January 2019�The objectives of this proposal are 1) To determine the temperature and relative humidity optima for Guignardiacitricarpa pycnidiospore infection and production on citrus twigs, leaf litter, and fruit; 2) To determine the relative potential of Guignardia citricarpa to form pycnidiospores on citrus twigs, leaf litter, and fruit; 3) To determine whether Guignardia citricarpa can survive and reproduce on citrus debris on grove equipment.�In this project we established that Phyllosticta citricarpa (syn. Guignardia citricarpa) conidia are being formed in dead twigs in the field.� We were only able to collect low numbers but suspect this may have been influenced by the methods used.� The conidia are less likely to be present in the dry winter months but are frequently present at other times of the year.� The amount of conidia was found to be influenced by rain in the preceeding two weeks, the average temperature, days with measurable rainfall in the last 7 days and an interaction between the accumulated rainfall and average temperature.� When we investigted the presence of P. citricarpa DNA in the bark of the same twigs we found that the amount was influenced most by the average temperature, days with measurable rainfall in the last 7 days, days with measurable rainfall over 28 days and an interaction with average temperature and days with measurable precipitation.� We also were able to link the bark DNA quantities with areas in the grove known to have a high incidence and severity of symptomatic fruit and trees.� We found that the more symptomatic trees had a greater amount of P. citricarpa DNA in their twigs than those with low severity.� This establishes that there is a connection between the amount of P. citricarpa in the canopy with the severity of fruit infection.� What remains to be shown is whether the twigs are the initial inoculum source for the fruit or a sign of the overall infection level.�We also conducted a large factorial experiment to look at the effects of temperature and relative humidity on the infection of twigs and leaves by P. citricarpa.� We found that relative humidity was very important for the development of pycnidia and conidia on twigs.� Relative humidity levels below 90% greatly reduced the number of pycnidia or conidia formed.� However, the amount of P. citricarpa DNA present in the twigs showed that the fungus did not die but in some cases continued to increase it’s biomass greatly.� Temperature was also very important and extremes in the temperature profiles did not allow for structure formation or in some cases biomass growth.� Sporulation was more affected by temperature than relative humidity.� This allows us to see when production of conidia is most likely to estimate the greatest inoculum potential.�We also investigated the effect of disinfectants on spores in debris.� It is known that debris can cause disinfectants to loose their potency and so it was shown in this project.� However, it was found that the potency could be regained if a large enough volume of the disinfecant was used.� Results of the completed research are consistent with recommendations from FDACS in regards to efficacy of recommended disinfectants.� The finding that efficacy diminishes when spores are associated with citrus debris offers an opportunity to update recommendations for hedging operations and other activities that may generate significant amounts of fine debris to ensure that debris is fully saturated with disinfectant solutions.