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United States Department of Agriculture

Agricultural Research Service

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Research Project: An Assessment of the Viability of Black Spot-Blemished Citrus Fruit as a Pathway for Disease Dispersal via Domestic and International Trade

Location: Subtropical Plant Pathology Research

2013 Annual Report

1a. Objectives (from AD-416):
To examine if citrus fruit is a viable pathway for dissemination to, and establishment of, G. citricarpa in areas currently free of Black Spot in other states and overseas.

1b. Approach (from AD-416):
To accomplish this, we plan to conduct a non-regulatory motivated probabilistic risk assessment of the life history of G. citricarpa and examine the survival and persistence of G. citricarpa lesions on developing and mature fruit. This will be done via a stochastic model and Monte Carlo simulation with sensitivity analyses to assess the true risk of G. citricarpa moving through all steps in the pathway and becoming established in the termination point of the pathway, i.e., new previously uninfected areas.

3. Progress Report:
This research relates to inhouse project objective 3a: Develop and use stochastic models to test various disease control strategies for HLB, ACC, CBS and diseases caused by other exotic pathogens. During the initial phase of the project we organized a 3-day meeting of the collaborators. We designed a framework for the model based on multiple parameters broken into discrete phases of the distribution pathway, i.e., disease/infection, packinghouse/transport, and distribution/marketing. Each phase was them populated with data extracted from the existing literature on black spot. The data was organized and fitted to likelihood distributions. Each distribution was linked to the next distribution or step in the pathway in series. The result was a simplified, functional model for the entire pathway. Additional steps, i.e., data distributions, can then be added as more data are gleaned from literature and/or via gap-filling research. Where data was missing, we estimated distribution functions. We then ran the initial model to identify any programming issues and found none. We charged individual collaborators with collecting data we will put it into the model. We came to consensus on estimates of missing distribution functions. We identified data gaps and assigned individual collaborators to commence the research to fill them. Post conference, we began to modify the model to include additional pathway components identified and data provided by the international research team. No changes to the approach were needed at this time. During the second phase, the model continued to be refined by identifying and adding data for individual steps in the marketing and distribution pathway. We identified, clarified and initiated gap filling research on: 1) Testing inoculum viability of packinghouse treated and untreated fruit through time; 2) Examination of the mitigating factors that control field spread of black spot and its rate of increase; 3) Determining the proportion of fruit culled in packinghouses in South Africa and Brazil; 4) Studies to determine the infectious period of conidia from fruit collected from groves; and 5) We are also continuing to review the literature to exhume for the bits of data on fruit susceptibility, culling practices, storage and shipping losses, marketing and repack losses, consumer discards, dispersal patterns in wind and rain, disease susceptibility of various cultivars, import and export information including flowering dates, harvest., fruit distribution and transport patterns, and per capita consumption in various countries. During the third phase, data was gleaned from literature on black spot now has been incorporated into the model and the model continues to be refined. This data was incorporated by defining new mathematical distributions relative to each data parameter and incorporating these distributions into the various steps of the model pathway. We determined/modified locations as points of origin and end points of fruit distribution and obtained, and these have now been incorporated into the model framework. During the third phase of the grant, we had our 2nd meeting and spent three days going through all steps of the model verifying each step in the pathway and discussing it in detail. We identified additional gap filling research that was needed to either complete the model or strengthen the model at various points in the pathway. Assignments of gap filling research were accepted by Brazilian, South Africa, and U.S. members of the team. The Brazilian team members conducted gap-filling research on port of entry rejection and the number of fruit disposed of and/or diverted at that point in pathway; and examined the incidence of disease fruit pre-and post-packinghouse to determine the culling rate for CBS for that step pathway. They also explored harvesting of entire trees and the number of infected fruit that occur at the center of the tree versus at the periphery and the whole tree. The South African team conducted a series of experiments to determine the percentage of culls due to CBS postharvest, number of CBS symptomatic fruit that are packed, i.e. lesion fruit that are missed during packinghouse, lesion development during cold storage and shelf life, and picnidiospore production and viability in packinghouse treated versus nontreated fruit. South Africans also explored estimates of the incidence increase due to latent infections. Members of the U.S. team examined fruit disposal from prior studies conducted by USDA APHIS to determine how and at what proportion consumers discard fruit and explored the susceptibility of infection of various endpoints of pathways, i.e. regions within various countries, from the CBS data from NAPFAST. The group talked with a local fruit packer to understand not only the packinghouse but various steps within the pathway including shipment from the field to the packinghouse, storage into greening rooms, shipping to international ports, redistribution and transshipment to end points, distribution to endpoint losses, market losses, consumer losses, and consumer discard of declining fruit as well as peels from consumed fruit. We also discussed how to obtain better data for post-packinghouse distribution, shipment and transshipment, marketplace and consumer losses. During the fourth phase of the project, we had our 4th meeting. Once again we went through all steps of the model verifying each step in the pathway and discussing it in detail and identified additional gap filling research that was needed to either complete the model or strengthen the model at various points in the pathway. Data were provided by Brazil to uncover a distribution where a probability is assigned to x number of days with “suitable” conditions for infection. Brazilian members ask Spanish colleague for more quantification associated with the how much cull from packinghouses in Spain go to juice, feed animals, or just sit in the grove. Brazilians shared studies: 1) Conducted to evaluate the efficiency of the packinghouse process; 2) Looked at the effect of the packinghouse process on reduction of fruit (for the domestic market) with CBS symptoms; and 3) CBS incubation period in different varieties of sweet orange. Florida team: 1) sent a list to the group of all current sites that he has the weather data; 2) Created a simulation model and come up with a distribution for the likelihood that backyard composting will be done at a location in the yard that is close enough to a tree to cause an infection; and 3) Found the proportion of fruit that goes to restaurants versus the amount that goes to markets for personal households. Both South Africa and Brazil members - All agreed that more research was needed on survival period of CBS in a compost pile; it was suggested these results will be about 2 weeks. Florida team members examined: 1) Appropriate quantification of asymptomatic fruit; 2) Include the HLB/CBS interaction into the model; and 3) Viability of lesions in various climates and after various amounts of time. The major results of a South African studies were provided. Percent of lesions was evaluated before fruit went to the packhouse, fruit that was culled from the packhouse, and fruit that was determined to be “export quality”. This data was use for the efficiency of the packinghouse process to reduce the numbers of fruit with CBS lesions that continue through the model chain. We discussed is a study could be done to better quantify the accuracy of visual inspection of fruit for lesions versus testing via PCR. South Africa discussed: 1) A manuscript where ascospores were modeled from trap data in relation to whether events; 2) The history and the current situation related to the EU’s decision to impose trade barriers; and 3) Introduced CLIMEX model to and discussed if/how results could be incorporated into the model. Contemplated 10 pathways that should be highlighted in the manuscript. For each we need: 1) Weather at the start location; 2) Production numbers at the start location. Exports from that location and how much is going to the endpoint; and 3) Weather data at the endpoint location. Discussed that some pathways should highlight locations where citrus is grown (e.g., Brazil) and shipped to a suitable location for CBS (e.g., South Florida) and a non-suitable location (e.g., parts of Europe). The team decided to develop two models for pathogen introduction: 1) Fruit; and 2) Other plant material (budwood, propagating material, seedlings.

4. Accomplishments

Last Modified: 06/28/2017
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