Vapour action of fungicides against powdery mildews

Certain fungicides prevented the growth of powdery mildews at distances extending several millimetres beyond the edge of localized deposits on leaves. Most of the materials also acted at a distance when separated from the leaf by glass cover-slips, indicating that the effect was due to the emission of vapour. In addition to sulphur and lime-sulphur, whose vapour action is already known, fungicides based on drazoxolon, oxythioquinox, binap-acryl, dinitro-octyl phenyl crotonates and a phthalimido-phosphonothionate were found to act in this way. Two systemic fungicides, griseofulvin and triamiphos, did not give a detectable vapour action. Tests were conducted mostly on open glasshouse benches. A draught from a fan shifted the zones of inhibition to one side of the deposits, but did not reduce their areas. Vapour effects were similar in extent on plants maintained at 18–25° C and at 27–32° C. Variation in the areas protected was studied in relation to the size and fungicide content of the deposits, to different powdery mildews, to time of incubation and to different types of formulation. Deposits applied to leaves by high- or low-volume sprays at concentrations used in the field gave significant protection at a distance. Vapour effects were demonstrated also on mildew conidia incubated on glass slides bearing a spot of fungicide, and on infected plants placed in beakers coated on the bottom with fungicide. Movement of fungicides in the gaseous state is discussed in relation to the control of foliage diseases in the field.