Occurrence of the rice root nematode Hirschmanniella oryzae on monsoon rice in Myanmar

During May-October 2007, soil and root samples from 539 fields were collected from 11 monsoon rice varieties in 12 regions in Myanmar. All regions surveyed and 90% of fields sampled were infested with the rice root nematode Hirschmanniella oryzae. The average H. oryzae population was 10/100 mL soil and 419/20 g roots respectively. In 6.9% of the fields sampled 50 H. oryzae/g root were found. The average root population densities were the highest (640/20 g roots) in Taungpyan variety and the lowest (155/20 g roots) in Immayebaw variety. The lowest soil and the second-lowest root populations of H. oryzae were observed in Shwethweyin which may indicate that this rice variety is less susceptible to H. oryzae. Among three cropping sequences, the highest frequency of occurrence (94%) was found in the rice-rice cropping sequence. Based on the prominence value (a combination of the frequency of occurrence and abundance) of H. oryzae, Hlaingtharyar was the most infested region where susceptible rice varieties are grown and rice-rice cropping sequence is practiced. The rice production in this region may be the most at risk of suffering important yield losses due to H. oryzae.