Biomass of Nematode and Associated Roots: A Determinant of Symptom Production in Root Knot Disease of Rice (Oryza sativa L.)

The biomass of Meloidogyne graminicola developing in rice roots was studied in relation to the expression of disease symptoms. The biomass of invading second stage juveniles (0.09 lg) increased to 33 lg on day 16 when adult females were in advance egg laying stage, with an increase of approximately 360-fold. Initiation of leaf yellowing was related to the ratio between total nematode and total root biomass of rice seedlings. Plants with nematode-to-root biomass ratios above 1 : 161 did not show any symptom while those with ratios between 1 : 138 and 1 : 121 exhibited yellowing. Plants with nematode-to-root biomass ratios between 1 : 115 and 1 : 60 showed moderate stunting while those with ratios between 1 : 43 and 1 : 20 exhibited severe stunting. The wilting symptoms occurred at or below 1 : 14 nematode-to-root biomass ratio. The yellowing of leaves in seedlings inoculated with graded inocula was expressed when the nematode-to-root biomass ratios reached to 1 : 136 on day 15 at 1000 J2, 1 : 138 on day 9 at 3000 J2, 1 : 134 on day 7 at 6000 J2 and 1 : 129 on day 5 at 9000 J2 per pot. In rice nurseries, seedlings showing moderate stunting, severe stunting, wilting and wilting with single gall recorded nematode-to-root biomass ratios of 1 : 92, 1 : 20, 1 : 12 and 1 : 14 respectively. In severely stunted transplanted rice, the nematode-to-root biomass ratio ranged from 1 : 84 to 1 : 75.