Rice cultivars susceptibility/resistance to the root-knot nematode Meloidogyne graminicola in Rwanda

The intensive monocropping of rice in Rwanda has gradually built up the pressure of pests and diseases to alarming levels in several marshlands. The most common epidemics are blast diseases and diopsis pest. The status of nematode problems in rice fields in Rwanda has not been investigated so far. We carried out an explorative study on the presence of M. graminicola in rice fields in Rwanda and no symptom of root-knot nematode infection has been identified on rice. The hypothesis was that either the rice cultivars in Rwanda were resistant to M. graminicola or the lower night temperature combined with the soil texture and composition were the limiting factors to M. graminicola reproduction. The results of the infection experiments carried out in laboratory on 15 popular cultivars from Rwanda showed that those cultivars are not resistant to M. graminicola. However, several New Rice for Africa (NERICA) cultivars that were also tested were (not significantly) less susceptible than the control Nipponbare (Np). This agrees with the resistance of NERICA against a range of root-knot nematodes, resistance acquired from its parent Oryza glaberrima. Infection experiments carried out in simulated conditions of temperature (22°C day and 15°C night) that prevail in Rwanda marshlands showed that the mean number of galls developed per plant was reduced compared with the results at 28°C (optimal temperature for M. graminicola reproduction in general). Our results lead to the conclusion that, due to the fluctuating temperature, that varies in a regular way between 22°C and 15°C in Rwanda marshlands, combined with the clayey character of the soil, the reproduction rate of the nematode was slowed and the possible infection was hampered.