Classic Genetics and Breeding - Genetics, Genomics and Breeding of Crop Plants - page 72

improvement obtained by combining two genotypes. Recurrent selection
for resistance to Sclerotinia head rot was carried out for 15 cycles on a restorer
sunflower population created in 1978. For the first three cycles a test
measuring rate of extension of mycelium on the back of capitula was used;
from the fourth cycle onwards, it was combined with a test based on ascospore
infections, which repeat more closely to natural infections ( Fig. 2-9 ) . An
80% reduction in diseased area was obtained in four cycles, with the first
test, thereafter the population remained stable and homogeneous for this
character. In 12 cycles the latency index (measure of incubation period) of
the ascospore test doubled, and the best relation with cycle was a simple
regression, with a significant slope, indicating that further improvements
should be possible. The hybrids made with the first, sixth and fifteenth
generations of the population showed a halving of percentage attack in the
field and hybrids with some of the best lines bred from several cycles
presented even greater levels of resistance (Vear et al. 2007).
mycelium test
Cycle
ascospore test
cycle
natural attack on hybrids
Susceptible control: GB PR56
Resistant control: SD PAC1
cycle
Figure 2-9 Reaction to Sclerotinia capitulum resistance tests over 15 generations of recurrent
selection (from Vear et al. 2007).