This report marks the beginning of the third and final year of a project on protecting newly planted citrus trees from Asian citrus psyllid (ACP) and huanglongbing (HLB). The project is collaborative between USDA-ARS (Hall) and University of Florida (Stansly). Update on USDA-ARS activities. Two experiments are being concluded by USDA-ARS in the Indian River citrus area at a grove just west of Fort Pierce. For research purposes, most of the grove is under a minimal ACP management program and HLB-infected trees are not removed. A detailed summary of the two experiments (as of late summer 2011) was presented in the September 2011 Annual Report. Since that time, notable is that regardless of the particular psyllid management program studied in each experiment, the percentage of trees infected by HLB increased to more than 65% within 39 months after planting in one experiment (infected trees removed as they were found and not replanted) and to nearly 100% within only 26 months after planting in the second experiment (infected trees not removed). The results support that a grower will have difficulty protecting new plantings from ACP and HLB if concerted efforts are not made to control ACP and HLB on an area-wide basis in the vicinity of a new planting. For more information on the specifics of the two experiments, refer to the September 2010 progress report. Update on UF activities. Insecticides currently remain a principle tactic used for ACP/HLB management. Growers need information on when it is best to apply insecticides, how to apply them, and the best insecticides to use. Minimizing the use of a particular pesticide and rotating modes of action (MOA) are considered critical in resistance management. New insecticides with novel modes of action are desired. Research in newly planted citrus showed that a new DuPont insecticide, Cyazypyr (DPX-HGW86, cyantraniliprole), is very active against both ACP and citrus leafminer, being as good as or better than Platinum or Admire Pro. In one experiment, a comparison of young trees treated four times annually with different rotational schedules of Admire, Platinum and Cyazypyr indicates that, regardless of the order that the insecticides were applied, significant control of ACP was achieved as compared to ACP levels in untreated trees; also, lower percentages of trees tested HLB positive after 1.5 years in plots receiving insecticides (0 to 8% trees infected) than in plots of trees untreated (28% trees infected). This insecticide is of a novel MOA (diamide chemical class) and is effective against a number of important types of pests including fruit flies, leafminers, leaf-feeding beetles, whiteflies, hoppers, thrips and weevils. A citrus label for Cyazypyr is anticipated.
Experiment 2 (within HLB-Area-Wide Management AWM). The incidence of HLB did not increase in the last 3 months during spring season (October to December/11), even in plots without insecticide application. The same was observed for CVC incidence. The HLB progress rate still low (cumulative incidence from 6.3% in November/10 to 7.3% in December/11 for plots without ACP control; and from 4.5% to 5.3% for plots with ACP control). The AWM of ACP has kept the ACP population density very low (almost zero) and deccelerated in the last two years, even in the plots without ACP control and with abundant new shoots at spring. A manuscript with the results of Experiment 1 combined with experiment 2 was writen and has been revised by all co-authors and will be submmitted to scientific journal until February. For Year 3, we will continue collecting data from Experiment 2 under regional management, analyzing the spatio-temporal disease and economics data from both experiments.
This is a one-year project with the objective of developing a method for controlling the Asian Citrus Psyllid (Diaphorina Citri Kuwayama) population which vectors the disease Huanglongbing (HLB). This phase of the project involves screening dsRNA constructs that will knock-out essential mRNAs, following feeding, by RNA interference. More than 50 targets have been pre-selected by experts in the field. The initial screening phase, which began in August 2011, will continue through the first three quarters (Aug. 2011 to April 2012). Repeat experiments will occur in the last quarter (April 2012 to July 2012). Publication of findings will also be performed in the final quarter. Preliminary data show a remarkable increase in the mortality of Psyllid populations in a controlled environment, following dsRNA feeding. The mortality rate of insects feeding on water alone was much greater than those feeding on an artificial diet. Alternatively, diets containing several of the selected dsRNAs were shown to increase mortality above diet alone, but did not exceed water alone. Diets containing 3 – 48ng/uL of a specific dsRNA that show 20% higher mortality than diet alone after 3 days and 40% higher mortality than diet alone after 6 days were found to include: nutrient transporters such as Aquaporin (Water Transporter) and a Sugar Transporter as well as Electron Transport-Respiratory and Iron sequestering proteins such as Succinate Dehydroginase and Rieske. Higher than expected mortality using non-specific dsRNA sequences has also been observed obscuring a definitive mechanistic interpretation. With further trials we hope to better understand this process.
