Protocol for RYMV Inoculation and Resistance Evaluation in Rice Seedlings

Background

RYMV is a major biotic constraint for rice production in Africa (Séré et al., 2013) and has been reported in most rice-growing countries in Africa and Madagascar. It is not transmissible through seed (Konaté et al., 2001; Allarangaye et al., 2006) but by insect vectors (particularly beetles) and by contact during agricultural operations (Bakker, 1974; Traoré et al., 2006), especially while transplanting seedlings from seedbeds to the field. The virus is highly stable and capable of multiplying at high concentrations in its rice and wild Poaceae (Bakker, 1974) hosts.

Monitoring of RYMV incidence in seedbeds and varietal selection are the most efficient and sustainable ways of managing RYMV. There are two phenotypes of resistance – partial resistance, characterized by a delay in the appearance of symptoms (Albar et al., 1998), and high resistance, characterized by the absence of virus detection using DAS-ELISA (Ndjiondjop et al., 1999). Although partial resistance is widely distributed among Oryza sativa japonica varieties, only a few varieties from the cultivated rice species O. sativa and O. glaberrima express a high level of resistance to RYMV. Three major resistance genes – RYMV1, RYMV2 and RYMV3 – have been reported (Ndjiondjop et al., 1999; Thiémélé et al., 2010; Pidon et al., 2017).

Evaluation of the level of resistance to RYMV in rice varieties or lines and the comparison of the outcomes of different experiments require the use of a standardized protocol. This paper describes such a protocol which is based on DAS-ELISA and symptom severity. Reference accessions (susceptible, partially and highly resistant) have been included in the protocol to enable the drawing of reliable conclusions by comparing the test entry with reference materials. The number of plants tested for each variety or line is, however, not specified as it depends on the genetic material being tested and the objective of the experiment.