Glasshouse experiments on apple scab

Abstract

SUMMARYDilutions of various fungicides were compared on potted MM 109 clonal rootstocks in the glasshouse for protective (pre‐infection) and curative (post‐infection) activity against apple scab. The materials, as commercial preparations, included captan, copper (as ‘dry’ Bordeaux mixture), dichlofluanid, didecyldimethylammonium bromide (DDAB), dispersible sulphur, dithianon, dodine acetate, lime‐sulphur (LS), phenylmercury chloride (PMC), and thiram; they were assessed on the basis of percentage active ingredient in the dilutions.When acting at normal rates of use, many of the fungicides were much more effective protectively than curatively, but this contrast was less applicable to DDAB, dodine acetate, LS, and dichlofluanid. PMC, however, was an especially notable exception in proving curatively superior at unusually low rates. In addition, it was particularly active in vapour form, and was effectively translocated not only from one leaf surface to the other, but, especially at 0.005%, from sprayed to younger unsprayed leaves on the same shoots. Other standard fungicides showed little or no translocated activity, but isobutyl‐o‐coumarate was effective at the growing shoot‐tips in spring from a spray applied to the lower leaves.PMC reduced infection when applied once at any time during the scab‐incubation period, and especially 24 hr. after infection, when the control was almost complete. Longer intervals before spraying reduced the curative action, but not in proportion to the intervals; the infection pattern showed multi‐peak fungicidal activity and, after the later sprays, many lesions were practically infertile. The curative properties of PMC were matched by several other formulated phenylmercury salts compared at equivalent percentage mercury content, but the dimethyldithiocarbamate proved rather less effective. A preliminary attempt to supplement the curative properties of PMC with the protective properties of LS, by mixing these fungicides at low rates, showed evidence of fungicidal antagonism, and this needs further investigation.Several fungicides, notably the inorganic sulphurs, thiram, the phenylmercury compounds, dichlofluanid, and DDAB showed incidental activity against natural infection by powdery mildew.