Greenhouse and field evaluation of fungicides for control of olive leaf spot in New Zealand

Abstract

Olive leaf spot, a major disease of olive worldwide, is difficult to control in regions with cool and moist weather conditions such as New Zealand. The fungicides, boscalid, captan, carbendazim, copper hydroxide, copper sulphate, difenoconazole, dodine, kresoxim-methyl and a kresoxim-methyl/copper hydroxide mixture, were tested for efficacy in greenhouse and field trials. Greenhouse studies showed that all fungicides significantly reduced disease severity but the level of control was affected by the time interval between fungicide application and pathogen inoculation. For the field study, trees in commercial olive groves in three regions received two fungicide applications in each of three consecutive seasons: winter, spring and autumn. Fungicide type and time of application affected the disease incidence on the leaves. In winter, none of the fungicides, except copper sulphate and copper hydroxide, reduced disease incidence on the leaves compared with unsprayed controls. Most of the fungicides reduced disease incidence after spring and autumn applications, with the autumn applications being the most effective. Of the fungicides tested, copper sulphate and a mixture of kresoxim-methyl and copper hydroxide were the most effective, reducing disease incidence by 85–96% and 63–93%, respectively. Fungicide efficacy was greater in the drier climatic regions of New Zealand than the wetter regions. The results of this research facilitated the development of an improved fungicide spray programme for control of olive leaf spot in New Zealand.