Morphological characterization of the interaction between Diplocarpon rosae and various rose species

Blackspot, caused by Diplocarpon rosae , is the most severe and ubiquitous disease of garden roses, but information is lacking about genotype‐specific forms of resistance and susceptibility of the host. Macro‐ and microscopic analyses of 34 rose genotypes with a defined monoconidial culture black spot inoculum identified susceptible and resistant rose genotypes and further genotype‐specific subdivisions, indicating the presence of partial forms of resistance and different resistance mechanisms. In total, eight interaction types were characterized, five representing compatible (types 1–5) and three representing incompatible interactions (types 6–8). The incompatible interactions were characterized by the lack of any visible fungal structures beneath the cuticle (type 8), single‐cell necroses (type 7) or necroses of larger cell clusters (type 6), the latter two types with penetration hyphae and haustoria in epidermal cells.