3.4.27
THE EVALUATION OF BETA GERMPLASM FOR DISEASE RESISTANCE

MC LUTERBACHER, JM HANDSLEY and MJC ASHER

IACR-Broom's Barn, Higham, Bury St. Edmunds, Suffolk, IP28 6NP, UK

Introduction and objectives
Sugar beet production in Europe is affected by several diseases including beet yellows virus (BYV), beet mild yellowing virus (BMYV), seedling damping-off (caused by Aphanomyces cochlioides and Pythium ultimum) and powdery mildew (Erysiphe betae). Currently, varieties resistant to these diseases are not available commercially and farmers resort to pesticides to achieve control. To rectify this situation work is under way to evaluate 600 Beta accessions for resistance to these diseases. New sources identified will be incorporated in future sugar beet breeding programmes.

Materials and methods
Many of the Beta accessions under evaluation are cultivated types (B. vulgaris spp. vulgaris) but others, notably B. vulgaris spp. maritima, are also well represented. Most are of European origin but there are examples collected from the rest of the world. With the exception of powdery mildew, artificial inoculation methods are used for evaluation and testing is conducted in glasshouses or controlled environment rooms to ensure uniform infection and to minimize environmental variation. With powdery mildew, natural epidemics at Broom's Barn are sufficiently reliable to allow testing in the field. The methods used are as follows:
BMYV and BYV: virus-carrying aphids (Myzus persicae) are placed on the leaves (at the two leaf stage) of 30 seedlings of each accession. After feeding for 4 days the aphids are killed by fumigation and the plants left to grow for 4 weeks in conditions suitable for virus multiplication. Subsequently, the virus content in leaf discs from each plant is quantified using ELISA; for each plant a mean virus content is calculated relative to a predetermined standard curve.
Seedling damping-off: the method used is a modification of previous work [1]. lnoculum of A. cochlioides and P. ultimum is produced in cultures. Subsequently, the colonized medium of each pathogen is combined at an appropriate rate with partially sterilized soil and the resultant mix is placed in cellular seed trays. 96 seeds per accession are sown individually in each seed tray cell. The trays are watered daily and after 2 or 3 weeks, for A. cochlioides and P. ultimum respectively, the emergent seedlings are scored for infection on a 0-4 scale. A percentage disease index for each accession is derived from the mean score.
Powdery mildew: evaluation is undertaken in nursery plots in the field. To ensure high and uniform infection pressure, plots containing accessions are separated by 'spreader' plots of a susceptible sugar beet variety; this variety also acts as a check. At a suitable time in the epidemic, disease levels are quantified on a 0-5 scale. In 1997 the disease was first noted on plots in late July and assessments were made in mid-August.

Resuts and conclusions
Results from evaluating the first 200 Beta accessions for each resistance trait are promising. Several accessions were found to contain low amounts of BMYV (<10%) and BYV (<2%) in comparison with the commercial variety Saxon (BMYV 35%; BYV 16%). A similar result was observed in the powdery mildew trials; three accessions had mean scores under 0.5 compared with the variety Sandra at 3.0. Preliminary results from the damping-off evaluation also indicate notable variation between accessions. In all cases individual plants identified as resistant to a particular pathogen are retained, vernalized to promote flowering, and subsequently self-pollinated. The resultant seed will be tested to see whether the trait is heritable. In addition to providing breeders with new sources of resistant germplasm, crosses are being made to develop molecular markers for particular traits so that marker assisted selection may be possible in the future.

References
1. Williams GE, Asher MJC, 1996. Crop Protection 15, 479-486.