Organic and synthetic fungicides compared for control of leaf spot diseases and powdery mildew on crapemyrtle and hydrangea

While consumers are interested in organic or biorational alternatives to synthetic fungicides for the control of diseases of ornamentals as well as vegetables and tree fruits in residential landscapes, relatively few field trials have been conducted to establish their performance against ornamentals diseases. Weekly applications of Armicarb® 100 (potassium bicarbonate) controlled powdery mildew on flowering dogwood as effectively as the synthetic fungicides Heritage 50WDG (azoxystrobin) and Banner MAXX (propiconazole) (6). Control of powdery mildew on deciduous azalea (8), hydrangea (4), and monarda [bee balm] (7) as well as black spot on rose (5) has also been obtained with commercial potassium bicarbonate products. On flowering dogwood, the organic fungicides SunSpray Ultra Fine Oil (paraffinic oil), Neem Concentrate (neem extract), and Rhapsody biofungicide (Bacillus subtilis strain QST713) had to be applied twice as often as a synthetic fungicide to obtain a similar level of powdery mildew control (1). In contrast, SunSpray Ultra Fine Oil [discontinued] and to a lesser extent MilStop 85W (potassium bicarbonate) proved nearly as effective as the synthetic fungicides Heritage 50WDG and Eagle 40W (myclobutanil) in controlling powdery mildew on hydrangea (4). Efficacy of organic fungicides for the control of aggressive leaf spot and blight diseases on woody ornamentals is not as well known. Hagan and Akridge (1) found that SunSpray Ultra Fine Oil, Neem Concentrate, and Rhapsody failed to give effective spot anthracnose and Cercospora leaf spot control on field or container-grown flowering dogwood.