October 2019
The objectives for this proposal are 1) Conduct field trials of new products and fungicide programs for PFD management as well as validation trials for the Citrus Advisory System (CAS); 2) Investigate the reasons for the movement of Postbloom fruit drop (PFD) to new areas and recent major outbreaks; 3) Evaluate methods for initial inoculum reduction on leaves so that early fungicide applications could be more effective and identify the constituents of the flower extracts using “omics” techniques.
The spray trial and CAS validation trial data were collected and analyzed. The data were reported as part of the comprehensive final report for project 16-010C in September. Planning for the next season will be underway in the next quarter.
The wind tunnel experiments continued throughout this quarter. The majority of the data for spores from leaves has been completed. Leaves were inoculated in planta with Colletotrichum acutatum and allowed to form appressoria. The approssoria were stimulated to germinate and for secondary conidia. The trees were then exposed to three windspeeds: 5, 10, and 20 m/s, with and without rain within the wind tunnel at the USDA Tropical Research Unit in Fort Pierce. Petri dishes with selective media were placed a different distances from the inoculated plants prior to the treatments. The number of CFU were counted to assess how well the inoculum moved. The data are in the process of being modelled to select the best fitting models. Even though the data are still being analyzed, it is evident that some inoculum can move up to 15m with just wind and even more inoculum moves when there is wind and rain. This has not been reported for C. acutatum before which is considered to be an exclusively splash dispersed organism. The secondary conidiation inoculum has not been studies in detail previously but is the primary PFD inoculum and responsible for initial outbreaks of disease. We plan to study the movement of the primary inoculum produced on flowers when they become available in the spring.
Little progress has been made with the modeling to better predict PFD outcomes because of the focus on the inoculum movement. The movement was a priority because of the limited access to the USDA facilities.
There was also little work on the germination stimulation with the flower extracts because of the reasons stated above. The experiments have been restarted and progress with the different extracts are expected in time to inform our efforts at flowering this spring.
Dr. Yu Wang’s lab has been busy analysing the constituents of the water extracts of the flowers at pin head, button bloom, and open flowers. All the standards and chemical techniques were obtained and methods optimized. The sugars and sugar alcohols in the flower stages were identified. In agreement with previous studies that sucrose stimulated the germination of appressoria, glucose and fructose concentrations increase with flower maturity. Some sugars and sugar alcohols did not change signficantly among flower maturity stages while some increased at button stage but decreased at open bloom.