May 2022Objective 1: Evaluate the optimal spray timing for Florida and investigate if tree skirting or alternative products improves fungicidal control of citrus black spot.Objective 3: A MAT-1-1 isolate may enter Florida and allow for the production of ascospores. The industry needs to know if this happens, as it will affect management practices. Additionally, the existing asexual population may be more diverse than currently measured. If multiple clonal linages exist, then there may be different sensitivities to fungicides or other phenotypic traits. We also need to determine whether P. paracitricarpa or P. paracapitalensis are present in Florida for regulatory concerns due to misidentification. We plan to survey for the MAT-1-1 mating type, unique clonal lineages, and two closely related Phyllosticta spp. In this project, we conducted a two year skirting and fungicide timing trial in a grove with a moderate severity of citrus black spot. We found that skirting had a non-significant effect on CBS incidence and severity. The fungicide timing programs did improve the CBS management but only in the second year when the disease intensity was higher. We can conclude that skirting does not provide additional benefit to a fungicide program and that the fungicide program as currently recommended is sufficient for CBS management in Florida. The opitmal timing of our fungicide programs had not been confirmed in Florida previously and were based on our best estimates from the literature from other countries, particularly Brazil. We were able to confirm that azoxystrobin mixed with difenoconazole is a good product for CBS management in Florida and South Africa. We had similar results with febuconazole as well where the activity was inconsistent between seasons. The reason is not easily determined. The mixture of fluopyram and tebuconazole did not perform well in either country. We had promising results with zinc polyoxin-D rotated with pyraclostrobin in Florida but was unable to try it in South Africa. We also had good results with the adjuvant Goodspray One with pyraclostrobin rotated with copper hydroxide and thyme oil rotated with copper hydroxide in the 2nd year. The product with only one year of data will need confirmation in another season. The results with two years of data will be used to make chemical recommendations in the Florida Citrus Production Guide. We screened our collection Phyllosticta citricarpa isolates and did not find any MAT1-1 isolates, meaning that there appears to be only one mating type here in Florida at this time. We also did not find the new pathogen P. paracitricarpa in our collection. In screening the collection, we did find a species previously not associated with citrus, P. hymenocallidicola, which was not pathogenic on citrus. It appears to be an endophyte on citrus but this was not confirmed. The species was poorly described in the literature and it was unclear as to whether it was a pathogen on the host it was originally described from Hymenocallis littoralis. We did find it was a pathogen on H. littoralis, causing leaf spots. We worked with colleagues from Cuba and were able assist the characterization their isolates. It was found that their isolates were very similar to those in Florida and also only had the MAT1-2 mating type idiomorph. It appears that these populations may be linked, but how is uncertain. This work was published in 2022. Further work from the South African group showed that whole genome analysis of isolates from Florida could detect isolate differences even among this clonal population. This allows for better differentiation among populations in larger population studies. This work was published in 2021. Further work on the global populations of CBS confirmed that Florida is a clonal population based on the number of genetic differences and that the Cuban population was indeed very close to the Floridian one. The next closest population is from eSwatini, but it is quite distant. Again China and Australia were identified to have the most population variation. The remaining populations were regions within South Africa, Argentina, Brazil. These populations had less diversity than China and Australia but had more diveristy than the North American population. The populations outside of North America and China were interconnected and relatively close to each other. This work shows that more collections of isolates needs to be done, especially in Southeast Asia to better understand how P. citricarpa has been moved globally, likely in plant materials.