Evaluation of systemic fungicides as pre-harvest treatments of avocados

Various systemic fungicides and a non-systemic fungicide (copper ammonium carbonate) were evaluated as possible alternatives to copper-oxychloride and benomyl, for control of preand postharvest diseases of avocados. Foliar applications of cyproconazole, flusilazol or copper ammonium carbonate were as effective in controlling Cercospora spot as copper-oxychloride and benomyl, but were less effective in controlling sooty blotch. Visible spray residues were significantly lower where the systemic compounds cyproconazole, flusilazol, triadimenol and the non-systemic fungicide (copper ammonium carbonate), were applied in the final January spray round, instead of copper-oxychloride. INTRODUCTION Avocados are susceptible to numerous diseases. Cercospora spot caused by Pseudocercospora purpurea (Darvas, 1982) and sooty blotch caused by an Akaropeltopsis sp (Theron et al, 1981), are important preharvest diseases. The major postharvest diseases are stem-end rot, caused by any of 10 organisms, with Thyronectria pseudotricha the most important (Darvas, 1978); anthracnose caused by Colletotrichum gloeosporioides (Kotzé, 1978) and the Dothiorella/Colktotrichum rot complex (Darvas, 1978). In all the above instances, infection of fruit takes place in the orchard prior to harvest. Pre-harvest applications of fungicides are therefore a feasible means of partially controlling these diseases. Copper-oxychloride and benomyl are currently registered for the control of preharvest diseases of avocados in South Africa (Vermeulen et al, 1990). As yet, no fungicides have been registered for the pre-harvest control of postharvest diseases. However, preharvest applications of copper-oxychloride and benomyl are reported to control these diseases to some degree (Darvas, 1981; et al, 1981; Kotzé et al, 1982; Labuschagne & Rowell, 1983 and Lonsdale & Kotzé, 1989). The disadvantages in using copperoxychloride and benomyl are: Firstly, copper-oxychloride leaves unsightly residues on the fruit, which have to be removed in the packhouse prior to packing. It is estimated that the removal process slows the packing tempo by as much as 50%, according to Dr J J Bezuidenhout (personal communication, 1991). Secondly, Darvas (1982) reported tolerance of Pseudocercospora purpurea to benomyl. The purpose of this study was to screen various new systemic fungicides as well as a non-systemic fungicide (copper ammonium carbonate), as possible replacements for pre-harvest copper-oxychloride and benomyl sprays. A randomised block design was used, with five single tree replicates per treatment. A high volume applicator with hand lances was used to apply the fungicide sprays. Fruit were evaluated in the first week of April 1991 for the incidence of pre-harvest diseases. Fruit was also evaluated for the presence of visible spray residues at time of picking on a 0-3 scale, where: MATERIALS AND METHODS Three trials were carried out during the 1990/91 season at Westfalia Estate on 10and 15yearold Fuerte trees. The chemicals used in the trials were: A randomised block design was used, with five single tree replicates per treatment. A high volume applicator with hand lances was used to apply the fungicide sprays. Fruit were evaluated in the first week of April 1991 for the incidence of pre-harvest diseases. Cercospora spot was rated on a 0-3 scale, where: