High initial population densities of Meloidogyne graminicola result in loss of resistance and tolerance of recombinant inbred lines (RIL) of Asian rice (Oryza sativa)

A glasshouse experiment was conducted to examine the effect of a range of high initial populations densities (Pi = 25,000, 50,000 and 100,000 second-stage juveniles (J2) plant(-1)), equal to an initial pathogen pressure of 5, 10 and 20 J2 (g soil)(-1)), respectively, of a Meloidogyne graminicola population from Batangas (Philippines) on recombinant inbred lines (RIL) that had been classified as either (partially) resistant or tolerant to a lower initial pathogen pressure (1 J2 (g soil)(-1)) of this M. graminicola population. These RIL were derived from a cross between the Asian rice (Oryza sativa) genotypes 'IR78877-208-B-1-2' (resistant and tolerant donor parent) and either 'IR64' or 'Dinorado' (susceptible and highly sensitive recipient parents). Pi resulted (with few exceptions) in M. graminicola root population densities > 10,000 J2 (root system)(-1) and > 1,000 J2 (g roots)(-1), and in root gall indices >= 4.5 in the RIL and 'IR78877-208-B-1-2'. Based on the high final nematode population densities and root gall indices, the RIL and 'IR78877-208-B-1-2' cannot be considered as either resistant or partially resistant to M. graminicola. The effect of Pi on plant growth and yield varied among the variables measured and among the RIL. Plant height was least affected but root and shoot weight, number of panicles plant(-1) and % filled grains plant(-1) were significantly reduced. If a % yield reduction of < 10% and 10-20% (compared with non-infected control plants) is used as an indication of tolerance or less sensitivity, respectively, to M. graminicola infection, then none of the RIL and 'IR78877-208-B-1-2' can be considered as either tolerant or less sensitive to M. graminicola. These results underline the necessity when selecting donor parents to introgress either resistance/partially resistance or tolerance/less sensitivity to M. graminicola infection into Asian rice breeding lines with good agronomic characteristics to test the host response of these candidate donor parents under a wide range of environmental conditions that may influence the resistance and/or sensitivity of these candidate donor parents.