Effect of Copper on Growth Characteristics and Disease Control of the Recently Introduced Guignardia citricarpa on Citrus in Florida

Abstract

Guignardia citricarpa, the plant pathogenic fungus that causes citrus black spot, was recently introduced into the United States. The development of this disease in the presence of multiple applications of copper per year to manage citrus canker warrants an investigation into the effects of copper on growth of isolates of G. citricarpa from citrus in Florida. Guignardia citricarpa and G. mangiferae isolates, confirmed by internal transcribed spacer (ITS) sequencing of ribosomal DNA and DNA homology, were inoculated on non-amended media and media amended with 50 and 500 μg·ml-1 copper sulfate. Radial colony growth was assessed over a 26 to 59 day period. Copper reduced the growth of G. citricarpa isolates in media amended with 500 μg·ml-1 copper but had variable effects on radial growth in media amended with 50 μg·ml-1 copper. There was little effect of copper on the in vitro growth of G. mangiferae isolates. Field application of copper with and without an adjuvant for the control of citrus black spot was undertaken in a commercial grove in Florida in 2011. Spray applications were made on a 23.3 ± 4.7 day interval and fruit accessed between December 2011 and March 2012 for black spot symptoms. Copper failed to reduce the proportion of fruit exhibiting symptoms compared to that of the controls.