Quantification of Mirafiori lettuce big‐vein virus and its vector,Olpidium virulentus,from soil using real‐time PCR

Big vein disease of lettuce (Lactuca sativa) is an economically important disease transmitted through soil by Olpidium virulentus, and has occurred in most production areas worldwide. The disease is assumed to be caused by Mirafiori lettuce big‐vein virus (MiLBVV). To understand the dynamics of the virus and its vector, MiLBVV and O. virulentus were directly detected in soil. DNA and RNA were extracted from 5 g soil using a bead beating method, followed by purification using adsorption to a column. Detection and quantification were performed using real‐time PCR and a TaqMan probe that was prepared based on the CP region of MiLBVV and the rDNA‐ITS region of O. virulentus, respectively. Furthermore, using a visual assessment of the incidence rate of big vein disease on lettuce in agricultural fields, the C_t values of MiLBVV and O. virulentus from soil were also determined using real‐time PCR. The results showed that MiLBVV concentrations in the soil were high in the field, as also determined by a visual assessment of the incidence rate of big vein disease on lettuce. However, the amount of O. virulentus in soil was not directly correlated with the incidence of MiLBVV. From these results, it is suggested that the risk of lettuce crops developing big vein disease can be estimated using an index of the amount of MiLBVV in the soil.