Control of powdery mildew in cut roses using natural products in the greenhouse.

Roses grown under greenhouse cultivation are commonly affected by diseases and regular fungicide applications are commonly used. The aim of this research was to identify natural products that can substitute and reduce the health and environmental impacts of typical chemical treatments in the control of powdery mildew [(Podosphaera pannosa (Wallr. Fr.) de Bary] and grey mould (Botrytis cinerea Pers.). Treatments were applied in the greenhouse on the cut rose cultivars Sunluck (Gold Strike®) and Nirpbredy (New Fashion®) growing in a soilless system. Oregano and clove essential oil at 0.5 ml/l (an emulsifier was added) and sodium bicarbonate (NaHCO3) at 4 g/l were sprayed on rose plants to runoff. The controls were untreated roses and a standard chemical control (the fungicides Dinocap, Triadimenol, Dimethomorph+Sulphur, Bupirimate, Dithianon, Iprodione and Thiram) was applied in rotation. Sprays were applied every 7-10 days after the first symptoms of disease appeared. Disease incidence was checked on 100 leaflets and symptoms were evaluated using a scale from 0 (no disease) to 7 (76-100% infection). At three different times (June and November 2005 and January 2006) biometric data (stem height, stem diameter, stem flexibility, flower diameter, number of petals/flower and thorniness) and colorimetric analysis of the leaves were evaluated in order to determine the effect of treatments on cut rose quality. Treatment with essential oils and NaHCO3 was able to control the incidence and the severity of powdery mildew on roses. Ambient infection of grey mould was low and comparisons between treatments were not possible. Significant differences among the chemical and natural product treatments were found for all traits examined. Although NaHCO3 treatment controlled powdery mildew to a greater extent than essential oils, it resulted in slight phytotoxicity. These results indicate the potential use of natural products to control powdery mildew of roses and could be a good alternative to chemical fungicides. However, differences in response to powdery mildew depend on the cultivar susceptibility, period of treatments and level of control needed.