Latent entry and spread of Colletotrichum acutatum (species complex) in strawberry fields

Anthracnose, caused by Colletotrichum acutatum (species complex), has become a troublesome problem in strawberry production worldwide. This paper reports (i) an optimized sampling method combined with a real-time PCR technique to detect the latent presence of C.acutatum in cold-stored strawberry plants used as planting material in several European countries, and (ii) a study of the spread of C.acutatum following a point inoculation under field conditions. Screening of different parts of planting material suggested that C.acutatum is most likely to be present on runners and old petioles. In addition, in seven out of nine batches of planting material from different nurseries, latent infection by C.acutatum was detected in at least one of five replicate samples. Field experiments in 2009 and 2010 showed extensive latent within-field spread of the pathogen on strawberry leaves, with a within-row dispersal distance up to at least 175m in 1week. A straw ground cover between the rows did not decrease C.acutatum spread, probably because introduced (and/or subsequent) inoculum was confined to the plant bed (within the row) and was not present between the beds. Moreover, the number of C.acutatum spores on the symptomless leaves, as estimated using a real-time PCR method, was significantly (P<005) correlated with the incidence of fruit rot at harvest and post-harvest (r=056-066). These results illustrate the importance of detecting latent infections in planting material and strawberry leaves in the field.