Effect of frequency of copper applications on control of citrus canker and the yield of young bearing sweet orange trees

The efficacy of different copper spray intervals for control of citrus canker caused by Xanthomonas citri subsp. citri (Xcc) was investigated in 3-to-4-year-old commercial citrus groves of ‘Pera’ sweet orange in a citrus canker endemic area in southern Brazil. Three independent trials were conducted in 2004/2005, 2005/2006 and 2006/2007. The first trial was located in Ourizona, PR and the two following were established in Paranavaí, PR, in different locations. Trees were treated with copper oxychloride (1.8 g/L) at intervals of 7, 14, 21, or 28 days. Control trees were sprayed with water every 28 days. Control of canker was evaluated as incidence of canker on leaves and fruit and as the amount of dropped and harvested fruit. Regardless of the spray interval, copper significantly decreased the incidence of citrus canker on leaves and harvested fruits, and reduced the number of prematurely dropped fruits and increased yield. Disease incidence on leaves of untreated trees in each season peaked at 37, 51, and 43% of infected leaves, whereas the incidence of canker on foliage of copper-treated trees was no higher than 12, 16, and 11%, respectively. For the second and third year trials, when disease incidence was comparatively higher, the shorter the spray interval, the lower the disease incidence and number of dropped fruit and the higher the yield per tree. Citrus canker incidence on the leaves was inversely related in a linear fashion to the total number of copper sprays in each trial. Coefficients of determination (R2) between disease incidence and number of sprays were 0.70, 0.92 and 0.80 in the respective seasons. The financial return due to increases in yield from copper sprays was significantly related to the number of sprays and disease levels only in the third trial. Although copper sprays at a 28-day-interval was satisfactory for reduction of citrus canker incidence on leaves and fruits, a shorter spray interval was required to significantly reduce yield loss.