Location: Subtropical Plant Pathology Research
Project Number: 6034-22000-045-027-R
Project Type: Reimbursable Cooperative Agreement
Start Date: Jan 1, 2025
End Date: Sep 30, 2027
Objective:
The main deliverable from this project will be an understanding of the dispersal potential of conidia of Phyllosticta citricarpa, cause of citrus black spot (CBS) from tree canopies or leaf litter in citrus multiblocks, and how far Phyllosticta citricarpa, cause of CBS, can spread in a multiblock within a season in FL. The specific objectives are:
1. Characterize spore production and splash dispersal distance for P. citricarpa.
2. Determine the likely distances of CBS spread. Information is lacking on the dispersal of spores from CBS sources.
3. Ascertain the role of leaf litter or fruit peel as a source of conidia.
4. Ascertain the spatial and temporal development of CBS epidemics in FL.
Approach:
1. Characterize spore production and splash dispersal distance for P. citricarpa. Although some information exists on asexual spore dispersal it has not been studied under FL conditions. Passive sampling devices will be placed in tree canopies and at different distances from healthy and diseased trees before dispersal events (rain). Conidia dispersed will be quantified using microscopy and PCR. Leaf and fruit samples will be collected, and spore production and viability quantified in lesions over time (from early March to June). Weather conditions will be monitored to relate to spore production and dispersal. A wind tunnel facility at USHRL will be used to study spore dispersal under severe conditions. Although P. citricarpa cannot be used directly due to quarantine regulation, we will explore dispersal of spores of a related native fungus (P. capitalensis) that has similar sized spores to mimic P. citricarpa.
2. Determine the likely distances of CBS spread. Information is lacking on the dispersal of spores from CBS sources. Diseased fruit or twigs will be obtained from CBS-infected groves and will be placed at different positions in a grove in a quarantine area (on the ground or in the canopy of apparently healthy trees). Subsequently, the numbers of symptomless and symptomatic fruit as well as the maximum distance of symptomless and symptomatic fruit from the inoculum sources for each position will be assessed. Ascertaining the distance of spread will provide more certainty of the risks involved in spread of CBS in FL.
3. Ascertain the role of leaf litter or fruit peel as a source of conidia. Neither the role of infected, discarded fruit on the ground or leaf litter has been studied in FL as an inoculum source. The role of leaf litter as a source of conidial inoculum will be tested by placing leaflitter from diseased trees under healthy trees in groves in the quarantine zone. The leaf litter will be placed under developing fruit and the fruit and leaves in the trees monitored for development of CBS.
4. Ascertain the spatial and temporal development of CBS epidemics in FL. Although limited information exists in FL, the spatial scale of spread is not determined, and only limited information is known on the temporal development of epidemics. To better understand the spatial scale of CBS spread in FL, the numbers of lesions will be counted on 5 randomly selected fruit in 6 x 1 m2 quadrats in a minimum of 4000 tree canopies. The binomial dispersion index will be calculated, which combined with Taylor's power law analysis can be used to ascertain whether the distribution is random or aggregated. Temporal analysis assessments of CBS on leaves and fruit will be made 6 times from Sep to the following Jun. The data will provide information on the incidence and severity of CBS and provide a basis to ascertain CBS severity and spread risks over time within groves.
