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United States Department of Agriculture

Agricultural Research Service

Research Project: RESEARCH ON THE EPIDEMIOLOGY OF CITRUS HUANGLONGBING AND OTHER EXOTIC PATHOGENS
2013 Annual Report


1a.Objectives (from AD-416):
The overall objective of this research project is to conduct a series of epidemiological studies to elucidate those biological factors that are not well understood. Utilizing this new information, develop enhanced control strategies for citrus HLB. The sub objectives employed in support of the over-arching main objective are: 1. Develop new real time PCR diagnostic methodologies for Ca. Liberibacter asiaticus and Liberibacter americanus to the use for detection in both plants and insect vectors. 2. Conduct greenhouse epidemic trials using both pathogens, L. asiaticus and L. americanus, studying their spatiotemporal dynamics and interactions between the isolates. 3. Using spatiotemporal analyses monitor and estimate HLB epidemics in commercial orchards. 4. Determine acquisition and inoculation. It's a L. americanus and L. asiaticus for Diaphorina citri. 5. Examine the effect of temperature in geographic regions on HLB progress. 6. Determine the geographic distribution of the two HLB bacterial strains in the state of São Paulo Brazil. 7. Sam the progress and colonization of the two HLB strains in various citrus cultivars. 8. Determine the infectivity D. citri in commercial orchards for the two bacterial strains. 9. Based on information extracted from the above objectives, develop augmented and potentially new strategies for HLB control in commercial orchards.


1b.Approach (from AD-416):
Develop a coordinated, multiphased research effort on exotic citrus diseases of common concern to the US and Brazilian citrus industries with the Fundação de Estudos Agrários Luiz de Queiroz (FEALQ), Piracicaba, SP, Brazil and ARS personnel USDA, ARS, Horticultural Research Laboratory, Fort Pierce, Florida. The cooperative agreement will be developed to implement the research effort between these parties within the USA and abroad for research that cannot be performed in the U.S., or more easily facilitated outside the U.S.


3.Progress Report:

This research is related to inhouse project objective 3a. Develop and use stochastic models to test various disease control strategies for huanlongbing (HLB), Asiatic citrus canker (ACC), citrus black spot (CBS) and diseases caused by other exotic pathogens.

The intent of this study is to examine the effect of windbreaks, copper sprays to reduce infection, and leafminer treatments to determine their individual and combined effects on control of citrus canker in Brazilian commercial citrus and the applicability of this strategy to the U.S. commercial citrus industry. Via a USDA/ARS specific cooperative agreement with the University of Sao Paulo, and the Brazilian cooperator, new replicated plots have now been established at the IAPAR farm, in Xambrê, Parana state, located 350 km west from Londrina and 250 km west from Maringá. The plots consist of the cultivar Pêra on Rangpur lime, two years of age at the beginning of the experiment. Windbreaks have been completed and plants were established in Mid April 2010. Plots are progressing and the following treatments are being applied:.
1)no sprays (control),.
2)Cu++ sprays to reduce citrus canker incidence, and.
3)insecticide sprays to inhibit infestations of Asian leafminer (secondary effects). Main effects are windbreak versus no windbreaks. Citrus canker incidence is being estimated on multiple branches on each tree treated as the number of leaves per branch infected.

Xambrê Paraná Brazil Plots: To quantify the effect of windbreaks, copper sprays, and insecticide sprays individual and combined effects on citrus canker management. In 2010 replicated field plots were established. I was soon evident that the screens used as windbreaks were not strong enough to provide the required effect. Thus in addition to the screens, we established natural windbreaks using Casuarina. Casuarina trees that are currently ~6 m high. There are a few canker-affected plants in the experimental area that will be pruned every 3 weeks. We forecast that in September/October 2013, trees will have developed enough canopy to allow inoculation and to start the experiment. During May, 2013 inspection, 25 plants showed HLB like symptoms.

St Lucie Co. windbreak studies: Data from the windbreak system at the first location (5-y old Ray Ruby grapefruit, Estes Orchard, 11 acres, surrounded by 25-ft Corymbia torelliana) demonstrated a windbreak effect on canker incidence on fruit in August 2012. The weather stations (deployed in east-west and north-south directions) measured the effect of proximity to the windbreak on wind speed expressed as the number of wind gusts =11 mph. The relationship with incidence of fruit canker lesions was explored. In the period May -August 2012, the greater frequency of wind gusts = 11 mph, the higher the canker fruit incidence. The number of wind gusts = 11 mph increased with distance from the windbreak whether measured within the north to south rows or across the rows from east to west, resulting in a relationship between wind and disease incidence on fruit (R2=0.70). In August, the highest incidence of fruit disease (3.5%) was in the center of the 11 acre bock and the lowest incidence (0.4%) was on the end of the eastern most row nearest to the windbreak.

Programmable leaf wetness controller (PLWC): During the previous quarter we developed a complex sensor that closest emulates a leaf. This pin sensor, is a flat surface, made from wax. Contact wires are then connected together in a pattern, so that a drop of water will bridge a “+” to a “-”, creating a connection. The more drops of water, the more connections; the more connections, the more current flow. Results from tests of the new leaf wetness “pin sensor” show that this arrangement is way more sensitive to water amount than any previously tested sensor. After some tweaking of the gain circuit on the DC-AC-DC board, we were able to show that with each successive drop of water, the voltage increased until there was a saturation of water. This is what we have been looking for as this will produce a scalar approximation of leaf wetness as opposed to the current Boolean systems. The drying time is a bit faster than a citrus leaf. To correct this we will add layer of silk to the top of the pin sensor. Previous work with the silks has shown that it dries in a similar amount of time to a leaf. In theory, this thin layer of silk will hold water to the sensor for a more realistic time.


Last Modified: 10/24/2014
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