Verticillium longisporum – identifying resistance to a new threat in winter oilseed rape

Verticillium in oilseed rape (OSR) is known since the 1930s in Sweden and the 1980s in Germany. The pathogen, Verticillium longisporum (VL), is host-specific to the Brassica species and particularly threatens the production of OSR in intense crop rotations. In contrast to diseases induced by other Verticillium species, VL does not induce wilt symptoms in OSR but stunted growth and premature bloom and maturity. The colonization of the plant by VL in the field is characterized by an extended latent phase until the beginning of maturity after which the pathogen bursts out of the xylem vessels to produce ample masses of microsclerotia underneath the stem epidermis, in the stem pith and roots. We transformed VL with the green fluorescent protein (GFP) or used conventional fluorescence dyes in interaction studies on roots of Brassica napus with the confocal laser scanning microscope. The pathogen infects the roots in the root hair zone by direct penetration of the rhizodermis without formation of appressoria-like structures and invades the vascular system, where it spreads by hyphal growth and the production of conidia. In the field, detection of VL in infected plants by ELISA (or PCR) revealed the spread of infection being retarded until early maturity stages of the crop (> GS 85). As chemical control of VL is not possible, a broad search for sources of resistance in B. napus lines and parent B. oleracea and B. rapa genotypes has been started. In this screen based on a greenhouse bioassay a considerable variation in disease severity among Brassica genotypes was found. Resistance predominantly originated from B. oleracea accessions, but also from a few B. rapa and B. napus lines. Resistance was due to inhibition of fungal spread in the vascular system rather than through the prevention of root penetration. Reduced pathogen spread in less susceptible genotypes was associated with distinct phenolic responses in the adjacent xylem parenchyma of the hypocotyl. Resistant genotypes detected in the greenhouse were evaluated under field conditions at several locations in Germany in 2006. The final objective of these studies is to characterize and evaluate resistance factors against V. longisporum and to identify sources of resistance potentially useful in breeding programmes.