Population dynamics of Pythium sp. and Meloidogyne graminicola from an aerobic rice field planted to variety APO

Field sampling and pathogenicity tests were conducted during the planting seasons of 2007-2008 to assess the biotic factors which could contribute to poor seedling establishment, stunting, and yield decline of rice variety Apo grown under aerobic conditions in field B912 at IRRI. Based on soil analysis, it is a clay loam soil with 36% clay, 38% silt and 26% sand, and it has more than adequate supply P and K but deficient in N and has low organic C. Four fungal genera namely, Pythium, Rhizochtonia-like, Fusarium, and Sclerotium-like were isolated from discolored roots. Among these, Pythium isolates were most predominant. Succeeding pathogenicity test on rice variety Apo, however, revealed that only a few of these isolates are actually pathogenic. This is contrary to previous reports in 2005 where most of the Pythium isolated tested pathogenic to rice variety Apo. This apparent shift in the Pythium population composition may have been caused by the inundation of top soil from an unknown off-site location during a strong typhoon in late 2006. Among the recent isolates, only one was highly pathogenic causing pre-emergence damping-off and stunting of seedlings that were able to emerge. A few of the isolates are only slightly pathogenic while the rest are non-pathogenic with some apparently exhibiting growth-promoting activity. Pythium dynamics in the field for both 2008 dry and wet seasons showed increasing population up to 52 days after sowing (DAS) followed by a gradual decline. The population levels were higher during the wet-season. M. graminicola population, on the other hand, showed increasing trend throughout the growing season. Population levels of M. graminicola were also higher during the wet season. Between Pythium and M. graminicola, the latter appears more widespread and able to maintain high population levels until harvest in this field condition.