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Epidemiological concepts and methods to predict the effects of plant morphology and partial resistance components on sunflower black stem epidemics.
A. A. SCHWANCK (1), S. Savary (1), L. Willocquet (1). (1) Inst National de la Recherche Agronomique, Toulouse, France

Experimental, modeling, and statistical approaches may be combined to mobilize epidemiological concepts to assess host plant resistance and its components. A set of 21 sunflower genotypes was used to assess morphological traits (MT) and sunflower black stem intensity (BS; caused by <i>Phoma macdonaldii</i>) in field experiments, as well as disease cycle components in greenhouse (GHE) and controlled conditions (CCE) experiments. The identification of predictors of BS field resistance was based on analyses performed at two scales. First, relationships between MT and BS intensity were analyzed at the field plot scale with a range of methods (cluster analyses of morphological and epidemiological variables, contingency and correspondence analyses, and logistic regressions). Second, predictors for field resistance were identified using genotype-scale data from MT, GH and CC measurements, using Kendall’s tau and <i>W </i>tests, and binary logistic regressions. The first approach showed significant (<i>P</i><0.05) associations of number of green leaves (GL), plant height (PH) and stem diameter with disease intensity in the field. The second approach showed significant (<i>P</i><0.05) association between field BS measurements and MT (GL and PH), GHE (stem lesion length and expansion) and CCE (infection efficiency) predictors. CCE predictors were better associated to BS incidence, and GH predictors to BS severity. These results point at potential phenotypic markers for partial host plant resistance.

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