Abstract

Soybean oil formulations developed in our laboratory were evaluated for control of powdery mildew on flowering dogwood Cornus florida. In a preliminary trial in a field nursery, two formulations, TNsoy1 and TNsoy2 [0, 1 or 2% (v/v)], were applied to field-grown flowering dogwood at 2 wk intervals from June 10 until August 19. Soy oil-treated trees had less powdery mildew, higher net photosynthetic rate and growth rate than unsprayed trees, without apparent phytotoxicity. Leaves sprayed with Banner MAXX had less powdery mildew and higher photosynthetic rates than oil treated leaves. These same treatments applied the next year under more frequent rain showers did not provide satisfactory control. Newer soybean oil formulations using natural or food grade emulsifiers, were compared to three commercial botanical oil formulations and to SunSpray Ultra Fine Oil in greenhouse trials. All oil formulations provided some protective and eradicative control of powdery mildew. Photosynthesis of oil treated leaves was usually depressed, compared to controls, for several days but recovered. In greenhouse trials where the oil was not washed off by rain or irrigation, the oil residue was visible on leaves, reduced powdery mildew severity, and depressed photosynthesis for several weeks. In a second greenhouse trial, soybean oil formulations provided greater control if applied after rather than before initial inoculation.