Effect of different water regimes on nematode reproduction, root galling, plant growth and yield of lowland and upland Asian rice varieties grown in two soil types infested by the rice root–knot nematode Meloidogyne graminicola

In a screenhouse experiment, plants of the lowland rice variety Thihtayin and the upland rice variety Kone Myint 2 were grown in two soil types (clay loam and sandy loam), inoculated with 3,000 Meloidogyne graminicola second-stage juveniles (J2) per plant and. from 6 weeks onwards maintained until harvest under three water regimes: permanently flooded, intermittently flooded and upland (monsoon rainfed) conditions. Both varieties were susceptible to M graminicola infection under all three water regimes and in both soil types but differences in susceptibility were observed between the two varieties and among the treatments. The effect of water regime on the number of eggs and J2 of M graminicola inside the roots was lower than expected: with one exception no significant effects were observed of any of the water regimes on the root population density in both rice varieties in both soil types. This observation may be explained by the delayed flooding, which started 6 days after nematode inoculation for the permanent and intermittent flooding water regimes. In both varieties and in both soil types, the root galling index was significantly lower on permanently flooded plants (< 4.5) compared with plants that had been either intermittent flooded or grown under upland conditions (>= 5.0). The highest root galling indices were always observed on plants grown under upland conditions (7.0-8.5). Permanent flooding prevented the suppression of most plant growth and yield-contributing traits measured. Moreover, permanent flooding also prevented significant yield loss in plants of both varieties grown in the clay loam soil and in plants of variety Thihtatyin grown in the sandy loam soil. The results of our study confirm again the enormous impact M graminicola infection can have on the yield of both lowland and upland rice varieties. With the exception of one treatment, yield loss was always higher than 20% and even almost 100% (yield failure) in plants of both varieties grown in the sandy loam soil under upland conditions. Although yield losses caused by nematodes carried out under screenhouse experiments tend to result in an overestimation of these losses, the results of our screenhouse experiments show that yield losses caused on Asian rice by M graminicola must be very high also under field conditions in the farmer's fields.