March 2017 There were three main objectives for this project: 1) to determine if a) leaf litter biodegradation treatments reduce Guignardia spp. pseudothecia and improve control afforded by routine fungicide applications; b) if biodegradation is affected by the current fungicide application practices; and c) whether the biodegradation treatments will affect current citrus best management practices (BMP); 2) to determine the seasonal dynamics of leaf litter inoculum load in varying management regime intensities and how environment affects pseudothecia production in the leaf litter; 3) to test if the resistance to black spot in the leaves and fruit in sour orange is correlated and under simple genetic control through laboratory and field testing of progeny of sour orange crosses in both Florida and Australia. Progress was made on each objective. In objective 1, we determined that there was little observable effect on the number of Phyllosticta spp. structures in the leaf litter per se between the untreated control, urea, and soil-set. But there were significant differences in the incidence of fruit with black spot symptoms with the soil-set treatment having the lowest in 2015 and 2016 (P < 0.05) but equivalent incidence in 2017. However, the severity was lowest with the soil-set treatment for all three years. It was found that the high volume fungicide application practices used in Australia does slow the decomposition of leaf litter. In the small plot trials used from Australia, urea was not a preferred treatment choice as it did not improve decomposition but organic mulch like bagasse was excellent. Bagasse was also found to be an excellent mulch choice in small experiments in Florida. In objective 2, sporulation and structure formation was followed over three years in Florida and Australia. In Florida, leaf litter was collected all year where as in Australia it was collected during the fruit susceptibility period. We found that pycnidia formation preceded pseudothecia formation in both Australia and Florida. In Australia, the majority of the fungal structures observed were P. citricarpa but in Florida, the majority of structures were P. capitalensis. From 2014 to 2017, the level of P. citricarpa increased to nearly equivalent levels of P. capitalensis. In Florida, pycnidia and conidia were observed all year but pseudothecia and ascspores tended to be present in cycles and always at lower levels. It is hypothesized that the ascospores were all from P. capitalensis. We also demonstrated that P. citricarpa has two mating types required for ascospore formation and only one is present in Florida, unlike the rest of the world. This means that only conidia are present in Florida. Our team was able to demonstrate that the two mating types were needed for ascospore formation and were able to get them to form in vitro. Again a first. In objective 3, the Florida team refined and used a method to inoculate potentially resistant 'Chinotto' hybrid leaves. There were different levels of spore formation on the leaves but it is unknown how this relates to fruit susceptibility. In Australia, fruit inoculations were done in a citrus collections of many breeding lines. Sour orange hybrids were differentially susceptible but the most promising results were from pomelo lines where inoculated fruit had no disease expression.