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ARS Home » Southeast Area » Fort Pierce, Florida » U.S. Horticultural Research Laboratory » Citrus and Other Subtropical Products Research » Research » Research Project #437484

Research Project: Developing Pre- and Postharvest Methods to Control Diplodia Stem-end Rot to Reduce Mandatory Segregation of Florida Grapefruit Exports to Japan

Location: Citrus and Other Subtropical Products Research

Project Number: 6034-41430-007-009-R
Project Type: Reimbursable Cooperative Agreement

Start Date: Oct 1, 2019
End Date: Jun 30, 2023

In recent years, a new market barrier has been discovered regarding Lasiodiplodia (L.) theobromae (syns. Diplodia natalensis), causing Diplodia stem-end rot (Diplodia SER), on fresh citrus exported to Japan. Besides causing reduced yields in Florida, Huanglongbing (HLB) in Florida is also resulting in much more postharvest Diplodia SER. The overall objective of this project is to develop new or improved treatments/technologies that are economically sustainable and effective in reducing Diplodia SER while maintaining fruit quality and shelf life for fresh market distribution. This project will: 1. Measure L. theobromae populations and colonization levels on grapefruit button and calyx before harvest within different groves and determine if levels are correlated with postharvest Diplodia SER incidence and severity. This will help in developing a L. theobromae monitoring system to guide pre- and postharvest treatment to control the disease. 2. Test selected fungicides/formulations in vitro against the growth of different Diplodia fungal isolates. 3. Determine the efficacy of promising preharvest fungicides at reducing colonization of fruit stem-end tissues by L. theobromae in commercial grapefruit groves, and if such effects translate into expected reductions in postharvest Diplodia SER incidences and severity. Newer fungicides, such as fludioxonil, are known to be effective against L. theobromae postharvest, and while there are preharvest formulations of the active ingredient, none are yet registered for preharvest use on citrus. Likewise, Quadris Top (Azoxystrobin + Difenoconazole) was found to be effective on L. theobromae preharvest on orange, but not on grapefruit. Adjusting the application timing, method, etc. may allow such materials to be consistently effective.

1. Evaluate the efficacy of modified postharvest degreening treatments (i.e., using lower temperatures) on reducing Diplodia stem-end rot (Diplodia SER) incidence and severity on grapefruit. 2. Test new citrus postharvest fungicides (i.e., Natamycin and Propiconazole), essential oils, and chlorine dioxide (CIO2) on postharvest growth and development of L. theobromae in vitro and on inoculated fruit. 3. Develop delivery methods for promising materials identified in (2) above (e.g., aqueous drenches, incorporation into waxes, controlled-release formulations contained with sachets for volatile production, etc.) to control Diplodia SER on inoculated grapefruit or on fruit with natural infections. 4. Fruit samples will be inoculated with different levels of Lasiodiplodia (L.) theobromae. Biomarkers detected from inoculated samples will be used to predict the development of Diplodia SER induced fruit decay. 5. Identification of biomarkers: Samples from different inoculations proceed through a multi-step process including analysis using three different platforms: LC-MS/MS(±ESI), LC-TOF/MS and GC-TOF/MS.