The logic of cereal disease control with antifungal azoles

Quantitative structure-activity relationships have been performed by applying the Hansch approach to a series of known and new antifungal azoles, mainly 1,2,4-triazole derivatives, in their control of two diseases, caused by Erysiphe graminis and Puccinia graminis, on wheat plants. The residual protective control of E. graminis may be expressed as a linear function of a single parameter, represented by log P air/water, calculated by available data of solubility and vapor pressure at 20°C. The curative action against P. graminis follows a more complex logic and requires multiparametric equations. The related EC 80 values could be expressed as a linear function of the difference (log P air/water − log P octanol/water) = log P air/octanol, provided two dummy parameters were added to formally account for the presence of H-donor and H-acceptor groups present in the side chain of the azoles. The uncertainty associated with the latter of these parameters is discussed and tentatively ascribed to a possible masking of electrostatic potential associated with electronegative groups. The different complexity in the control of the two pathogens is thought to mainly depend on the different location of the related mycelia.