Understanding the underlying biology of citrus black spot for improved disease management

Understanding the underlying biology of citrus black spot for improved disease management

Report Date: 10/08/2019
Project: 18-006   Year: 2019
Category: Other
Author: Megan Dewdney
Sponsor: Citrus Research and Development Foundation

Sept 2019
Objective 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 the fungicide and skirting control trial, seven applications of fungicides have been completed. They were applied approximately every 28 days. Skirting was done on June 4, 2019. We plan to have one more application for one treatment. Activity on this part of the project will be minimal in the next few months as we will need to wait for enough symptom development to rate which is expected no earlier than February based on previous years.

Applications were continued at an approximate 28 day interval for the fungicide screening trial throughout the summer for a total of 6 applications. Again, this part of the project will have minimal activity because we will need to wait for symptom development. Since again the cultivar is Valencia, symptom expression is expected in early 2020.

The subcontracting process with CRI has been slow although some progress has been made and should be complete soon. Despite this, 23 isolates were sequenced (6 from South Africa and 17 from the USA) using the Ion Torrent System. The genomes of all the isolates have been successfully assembled and analysed using a customised bioinformatics pipeline. Previous genotypes obtained with SSR primers were confirmed and new SSR primers were developed in silico. To date, mapping and SNP variant statistics as well as in silico genotyping data revealed significantly less variation between the USA isolates than between the isolates from South Africa.

Jeff Rollins is still working on travel to Cuba because of changes to Federal rules and regualtions. Trees and fruit have been selected for on tree experiments and inoculum production is underway.

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