COMPONENTS OF RATEREDUCING RESISTANCE TO CERCOSPORA LEAF SPOT IN SUGAR BEET: CONIDIATION LENGTH SPORE YIELD
V. Rossi , P. Battilani, G. Chiusa, S. Giosuè, L. Languasco, P. Racca
Four sugar beet cultivars exhibiting increasing levels of rate-reducing resistance towards Cercospora beticola (‘Cyrano’, ‘Bushel’, ‘Monodoro’, and ‘Break’) were used to study two resistance components: length of conidiation period (CP, the time required to produce conidia on the spots incubated under optimal conditions for sporulation) and spore yield (SY, the number of conidia yielded per cm2 of necrotic area per time unit); morphology and germination of conidia produced on the different cultivars were also considered. Experiments were carried out over a three-year period, considering sporulation on naturally infected leaves, either under field conditions or under optimal laboratory conditions, and on artificially inoculated leaves. Level of host resistance did not influence CP significantly: all spots produced conidiophores bearing conidia extensively 3 days after they had been stimulated to sporulate. On the contrary, resistance affected SY: the more resistant ‘Monodoro’ and ‘Break’ yielded fewer conidia per unit of necrotic area than the susceptible ‘Cyrano’ and the less resistant ‘Bushel’; the former cultivars produced about 35% of the conidia yielded by the latter ones. Differences in the cultivar rating were consistent over the experiments. Spores produced on the susceptible ‘Cyrano’ were longer, wider, and had more septa than the conidia yielded on the resistant cultivars, irrespective of their resistance level; they germinated more quickly than conidia yielded on other cultivars, and germ tubes grew more rapidly.