A real-time PCR based assay as an alternative to a seedling assay to evaluate potential risk of Meloidogyne incognita

Abstract

To develop a rapid assay for evaluating the potential impact of Meloidogyne incognita on a crop, we investigated the relationship between its density and gall index in a seedling assay. Forty-eight soil samples were collected from 12 sites of four greenhouses at two depths (surface 0–30 and subsurface 30–45 cm) two times (after disinfection and at harvest of tomato). While the densities of M. incognita were measured with real-time PCR, an assay using cucumber seedlings was conducted to evaluate damage by M. incognita. No or very low damage was observed to cucumber seedlings in all the 13 soils in which no M. incognita was detected. No serious damage was observed in the soils with densities of M. incognita less than 48 J2eq (second-stage juveniles equivalent)/20 g soil, suggesting that this density may be a threshold for damage occurrence. Gall index was significantly correlated (r² = 0.4586, P < 0.001) with the densities of M. incognita. Meloidogyne incognita was detected in all the 12 subsurface soil samples collected after disinfection and damage to seedlings was observed in 10 out of the 12 samples, suggesting that it is necessary to assess the impact of disinfection not only on surface soil but also subsurface soil. The present results reveal that a diagnosis method of M. incognita using real-time PCR might be a rapid and reliable tool for risk evaluation after disinfection.