Relationships between the damage to radish caused by the root-lesion nematode Pratylenchus penetrans, its density prior to cultivation and the soil nematode community structure evaluated by polymerase chain reaction-denaturing gradient gel electrophoresis

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A preliminary survey using 20 conventionally farmed fields in which fumigants have been applied every year showed that the root-lesion nematode Pratylenchus penetrans was distributed both in the upper (0-30cm) and lower (30-60cm) soil layers. In six of the 20 fields, P. penetrans was detected in the lower layers exclusively, suggesting that the most appropriate depth to sample soil is 0-60cm to estimate the relationship between the density of P. penetrans and its damage to radish. There was a highly significant correlation (r=0.923) between the density of P. penetrans in the 0-60cm depth and the number of spots on a radish. No damage to radish was observed in soils with <2.5 individuals of P. penetrans per 20g soil before cultivation. However, in cases in which the density of P. penetrans was 3.4-6.2 individuals per 20g soil, the number of spots on a radish showed more variation (0-131.5 per radish) and there was no significant correlation between them. The nematode community structure of soils with 3.4-8 individuals of P. penetrans per 20g soil, evaluated by polymerase chain reaction-denaturing gradient gel electrophoresis, was significantly different (ANOVA, PC2, P<0.05) between soils with low (0-42) and high (more than 80) damage levels, suggesting that radish damage might be predicted on the basis of the prevailing soil nematode community structure.
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