3.4.28
DEVELOPMENT OF AFLP MARKERS FOR DISEASE RESISTANCE IN SUGAR BEET

SA FRANCIS, M REDFEARN and MJC ASHER

IACR-Broom's Barn, Higham, Bury St Edmunds, Suffolk, 1P28 6NP, UK

Background and objectives
Sugar beet (Beta vulgaris ssp. vulgaris) is attacked by a variety of pathogens during growth and root storage, which cause estimated annual losses of 30 M. At present there is no, or very little, resistance in commercial UK sugar beet cultivars to some important diseases. Inoculation methods and phenotypic screening for resistance, particularly to late-season root rots, are very difficult and time-consuming processes not suited to a large commercial programme. The development of molecular markers for disease resistance would allow traits of interest to be quickly and efficiently selected early in a breeding programme. Marker-assisted selection also has the advantage of eliminating the effect of the environment on the expression of resistance. We are currently developing AFLP1 (amplified fragment length polymorphism) markers [1] for resistance to the following fungal and viral diseases of sugar beet: powdery mildew (caused by Erysiphe betae), rhizomania (caused by beet necrotic yellow vein virus, BNYVV), root storage rots (caused primarily by Fusarium culmorum), virus yellows (caused by beet mild yellowing virus, BMYV) and Polymyxa betae, the fungal vector of BNYVV. This paper reports the development of markers for powdery mildew and BNYVV resistance.

Materials and methods
For powdery mildew, six segregating populations of plants were developed by crossing disease-resistant B. vulgaris ssp. maritima sources with genetic male-sterile breeding lines. Resistant individuals of the F1 progeny were backcrossed to the male-sterile line to produce segregating BC populations. The BC populations were grown in field trial plots surrounded by a susceptible commercial cultivar which acted as a disease spreader. Leaf disc samples for marker analysis were taken before disease had developed. The trial was scored visually (on 919197) using a 6-point scale to distinguish resistance/susceptibility. For BNYW, two populations segregating for the Rz gene ('Holly gene') for rhizomania resistance were grown in infested soil in a glasshouse trial. Leaf discs were taken for marker analysis. The virus content of roots, and hence resistance/susceptibility, was measured by ELISA. Marker analysis was initially performed by bulk segregant analysis (BSA) with bulks comprising of the most resistant and susceptible individuals from each population. Twenty-four different primer combinations were used. Any bands unique to the resistant bulks were investigated further by repeating the AFLP analysis on individual plants.

Results and conclusions
Powdery mildew (Erysiphe betae): field trial data suggested that the resistance was probably polygenically controlled in all six populations. Two of the populations showed lower levels of resistance than the others and contained no markers. Preliminary analysis revealed nine putative marker bands after BSA and analysis of individuals. One population contained four possible markers amplified using two primer combinations.
Rhizomania (BNYW): a marker for the 'Holly gene' for BNYW resistance was found. This marker was present in two populations carrying Rz in different genetic backgrounds. The marker is being further characterised using larger numbers of individuals.

Putative markers have been found for resistance to powdery mildew and to BNYVV. These markers will next be tested for linkage to the resistance trait. The identification of markers for resistance to all the pathogens listed above would allow genes to be identified in breeders' lines and wild accessions, and the use of marker-assisted selection in the production of sugar beet varieties.

1The AFLP technique is covered by patents owned by Keygene NV.

Reference
1. Vos P, Hogers R, Bleeker M et al., 1995. Nucleic Acids Research 23, 4407-4414.