Temporal and spatial spread of Lettuce mosaic virus in lettuce crops in central Spain: factors involved in Lettuce mosaic virus epidemics

Lettuce mosaic virus (LMV) is transmitted by aphid vectors in a nonpersistent manner as well as by seeds. The virus causes severe disease outbreaks in commercial lettuce crops in several regions of Spain. The temporal and spatial patterns of spread of LMV were studied in autumn 2002 in the central region of Spain. Symptomatic lettuce (var. Cazorla) plant samples were collected weekly, first at the seedling stage from the greenhouse nursery and later outdoors after transplantation. The exact position of symptomatic plants sampled in the field was recorded and then material was tested by enzyme-linked immunosorbent assay to assess virus infection. Cumulative spatial data for infected plants at different growth stages were analysed using spatial analysis by distance indices. For temporal analysis, the monomolecular, Gompertz, logistic and exponential models were evaluated for goodness of fit to the entire set of disease progress data obtained. The results indicated that the disease progress curve of LMV epidemics in the selected area is best described by a Gompertz model and that the epidemic follows a polycyclic disease progression. Our data suggest that secondary cycle of spread occurs when noncolonising aphid species land on the primary infected plants (probably coming from infected seed) and move to adjacent plants before leaving the crop. The role of weeds growing close to lettuce fields as potential inoculum sources of virus and the aphid species most likely involved in the transmission of LMV were also identifiedThis work was supported by the Spanish Ministry of Education and Science (Plan National grants nos AGL-2000-2006 and AGL-2003-07532-C03-01).Peer reviewe