Hexanal Vapours Suppress Spore Germination, Mycelial Growth, and Fungal-derived Cell Wall Degrading Enzymes of Postharvest Pathogens of Longan Fruit

Longan (Dimocarpus longan Lour.) fruit have a very short postharvest shelf life due to microbial decay and exocarp browning. The most common commercial means for prevention of longan fruit rot and browning is SO2 fumigation, but it is facing food safety regulations and alternatives are needed. The aims of this study were to determine if the natural volatile hexanal could inhibit mycelial growth and spore germination of common longan decay-causing fungi, and if hexanal could affect fungal-derived cell-wall degrading enzymes that are involved in pathogenicity. Four common longan fruit decay fungi, Lasiodiplodia theobromae; Pestalotiopsis sp.; Phomopsis sp.; and Curvularia sp., were exposed to hexanal vapour, and mycelial growth in vitro was measured. Also, hexanal vapour effects on spore germination, mycelial morphology, and activity of extracellular enzymes of L. theobromae were studied. L. theobromae growth was completely suppressed at a lower hexanal volume than the other fungi. Fumigation for 1 h with 15 mu L hexanal per Petri dish was enough to completely suppress L. theobromae, with lower volumes for 1 to 48 h resulting in variable levels of suppression but not fungicidal effects. Spore germination was inhibited at 5 mu L per dish for only 1 h, and the effect was fungicidal at greater than 15 mu L. Hexanal vapour appeared to induce cell swelling with greater vacuolation and more mycelial branching of L. theobromae. Extracellular cellulase activity was reduced more than 80% by hexanal, but pectin methylesterase, polygalacturonase, and cutinase activities were not affected.