The effect of nutrients on reduction of the germination and mycelial growth of Septoria nodorum by the fungicides propiconazole, fenpropimorph and prochloraz, and on the reduction of S.nodorum infection by prochloraz, was tested on wheat plants in a controlled environment. The nutrients tested were aphid honeydew, sucrose, a mixture of amino acids and combinations of sucrose with yeast extract and amino acids, all in different concentrations. All nutrients could stimulate S. nodorum infection in combination with the tested prochloraz concentrations (4.5–45 mg a.i./1), but amino acids did not significantly stimulate infection in the fungicide-free treatments. The stimulating effect of the nutrients was generally stronger with higher nutrient/fungicide ratios. Aphid honeydew had more effect than its main components sucrose and amino acids separately. Antagonism between nutrients and prochloraz was determined with the Colby method and from dose-response curves by comparing ED50 S and drawing antagonism isoboles. Aphid honeydew, sucrose and amino acids caused an up to 10-fold increase of the ED50 and can be considered antagonistic to prochloraz
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