COMPONENTS OF RATE-REDUCING RESISTANCE TO CERCOSPORA LEAF SPOT IN SUGAR BEET: INCUBATION LENGTH, INFECTION EFFICIENCY, LESION SIZE

V. Rossi, P. Battilani, G. Chiusa, S. Giosuè, L. Languasco, P. Racca

Abstract


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 cm2 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. IP50 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.

Keywords


Cercospora beticola; sugar beet; resistance components; incubation period; infection efficiency; lesion size

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DOI: http://dx.doi.org/10.4454/jpp.v81i1.1043

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