The census travel model can parameterize and weigh risk contribution of international travelers in various categories given there is a valid disease-associated concern. The census travel front-end allows users to run the model that best suits their needs without strong knowledge of background code and data. Risk maps have been generated for plant diseases and other human viruses . The model has been incorporated into risk-based Multi-Pest Survey programs to provide an augmentative risk factor for continued risk introduction from human-mediated disease spread. In addition, the model can be linked with survey scenario estimators to balance efforts and costs within agency constraints and among emergency programs. The underpinning for HLB control is early detection and early response, especially in Texas and California where incidence is low and Arizona where the disease has not been detected. Eventually Florida, which has been devastated by the disease, will likely replant large areas to reestablish the citrus industry. In all of these situations, the optimal control strategy is to inhibit HLB from entering and establishing in commercial plantings. Infected tree removal, to reduce inoculum in the early stage of the epidemic, remains the most effective deterrent to epidemic development. The earlier the detection of Candidatus Liberibacter asiaticus infections, especially when asymptomatic or better yet subclinical, the more efficacious infected-tree removal can be. A number of early detection technologies are being explored among which canine detection of CLas infected trees, shows considerable promise. Twenty dogs were trained for early detection via a USDA, APHIS HLB Mac grant. Using 10 dogs, each tested against 1000 trees in replicated randomized “field trials” in a gridded array with varying HLB-incidence,hydroponic gutter resulted in 99.17% overall detection accuracy. All dogs performed very well with statistically insignificant trends toward false negative or false positives.
Each dog has its own personality and interacts slightly differently depending upon the trainer-handler. However, there was no statistical difference in CLas detection by trainer-dog combination.When two or more dogs alert on the same tree, the tree statistically has a 100% probability of infection. Dogs were also capable of accurately detecting CLas-infected trees exclusively from 5-gm feeder root samples. Using 10 dogs in a time course experiment with ACP inoculated trees, dogs began to detect CLas infections within 2-3 weeks of inoculation, whereas, none of the trees became PCR-positive for CLas until three months post inoculation and the majority of CLas-infected trees required multiple months prior to PCR detection. This confirms that dogs are indeed a very early detection methodology; able to detect CLas in trees with subclinical infection, i.e., before symptom expression and considerably prior to the ability of PCR for detection/confirmation. In field trials of young and mature citrus plantations, canines trot along the rows with an average interrogation time of ~1 tree/sec, most rapid of all detection methodologies.Dogs are rewarded for detections by verbal praise and short duration play with handlers. Various deployment strategies will be discussed based on the known spatio temporal distribution of CLas infected trees. Using canines to detect HLB in citrus has shown extreme promise in the research environment. Tests on different aged infections, different cultivars, roots, and infections from both grafting and psyllids have all shown the dogs abilities to detect and differentiate an HLB infected tree from clean trees at accuracies exceeding 99%. The next logical step is the introduction of the canines to “real world” environments, i.e., commercial citrus plantings and residential or “dooryard” trees to determine if detection accuracy continued. The dogs were introduced to a young grove in Florida first, and trained over the course of several months. However, due to the high incidence of infection in Florida, finding a suitable low disease incidence training area for dogs in a mature grove is almost impossible. The dogs were therefore taken to other states, especially Texas, for training and testing.
This also allowed the dogs to work in different environments, different grove management practices, and different citrus species. Another aspect of early canine detection is the use of the dogs in residential environments. Again, using other states for a test bed, the dogs were run through various types of residential areas, including residential neighborhoods, mobile home parks, and even a Buddhist monastery. The dogs showed the adaptability to successfully work in these exceedingly diverse and sometimes environmentally hostile environments. Specificity includes the ability of dogs to accurately discriminate between Candidatus Liberibacter asiaticus infections and infections by other pathogens or extraneous scent profiles. To accomplish this, a group of the dogs were taken to Beltsville, Maryland and tested against trees held in the international pathogen collection composed of a number of different viral and bacterial citrus pathogen accessions from around the world. The canines consistently differentiated CLas-infected trees and did not alert on trees infected with other pathogens. One unique finding was that dogs trained on CLas were also intrinsically capable of detecting Liberibacter africancus and Liberibacter americanus infected trees as well, without additional sensitization or training. Asian citrus psyllid continues to spread in California both by natural means and human-assisted means. In areas of the state where the psyllid is well established, such as Southern California, growers are applying coordinated insecticide treatments to reduce populations. In these regions, urban areas receive parasitoid releases to establish Tamarixia radiata and Diaphorencyrtus aligarhensis. Insecticide treatments are only applied to residential citrus trees within 400 meters of commercial citrus. Studies are underway to determine the efficacy of these treatments and to improve the areawide programs in various regions of the state. In regions of the state where the psyllid is not well established, eradicative treatments of pyrethroids and neonicotinoids are applied to both commercial and residential sites around trap finds. These finds have a high correlation with traffic corridors, packinghouses and juice plants, indicating that humans are assisting psyllid spread by moving infested bulk citrus, equipment and plant material. Industry-led changes in how the commodity is handled are expected to reduce the impact of human assisted psyllid spread. Control of psyllid movement around the state is critical to prevent spread of huanglongbing. Early symptoms of HLB include fibrous root loss and leaf blotchy mottle, followed by premature fruit and leaf drop, and yield decline.
