Copper is the most efficient pesticide for the control of citrus canker (Xanthomonas citri subsp. citri). To mitigate the environmental impacts and costs, the copper sprays in citrus orchards are being optimized based on the tree-row-volume (TRV). A previous investigation allowed for significant reductions of the spray volume and copper rates. Nevertheless, the results also indicated the need for additional studies. The aim of this work was to assess whether both the spray volume and the metallic copper rate based on the TRV may be further reduced. A field trial was carried out during two seasons in a three-year-old commercial orchard of Pera sweet orange located in the municipality of Paranavaí, Paraná, Brazil. The volumes of 20 and 40 mL of spray mixture/m3 of the tree canopy were assessed in combination with the metallic copper rates of 10.5, 21.0, 36.8 or 52.5 mg/m3. Disease was measured as the temporal progress of canker incidence on leaves, cumulative dropped fruit with canker, and incidence of diseased fruit at harvest. The quality of sprays was assessed by measuring the copper deposition and leaf coverage. The treatment with the highest citrus canker control for the lowest use of water and copper was the combination of 40 mL and 36.8 mg/m3. Regression analyses indicated that the minimum threshold deposition of copper was ~1.5 µg Cu2+/cm2 leaf area. In addition, the lowest spray volume and copper rate necessary to achieve this deposition are 35 mL/m3 and 30 mg/m3. The use of 20 mL/m3 did not efficiently control the disease due to the deficient coverage of treated surfaces. This study demonstrated that is possible to use even lower amounts of copper and water without interfering with efficiency of control of citrus canker.
The rise in the productivity of sweet orange in Brazil has been related to the use of superior rootstocks and higher tree density, among other factors. In order to investigate whether the cropping system and the land use efficiency would benefit from more intensive cultivation, the performance of Valencia sweet orange was evaluated over nine years on four rootstocks, which induced contrasting vigor, at 513, 696 and 1000 trees·ha−1. Agronomic Institute of Campinas (IAC) 1697 and IAC 1710 citrandarins, and diploid and allotetraploid (4×) Swingle citrumelos were classified as semi-dwarfing, super-standard, standard, and dwarfing rootstocks, respectively. The fruit yield per tree was decreased at higher tree densities, notably for more vigorous rootstocks. Conversely, the cumulative productivity was increased over the evaluation period by 27% at 1000 trees·ha−1, irrespective of the rootstock, and the most vigorous rootstock resulted in 2.5 times higher production than the dwarfing one on average. Most fruit quality parameters were seldom influenced by the tree density, while the rootstock was a decisive factor in improving the quality and the soluble solids content. Dwarfing rootstocks allowed for harvesting 17% more fruit per minute by manual pickers. Because the tree row volume per area is lower with such rootstocks, even at higher tree density, spray volume can be reduced, although appropriate equipment should be developed for better spray coverage on smaller trees. Nine years after planting under strict vector control, the cumulative incidence of huanglongbing-symptomatic trees on IAC 1710 was double that on Swingle 4×. Taken together, the results suggested that the land use efficiency in the citrus industry can be further improved by planting vigorous rootstocks at moderate to high tree densities. Nevertheless, obtaining highly productive semi-dwarfing and dwarfing rootstocks is the sine qua non for making high-density pedestrian sweet orange orchards more profitable.
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