A model integrating components of rate-reducing resistance to Cercospora leaf spot in sugar beet

Four components of rate-reducing resistance to Cercospora leaf spot in sugar beet (infection efficiency of conidia RC1, incubation period RC2, size of necrotic spots RC3 and spore yield RC4), previously measured in single infection cycle experiments, were integrated into a model simulating the chain of infection cycles under field conditions, as influenced by weather. To integrate resistance components, variables accounting for infection frequency, incubation period, affected leaf area, and infectiousness - which are computed for a susceptible cultivar - were modified by means of coefficients which reduced (RC1, RC3, RC4) or increased (RC2) them. Outputs obtained by running the model and changing resistance components actually reduced the rate of disease progress and the area under the disease progress curve of epidemics (AUDPC), as happens at field level; therefore, the approach may be considered successful. Changes in single resistance components were closely correlated with changes in AUDPC: improvements in RC1, RC3 or RC4 reduced AUDPC by the same, over the whole range of variation in infection frequency, affected leaf area, and infectiousness; on the contrary, little improvements in RC2 were more effective than stronger ones. When components acted simultaneously, each of them reduced disease progress in proportion to its magnitude; when all components were improved by the same amount, they had about the same effectiveness in slowing the epidemic. Changing more components simultaneously reduced the disease development slightly more than additively. Advantages for plant breeders in improving their selection strategies are outlined.