoculated treatment in completely randomised design with three replications. Seedlings were inoculated with the virus by sap transmission at two weeks after sowing. Disease incidence and severity (scale 1-9: 1-3 = green leaves with sparse dots or streaks, 9 = yellow or orange leaves and some plant dead), yield, and agronomic traits were recorded. Data analyses included Area Under the Disease Progress Curve (AUDPC), independent t-test, and Analysis of Variance. According to differences in most measured traits control cultivars FARO 29 and Gigante were proved to be the most susceptible and partially tolerant ones, respectively. Cvs FARO 12, FARO 17, FARO 37, and FARO 52 were classified as partially tolerant. Uninoculated control plants performed better than the inoculated for all the yield and agronomic parameters. Reduction in plant height (6%) and number of tillers per plant (4.8%), increased days to heading (3 days), and reduction in paddy yield (6.5%) was lowest in cv. Gigante. Paddy yield per plant of the RYMV-inoculated was the highest in cv. Gigante (2.4 g). The rice cultivars which combined RYMV-resistance with high-yield could be utilised in rice breeding programmes in order to enhance food security.

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