Determining the relationship between pre-plant Verticillium dahliae inoculum densities and development of Verticillium wilt of tomatoes

Verticillium dahliae is an economically important soilborne pathogen of tomato of which the microsclerotial inoculum can survive in the soil for extended periods of time. Previous studies, including two studies on tomato, reported that pre-plant microsclerotia soil densities can sometimes, but not always, predict the incidence or severity of Verticillium wilt. The overall objective of the study was to determine if V. dahliae pre-plant microsclerotia densities can be used as a predictive tool for the development of Verticillium wilt of tomatoes. A published qPCR assay was optimised and could detect 4.20 fg V. dahliae DNA (0.16 microsclerotia/g soil) from tomato field soils. There were no noticeable variations in the average number of microsclerotia per hectare across five commercial tomato fields, regardless of whether a high density (four composite samples from 20 soil cores/ha), medium density (four composite samples from 12 soil cores/ha), or low density (one composite sample from five soil cores/ha) sampling method was employed. A highly intensive sampling strategy (20 independently analysed soil cores/ha) was investigated for its predictive value in disease development since it allowed for correlation analyses to be conducted. A significant correlation (r = 0.578, P = 0.008) was evident between the number of microsclerotia pre-plant and disease severity during the season in only one of the three investigated fields; no correlation existed with yield. A glasshouse plant bioassay, using specific quantities of inoculated microsclerotia, established that the microsclerotia threshold required for significant disease development was five microsclerotia/g soil (283.8 fg DNA/g soil). However, one and two microsclerotia/g soil, although not causing significant symptoms, were able to sometimes infect the plants. This study showed that pre-plant V. dahliae soil densities do not have a disease predictive value in tomato, and that plant infection can develop at very low soil inoculum densities.