Mapping quantitative trait loci (QTLs) for resistance to Cercospora leaf spot disease (Cercospora beticola Sacc.) in sugar beet (Beta vulgaris L.)

The breeding of sugar beet varieties that combine resistance to Cercospora and high yield under nondiseased conditions is a major challenge to the bre eder The understanding of the quantitative trait loci (QTLs) contributing t o Cercospora resistance offers one route to solving this problem. A QTL ana lysis of Cercospora resistance in sugar beet was carried out using a linkag e map based on AFLP and RFLP markers. Two different screening methods for C ercospora resistance (a field test at Copparo, Italy, under natural infecti on, and a newly developed leaf disc test) were used to estimate the level o f Cercospora resistance; the correlation between scores from the field (at 162 days after sowing) and the leaf disc test was significant. QTL analysis was based on F-2 and F-3 (half-sib family) generations derived from crosse s between diploid single plants of 93164P (resistant to Cercospora leaf spo t disease) and 95098P (susceptible). Four QTLs associated with Cercospora r esistance (based on Lsmean data of the leaf disc test) on chromosomes III. IV, vn and IX were revealed using Composite interval mapping. To produce po pulations segregating for leaf spot resistance as a single Mendelian factor , we selected fur plants heterozygous for only one of the QTLs ton chromoso me IV or IX) but homozygous for the others.