Very-long-chain aldehydes promote in vitro prepenetration processes of Blumeria graminis in a dose- and chain length-dependent manner.

Surface properties of aerial plant organs have been shown to affect the interaction of fungal plant pathogens and their hosts. Conidial germination and differentiation - the so-called prepenetration processes - of the barley powdery mildew fungus (Blumeria graminis f. sp. hordei) are known to be triggered by n-hexacosanal (C(26)-aldehyde), a minor constituent of barley leaf wax. In order to analyze the differentiation-inducing capabilities of typical aldehyde wax constituents on conidia of wheat and barley powdery mildew, synthetic even-numbered very-long-chain aldehydes (C(22)-C(30)) were assayed, applying an in vitro system based on Formvar(®)/n-hexacosane-coated glass slides. n-Hexacosanal was the most effective aldehyde tested. Germination and differentiation rates of powdery mildew conidia increased with increasing concentrations of very-long-chain aldehydes. Relative to n-hexacosanal, the other aldehyde compounds showed a gradual decrease in germination- and differentiation-inducing capabilities with both decreasing and increasing chain length. In addition to n-hexacosanal, several other ubiquitous very-long-chain aldehyde wax constituents were capable of effectively stimulating B. graminis prepenetration processes in a dose- and chain length-dependent manner. Other wax constituents, such as n-alkanes, primary alcohols (with the exception of n-hexacosanol), fatty acids and alkyl esters, did not affect fungal prepenetration.