As a consequence of initial bacterial infection of fibrous roots, a 30-50% reduction in fibrous root density and elevated soil Phytophthora populations were detected in field surveys. Continued sampling of Hamlin and Valencia orange trees on Swingle citrumelo root stock in different stages of HLB decline revealed that root loss occurs in two stages. The second phase of root loss begins at the early stage of tree canopy thinning resulting from leaf drop and branch dieback. A more extensive survey of HLB-affected groves indicated that greater decline in fibrous root health and expression of HLB symptoms is observed where irrigation water is high in bicarbonates and/or soil pH > 6.5. HLB symptom expression of trees on different root stocks follows the known intolerance to bicarbonate . Acidification of irrigation water in central ridge and south central flat woods Valencia orange groves on Swingle citrumelo root stock for three seasons has maintained soil pH below 6.5 on the flatwoods and 6.0 on ridge. Over the last three seasons of survey, root density as an index of root heath has been sustained. Phytophthora populations remain below the damaging level in ridge groves and in flat woods increase to damaging levels coincident with the fall root flush but drop back to non-damaging levels for remainder of the season. Compared to the 2013-14 season,hydroponic nft channel yields in the ridge blocks have increased up to 4% and on the flat woods have increased up to 22%. Growers using acidification treatments with sulfuric and/or N-phuric acid for the last 3 seasons report an average cost of $60 per acre. This cost will analyzed in relation to yield response to provide a cost benefit of acidification.Our field and greenhouse data shows that secondary and micro-nutrient deficiencies are much greater in roots than in leaves of HLB-impacted trees. Not only is there severe feeder root loss, but the ability of the remaining roots to mine and translocate essential nutrients is compromised. Restoring root health is imperative for keeping trees productive in an HLB endemic environment. In an effort to improve tree health by focusing on the roots, we have been experimenting with polymer coated nutrients and more recently TigerSul micronutrients in the field and greenhouse. In a previous greenhouse study, HLB-infected Valencia sweet orange on UFR-3 root stock showed greater feeder root growth when grown with Harrell’s CRF supplemented with a 3x overdose of TigerSul manganese, or the Schumann blend of TigerSul manganese, iron and zinc . Results suggested that trees in the HLB world have higher specific micronutrient requirements than what are currently being recommended. PCR testing of the greenhouse trees suggested that an overdose of boron could suppress the bacterial titer in the midribs, whereas an overdose of manganese could suppress the bacterial titer in the roots. Several experiments are ongoing in the field to test hypotheses developed from the above information. We define ‘hybrid’ nutrition programs as any program that combines multiple sources of nutrient delivery with a goal of providing a constant supply of all required nutrients year round at an affordable cost. Successful field experiments with industry cooperators to be discussed include the St. Helena Project, where we are testing 100% CRF; and the Post Office Block, where we are testing a 50% CRF/50% traditional dry fertilizer program with micronutrient overdoses. Other successful ‘hybrid’ programs successfully developed by citrus growers will also be presented, including the Ed English program and the Duda program. Data from these trials will demonstrate that enhanced ground nutrition can restore health and sustainable profitable production to HLB-infected trees. Additional fine-tuning of fertilizer composition, type and delivery method could result in less need for psyllid control and thus lower production costs.
Genetic variability for HLB tolerance/resistance has been identified in existing experimental root stock germplasm planted throughout Florida, with both sweet orange and grapefruit scions. New root stocks are being identified in these trials that show a reduced infection frequency, and less severe symptoms once infected, as compared to commercial root stocks. Such root stocks are also showing a stronger recovery following infection in commercial trials. This information suggests that it should be possible to substantially increase the level of tolerance by conventional breeding at both the diploid and tetraploid levels, with focus on direct selection for HLB tolerance in progeny from carefully selected parental combinations. For the past 5 years, we have developed and utilized our ‘Gauntlet’ screening program described below. Each year, crosses of superior parents are made at both the diploid and tetraploid levels. Following a preliminary calcareous soil/Phytophthora screen, selected individual hybrid root stock candidates are transferred to citripots in commercial potting soil. Tops of these trees are propagated by rooted cuttings to produce seed trees on their own roots. The remaining individual liners are grafted with HLB-infected budsticks of Valencia sweet orange. The remaining root stock top is then removed, forcing flush from the HLB- infected Valencia bud stick. Trees are monitored for HLB symptoms, and healthy appearing trees are entered into a ‘hot psyllid’ house until psyllid feeding damage is observed on their leaves , followed by field planting at a challenging field site . Root stocks capable of growing off healthy sweet orange trees are identified for further study. To date, several thousand individual hybrids have been screened. The oldest ‘Gauntlet’ trees have now been in the field for approximately 4 years, and several promising new root stocks have been identified. Although PCR+, individual trees appear to have a reduced bacterial titer and are showing normal healthy growth and fruiting patterns. Seed trees of two of the most promising selections are producing abundant polyembryonic seed, and propagations for large-scale field trials are underway. Patterns are emerging regarding successful genetic combinations, and this will be discussed. Our goal is to develop root stocks that will facilitate sustainable and profitable citriculture in an HLB-endemic Florida, and possibly eliminate the need for psyllid control. Antibodies form the basis for various assays designed to detect plant pathogens in many pathosystems, but until recently have not been available to detect ‘Ca. Liberibacter asiaticus’ .