Use of natural products for the control of powdery mildew of rose plants

PowDery mildew caused by Podosphaera pannosa (Wallr. Fr.) De Bary on roses in greenhouse cultivation is usually controlled by chemical products. Aiming to lower the use of chemical treatments, in vitro and in planta assays were carried out using some natural products. The inhibitory effect of essential oils extracted from oregano inflorescences (Origanum heracleoticum L.) and cloves (Syzygium aromaticum L. Merril et L.M. Perry), was evaluated by in vitro tests. Essential oil at 0.5, and 0.125 ml/L concentration was added to conidia suspension (8 x 105 conidia/ml). After 2, 4, 8, 12, 24 and 48 hours, 400 conidia for each treatment and control were evaluated and the germination percentage calculated. The treatments showed the full inhibition of P. pannosa conidia germination in all cases. In planta assay was carried out in a greenhouse on roses grown in a soilless system. Concentrations of 0.25 and 0.5 ml/L of oregano and cloves essential oils, emulsified, plus an additive compound at 0.5 ml/L; and NaHCO3 at 5 g/L were sprayed on rose plants in an experimental block design. Each treatment was repeated 4 times. All the natural products were compared with usual chemical treatments and with an untreated control. All sprays were applied every 7 days. Diffusion and incidence of powDery mildew were checked during the assay. At the end of the assay biometric data were evaluated. The results show an inhibitory activity of both essential oils at higher concentration and of NaHCO3 treatment; this last one showed the best inhibition. Significant differences were found in comparison with chemical treatments and the control. No toxicity phenomena were found at either concentration of the essential oils treatments. The NaHCO3 treatment showed slight phytotoxicity