2018 ASHS Annual Conference

Even with the best management practices during production and harvest, botrytis (Botrytis cinerea Pers. ex. Fr.) can cause significant damage to cut roses (Rosa × hybrida). Latent and new B. cinerea infections often become known only after storage and transport because conditions during postharvest handling are conducive to B. cinerea growth. Cut roses are typically treated with synthetic fungicides before storage/transport to reduce postharvest botrytis damage, but interest has grown in finding natural, organic compounds for this purpose for a few reasons: resistance of B. cinerea to synthetic fungicides, including strains with multiple resistances, increasing restrictions on use of conventional fungicides, and retailers and consumers preferring safer and biodegradable alternatives to synthetic compounds. Plant-derived essential oils (EO) have shown potential as alternatives to synthetic fungicides to mitigate postharvest disease in several systems. We assessed the degree to which 16 synthetic and non-synthetic fungicide treatments controlled botrytis damage in cut roses infected with B. cinerea. We investigated whether EOs of cinnamon leaf (CLO), clove bud (CBO), and thyme (TO) could reduce damage in botrytis-infected roses to the same degree as a successful, traditional, synthetic fungicide. The 16 treatments applied to ‘Light Orlando’ flowers protected against botrytis damage and caused phytotoxicity to differing degrees. Only the synthetic fungicide fludioxonil (0.250 g‧L^-1 Medallion®) resulted in the desirable combination of greatly reduced stem termination frequency due to flower botrytis damage and relatively minor flower phytotoxicity damage. When applied to cut rose ‘Freedom’ or light colored cultivars (Cool Water, Jessika, Polar Star, Tiffany), all EO aqueous solutions caused pronounced phytotoxicity damage, but only TO reduced botrytis damage compared to untreated flowers. Roses exposed to EO vapor rather than EO aqueous solution tended to exhibit less phytotoxicity. Vapor of CLO and CBO tended to reduce botrytis damage less and caused greater flower phytotoxicity than TO vapor and aqueous fludioxonil. The greater phytotoxicity caused by higher EO concentrations was not accompanied by greater reductions in damage with either application method. The lack of botrytis damage control may have been because the phytotoxicity damaged tissue allowed additional infection and/or more extensive penetration of botrytis. In summary, thyme oil vapor exposures of 4.6 and 9.1 µL‧L^-1 were the most promising of the EO treatments and warrant further investigation.