Components of rate-reducing resistance to Cercospora leaf spot in sugar beet: incubation length, infection efficiency, lesion size

Four sugar beet cultivars exhibiting increasing levels of rate-reducing resistance towards Cercospora beticola (‘Cyrano ’ or ‘Univers’, ‘Bushel’, ‘Monodoro’, and ‘Break’) were used in monocyclic infection experiments to study the following resistance components: incubation length (IP, the degree-day cumulation, base 5°C, during the time between inoculation and appearance of spots), infection efficiency of conidia (INF, the number of necrotic spots per cm 2 of leaf at the end of a single infection cycle), and lesion size (LS, the area of single spots). All the resistance components were influenced by cultivar significantly; whereas no differences in lesion type were found. Resistance delayed the appearance of spots by a maximum of 12 days compared to the susceptible cultivar. IP 50 was delayed by a degree-day cumulation of 28°C, 60°C and 100°C in ‘Bushel’, ‘Monodoro’, and ‘Break’, respectively. More resistant cultivars showed a reduced INF compared to both less resistant and susceptible cultivars, of about 44 % in ‘Monodoro’ and 79 % in ‘Break’. Furthermore, in ‘Break ’ the LS was 45 % smaller than that of the susceptible cultivar. Differences between cultivars were consistent from one experiment to another. Within cultivars variability was an important source of variation; it was attributed to both ‘between plants ’ and ‘between leaves within plants ’ variability. The former was due chiefly to the genetic heterogeneity of the cultivars used, which are open-pollened three-way cross hybrids; the latter, to differences in the age of the leaves of a plant.