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.
Highlights
Guignardia citricarpa (Anamorph: Phyllosticta citricarpa) is the causal agent of Citrus Black Spot (CBS) and is capable of infecting all economically important varieties of citrus [1,2]
DNA samples collected from fruit and isolated fungi with morphology similar to Guignardia spp. were confirmed to be that of G. citricarpa in all but four which were confirmed to be G. mangiferae
The use of copper fungicides to reduce CBS symptoms on fruit has been used successfully in field trials in Australia, South America and South Africa [2,15,17,21,22]. This was not the case in this study, where copper did not reduce the proportion of fruit with black spot symptoms measured either on fallen fruits, fruits within one-meter square of the canopy or in 100 picked fruit, measured throughout the growing season
Summary
Guignardia citricarpa (Anamorph: Phyllosticta citricarpa) is the causal agent of Citrus Black Spot (CBS) and is capable of infecting all economically important varieties of citrus [1,2]. The disease was initially described more than 100 years ago in New South Wales, Australia, and has subsequently been found in Africa, Asia, South America, and New Zealand [3,4,5]. Guignardia citricarpa was observed for the first time in North America when orange fruit exhibiting lesions typical of CSB were found in Southwest Florida grove and tested positive for the CBS fungal pathogen [6]. Guignardia citricarpa does not affect the quality of the fruit in areas where the disease is endemic untreated groves have been reported to experience up to 80% yield loss due to fruit drop [2]. Florida is the largest producer of oranges (69%) in the US, producing 6.4 million tons in 2010, followed by California, Arizona and Texas [7].
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