Variation in pathogenicity in the Fusarium verticillioides populations isolated from maize during the growing season

In Northern Italy the main etiological agent of Fusarium ear rot in maize is Fusarium verticillioides, the major producer of fumonisins. The role of F. verticillioides population variability, on disease incidence variation has not yet elucidated, and seems to be important for resistance breeding, evolutionary risk of the pathogen, and efficacy of the agricultural management practices. One hundred eighty one F. verticillioides strains were randomly selected among F. verticillioides populations associated with maize samples collected during different maize growth stages in three fields located in Lombardia region. Pathogenicity assays of the selected F. verticillioides strains was assessed on maize by evaluating seed germination percentages and calculating the percentage infection index (I%I) concerning seed colonization, radicle decay, and coleoptile rot. Seed germination occurred still when F. verticillioides colonized the entire kernel.To investigate variation in pathogenicity within the populations, cluster and principal component (PC) analyses were computed using I%Is obtained. Observed I%Is were very variable with significant correlations between seed colonization and both radicle decay and coleoptile rot. Cluster analysis showed that isolates were grouped in three main clusters. Bi-plot of PCA revealed that F. verticillioides strains isolated at seed and at maturity stages were gathered, while strains isolated from maize debris and at seedling and pre-silking stages showed a scattered distribution. F. verticillioides pathogenicity seemed to vary according to the maize growing stages, particularly when seedborne population and strains isolated at pre-silking stages are taken into account. Information gained from the combination of clustering method and PC analysis inferred that F. verticillioides pathogenicity showed very high intraspecific variability.