An in vitro study on the postinfection activities of copper hydroxide and copper sulfate against conidia of Venturia inaequalis

The postinfection activities of copper hydroxide Cu(OH)2] and copper sulfate (CuSO4) against apple scab (Venturia inaequalis) were evaluated in an in vitro study. Our intention was to support the aim of reducing copper application rates by appropriate timing of applications. Experiments were conducted at 20 degrees C with leaf disks and isolated cuticular membranes (CM) of Malus x domestica 'Gloster' and 'Elstar'. Conidia of V. inaequalis were used as the inoculum. In untreated controls, 7.9 and 33.2% of germinated conidia formed primary stromata 24 and 48 h after inoculation, respectively. Treatments with copper compounds were applied 24 and 48 h after inoculation, which was 16 and 40 h after infection had occurred. When working with CM and using fluorescein diacetate as a vital stain, vital and dead stromata could be distinguished. Treatment effects were assessed 72 h after inoculation by counting vital (fluorescing) primary stromata. With leaf disks, the number of stromata was counted using KOH-aniline blue fluorescence staining. Cu(OH)2 and CuSO4 showed postinfection activity and killed primary stromata, provided that the surface of the CM was kept wet. Cu(OH)2 was more effective than CuSO4 and was able to kill all primary stromata 24 h after inoculation at concentrations of 116 and 231 mg L(-1). When Cu(OH)2 was applied at 116 mg L(-1) to leaf disks 24 h after inoculation, the number of primary stromata did not significantly differ from the control. Results indicate different modes of action for the highly water soluble CuSO4 and the slightly soluble Cu(OH)2. This supports the hypothesis that spore exudates react with insoluble copper compounds and form highly toxic copper complexes. Application of Cu(OH)2 to dry CM did not kill primary stromata. Hence, for Cu(OH)2 to exert postinfection activity, leaves must be wet. In the field, this cannot be guaranteed and a postinfection application of Cu(OH)2 cannot be recommended.