Comparison of the damage potential and yield loss of the rice root-knot nematode, Meloidogyne graminicola, on lowland and upland rice varieties from Myanmar

In Asia, the rice root-knot nematode, Meloidogyne graminicola, is an important pathogen of Asian rice (Oryza sativa) in most rice producing countries including Myanmar. In the first part of our study, the damage potential of M. graminicola on commonly cultivated newly released high-yielding lowland rice varieties and upland rice varieties (traditional, improved and aerobic), which are being grown in different regions in the summer-irrigated lowland and rainfed upland rice ecosystems in Myanmar, was investigated under screenhouse conditions. In the second part of our study, a field experiment was carried out in the Ayeyarwady River Delta, the major lowland rice producing area of Myanmar, to investigate the impact of M. graminicola on plant growth and yield of the same lowland rice varieties included in the screenhouse experiment in a naturally-infested farmer's field. None of the 15 lowland varieties and none of the nine upland varieties included in the screenhouse experiments was resistant to M. graminicola. Although differences in susceptibility were observed among the lowland and upland varieties, and between the two types (lowland and upland) of rice varieties, with an Mfeggs (multiplication factor of the second-stage juveniles without counting the eggs) ranging from 13.2 to 52.8 for the lowland varieties vs 39.8 to 108.4 for the upland varieties, all varieties included in our study can be considered as highly susceptible to M. graminicola when the nematode population densities are assessed at harvest. Also in the field experiment, all the 15 lowland varieties were susceptible to M. graminicola infection. This part of our study shows that upland rice varieties are more susceptible to M graminicola infection than lowland rice varieties. The percentage reduction in lowland and upland varieties was very similar in six out of the ten plant growth and yield-contributing traits measured. The highest differences in percentage reduction were observed for fresh root weight (41.0 vs 26.1% for the lowland and upland varieties, respectively), number of tillers per plant (29.1 vs 14.1%, respectively), percentage filled grains per plant (11.7 vs 0.8%, respectively) and filled grain weight per plant (34.7 vs 47.6 g, respectively). Within the two types of rice varieties significant differences in percentage reduction of plant growth traits between uninoculated and inoculated plants were observed among the rice varieties, so we suggest that the effect of M. graminicola infection on the different vegetative and reproductive plant growth stages of rice varieties is highly genotype-dependent and that no general conclusions can be made. In the screenhouse experiments, infection with M. graminicola caused on average a yield reduction with 31.1% in the lowland rice varieties vs 44.9% in the upland rice varieties, which indicates that in the upland varieties the higher nematode population densities per root unit (1 g) and per root system resulted in a higher yield loss compared with the lowland varieties. In the field experiment, carbofuran treatment resulted, on average, in a 16.5% increase in yield of the lowland rice varieties.status: publishe