Virulence, sporulation, and elicitin production in three clonal lineages of Phytophthora ramorum

Phytophthora ramorum populations are clonal and consist of three lineages. Recent studies have shown that the clonal lineages may have varying degrees of aggressiveness on some host species, such as Quercus rubra. In this study, we examined virulence, sporulation and elicitin production of five P. ramorum isolates from each of the three clonal lineages. Virulence (lesion size) and sporulation (sporangia production) were determined on wound-inoculated detached leaves of Rhododendron catawbiense ‘Nova Zembla’. Lesion area differed between the clonal lineages (p < 0.001) with the EU1 and NA2 isolates producing significantly greater lesion areas than did NA1 isolates on inoculated leaves (approx. 4.2, 3.6, and 0.8 cm2 respectively). Similarly, lineages EU1 and NA2 produced significantly more sporangia per leaf (p < 0.001) than did lineage NA1 (approx. 800, 1000, and 300 sporangia per leaf respectively). Real-time PCR assays detected expression of the class I elicitins (ram-α1 and ram-α2) in all 15 isolates. Of the two elicitins, only the ram-α2 differed between lineage (p < 0.0001) with nearly 2-fold higher levels of expression in the EU1 and NA2 lineages as compared to the NA1 lineage. Ram-α2 expression showed a positive linear relationship with isolate virulence or lesion size (R2 = 0.707). A significant, positive, linear relationship was also observed between ram-α2 expression and sporulation although it was not as strong (R2 = 0.209). In summary, isolates belonging to clonal lineages EU1 and NA2 are generally more virulent, produce more sporangia, and produce more ram-α2 elicitin in vitro than isolates belonging to lineage NA1.