March 26, 2015 The objective of this project was to investigate three questions: 1) How long does a leaf needs to be infected by Guignardia citricarpa before ascospore production can be initiated; 2) How does infection and colonization of leaves by Guignardia citricarpa occur and potentially showing how pseudothecia, the sexual spore producing structures, are produced; and 3) what is the interaction between the common twig colonizing pathogen Diaporthe citri and the black spot pathogen Guignardia citricarpa and whether they can co-exist to successfully sporulate on dead twigs. Guignardia citricarpa, the causal agent of citrus black spot, is a newly emerging disease of citrus in Florida. The disease is concentrated in Southwest Florida. It has not been clear when the inoculum was being formed or where in the tree. The mating type structure of G. citricarpa was shown to be heterothallic unlike the common citrus endophyte G. mangiferae, which is homothallic. Thus far, it was shown that G. citricarpa conidia germinate, form appressoria, and penetrate leaves through the culticle. No further changes have been observed over several months and no symptoms have been observed over 10 months. When Diaporthe citri and G. citricarpa are co-inoculated on twigs, D. citri out competes in suboptimal growing conditions for G. citricarpa. The optimal relative humidity range for production of conidia at 24’C was between 82 and 100% but some detection of G. citricarpa DNA was possible at the 43 and 72% RH treatment.