Soluble and insoluble copper formulations and metallic copper rate for control of citrus canker on sweet orange trees

Copper bactericides have been widely studied and used for control of citrus canker caused by Xanthomonas citri subsp. citri. New formulations become available periodically, however, little is known about the relative efficacy of insoluble (ICuF) and soluble (SCuF) copper formulations and the performance of ICuF at the same metallic copper rates. In the present study, we compared three rates of five ICuF (syn. fixed copper) and two SCuF for control of citrus canker during two seasons in a commercial orchard of 'Valencia' sweet orange located in Paranavaí, Paraná, Brazil. Control of citrus canker was evaluated as the temporal progress of canker incidence on leaves, cumulative dropped fruit with canker, and incidence of diseased fruit at harvest. All copper formulations and rates reduced the progress of citrus canker incidence on leaves. However, SCuF were not as effective as the ICuF. The incidence of leaves with canker on trees treated with ICuF never exceed 7.3%. In contrast, incidence of leaf canker on trees treated with SCuF ranged from 9.9 to 44.8%. Fruit drop per tree due to citrus canker for trees treated with ICuF did not exceed 6.2 and 17.3 in seasons 1 and 2, respectively. These losses were significantly lower than 20.4 and 38.6, observed for the untreated trees. Fruit drop was 10.4 and 20.9 for trees treated with SCuF in seasons 1 and 2, respectively. The citrus canker incidence on harvested fruit followed the same trend observed for other variables assessed. A non-linear regression analysis showed that the metallic copper rate accounted for a large proportion of the variation in incidence of citrus canker on leaves, fruit and premature fruit drop up to 1 kg of metallic copper/ha/application. Rates exceeding l kg/ha of metallic copper did not consistently improve disease control or reduce the fruit loss.