Colonisation profiles of Thekopsora areolata and a co-existing Phomopsis species in Norway spruce shoots

Abstract

The difficulty in sub-culturing biotrophic fungi complicates etiological studies related to the associated plant diseases. By employing species-specific ITS sequence stretches, we used real-time PCR to investigate the spatial colonization profiles of T. areolata and a co-existing Phomopsis species in seedlings and saplings of Norway spruce showing bark necrosis. There was a strong gradient in the colonization level of T. areolata DNA along the lesion length, with the highest DNA amount levels being recorded in the area with dark brown phloem. The separate analysis of bark and wood tissues indicated that the initial spread of the rust to healthy tissues neighbouring the infection site presumably takes place in the bark. A Phomopsis species co-existing together with T. areolata in several cases showed very high DNA levels in the upper part of the lesion outside the brown phloem area, and even in the visually healthy proximal tissues above the lesions. This indicates that this ascomycete has a latent stage during early colonization of Norway spruce shoots. This mode of infection most probably explains the successful co-existence of Phomopsis with a biotrophic rust, as their mutual interest would be to avoid triggering host cell death.