Control of Penicillium expansum by plant volatile compounds

Nine plant volatiles were tested for their activity in vitro and in vivo against Penicillium expansum , the cause of blue mould of pear. In vitro spore germination and mycelial growth assay showed a consistent fungicidal activity by trans ‐2‐hexenal, carvacrol, trans ‐cinnamaldehyde and citral, while hexanal (‐)‐ carvone, p ‐anisaldehyde, eugenol and 2‐nonanone exhibited a progressively lower inhibition. trans ‐2‐Hexenal was the best inhibitor of conidial germination [MIC (minimum inhibitory concentration) = 24·6 µ L L −1 ; ED 50 = 10·2 µ L L −1 ], while carvacrol was the best inhibitor of mycelial growth (MIC = 24·6 µ L L −1 ; ED 50 = 9 µ L L −1 ). The four most active compounds in in vitro studies were tested in vivo as fumigants against blue mould on pear cv. Conference. Best control was achieved by trans ‐2‐hexenal vapour treatments (12·5 µ L L −1 ) when applied over a 24‐h period, beginning 24 h after inoculation. In contrast, carvacrol (12·5–200 µ L L −1 ), and trans ‐cinnamaldehyde (50–400 µ L L −1 ) were ineffective and citral (200 µ L L −1 ) showed only slight effect.