Split Applications of Copper Fungicides Improve Control of Melanose on Grapefruit in Florida

Citrus melanose, caused by Diaporthe citri, is a major disease of fresh market grapefruit which is controlled primarily by preventive applications of copper fungicides. The goal of this research was to improve melanose control without increasing the total amount of metallic copper utilized. Disease control was assessed in field experiments for 3 years using one, two, three, or four applications of copper fungicides in which the same total amount of metallic copper was applied per season in each program. Melanose was severe in 1995 and 1997, and the four-application schedule provided better control than the one-, two-, or three-application programs. In 1996, disease pressure was low and all programs and fungicides provided equally effective control. Analysis of copper residues on the fruit indicated that loss of protection was attributable more to increase in fruit surface area when fruit were growing rapidly than to removal of copper by weathering. On larger, slower-growing fruit, loss of copper residues was apparently due to both factors. Economic evaluation of improved packouts of fresh market fruit and additional application costs indicated that net returns would be increased by $650 to $1,250 per hectare during years in which melanose was severe when the four-spray program was compared to a single spray.