To assess the impact of insecticides used for psyllid control on non targets, replicated blocks of mature citrus untreated or treated with foliar sprays of insecticides, horticultural oils, and nutritionals were sampled for psyllids, citrus rust mites (CRM), citrus red mite, predatory mites, snow scale, chaff scale, Glover scale, purple scale, citrus blackflies, whiteflies, mealybugs, as well as their natural enemies and fruit damage at two conventional groves in Hendry County and one each in Lee and Collier Counties. Threshold levels of psyllid populations were not reached in two treatments of two Hendry county locations. However plots designated for calendar sprays at both locations received a rotation of insecticides recommended for ACP control applied every month: spirotetramat (Movento MPC) at 16 fl oz/ac + horticultural mineral oil 2% in October, carbaryl (Sevin 7XLR plus) at 0.75 gal/ac in November and phosmet (Imidan 70 W) at 1 lb/ac in December. These treatments provided significant suppression of psyllids and also impacted some non targets. In a block of ‘Earlygold’ orange, CRM, chaff scale and purple scale averaged 0.4 ‘ 0.1, 2 ‘ 0.3 and 1 ‘ 0.2 per lens field of fruit in the treated plots, respectively, compared to 1 ‘ 0.2, 2 ‘ 0.2 and 0.4 ‘ 0.1 in the untreated plots, respectively. In a ‘Valencia’ orange block, CRM, chaff scale and purple scale averaged 0 ‘ 0, 0.5 ‘ 0.1 and 0.3 ‘ 0.1 per lens field in the treated plots, respectively, compared to 0.4 ‘ 0.2, 0.7 ‘ 0.1 and 0.2 ‘ 0.1 in the untreated plots, respectively. Percent russeting from CRM averaged 21’ 2 and 35 ‘ 2 in the treated and untreated plots of ‘Earlygold’, respectively, and 14 ‘ 2 and 21 ‘ 2 in the treated and untreated plots of Valencia, respectively, indicating that insecticide treatments, probably spirotetramat, are also provided some protection from CRM. Similar impact was observed against leaf infestation with woolly whitefly. However, more scales particularly purple scale, were observed on fruits in the treated plots. Percent russeting averaged 35 ‘ 3, 36 ‘ 3, and 36 ‘ 3 in the plots treated with grower standard sprays, biweekly sprays of 435 oil or the untreated check in a block of ‘Valencia’ in Lee county, reflecting no treatment impact. Averages per lens field between treatments ranged from 0.1-0.4 for CRM and 0.2-0.3 for chaff and purple scales. Percent leaf infestation with woolly whitefly averaged 4-5% between treatments. The experiment in Collier county compares 4 treatments (1) Nutritional (2) Insecticides (3) Insecticides + Nutritionals and (4) Untreated control and uses insecticides currently recommended for psyllid control at threshold of 0.2 adults per tap sample and nutritional program developed by McKinnon Corporation averaging three sprays per year. CRM were high on fruits in the treatments using insecticides (0.2-0.5/lens field) compared to nutritionals only and the control (0.06-0.08/lens field), however, russeting was less in the insecticide treatments averaging 3-4% compared to 8-10% in nutritonal and control. Chaff scale averaged 0.5/lens field in insecticides only treatment compared to 1/lens field in the remaining three treatments. Purple scale averaged 0.2/lens field with the nutritional treatment and 0.1/lens field in the remaining three treatments. Percent leaf infestation with woolly whitefly averaged 10, 4, 4 and 2 in treatments 1, 2, 4 and 3, respectively, indicating some suppression with insecticides but increase with the nutritionals only treatment. Four studies comparing 43 foliar treatments of recommended and experimental insecticides against ACP and citrus leafminer were submitted for publication in the Journal of Arthropod Management Tests of the Entomological Society of America.
We continue to work on the objectives of 1) refinement of sampling methods, 2) testing the influence of adult density and shoot infestation on precision of estimated means and distribution of population within blocks, and 3) evaluation and integration of methods for assessing psyllid density, shoot density, and infestation rates into a user friendly system accessible to consultants and managers. The stem tap and vacuum sampling methods were compared for estimating ACP adults in two experimental blocks of two conventional groves in Hendry county, one a 35 acre block of ‘Early Gold’ and the other a 16 acre block of ‘Valencia’. A ‘tap sample’ count consisted of adult psyllids falling on a white clipboard placed under randomly chosen branches which were then struck 3 times with a length of PVC pipe. The vacuum sampler is made from a leaf blower and collects psyllids even at low populations. A single sample consists of 10, 1 second strikes on the canopy of a single tree. The number of ACP adults captured in suction sample were compared with average number of ACP adults detected in two tap samples conducted before and after the suction sample. Regression analysis indicated a strong positive linear relationship between the two methods on numbers of adults detected in both blocks. In the ‘Earlygold’ block, the relationship was best explained by the equation y = 9.816x + 1.522 (r2 = 0.61) where y = the suction sample and x = the tap sample. In other words, about 10 ACP via suction for each tap. Out of 109 samples conducted using each method, psyllids were observed in 29% (n=32) of the tap samples and 53% (n=58) of the suction samples. Psyllids averaged 1 ‘ 0.3 and 10 ‘ 4 per tap and vacuum sample, respectively. In “Valencia” block the relationship was best explained by the equation y = 14.79x + 0.044 (r2 = 0.74). Out of 96 samples conducted using each method, psyllids were observed in 34% (n=33) tap samples and 55% (n=53) vacuum samples. Psyllids averaged 0.7 ‘ 0.2 and 11 ‘ 4 per tap and vacuum sample, respectively. A pest scouting workshop in collaboration with Hendry county extension is planned for this spring to promote the use of proper sampling methods for effective pest management particularly ACP. Methods to monitor psyllids using tap sampling method and flush examination along with data sheets to record data have been posted at our website: swfrec.ifas.ufl.edu/entlab. Growers are using the sampling kits provided through SWFREC and IFAS extension and suggested data collected methods. Detailed comparisons of different sampling methods are provided in the EDIS document listed below. Arevalo, A. H, J. A. Qureshi and P. A. Stansly. 2011. Sampling Asian citrus psyllid (ACP) in Florida citrus groves. EDIS, http://edis.ifas.ufl.edu/in867.
Objectives of this project include: 1) scale up production of Tamarixia radiata to levels that will allow better assessment of the potential impact an augmentation program may have on the ACP population and ultimately the spread of HLB, 2) use genetic techniques to identify parasitoids recovered from the field to demonstrate establishment and effectiveness of released strains, and 3) pass on rearing technology to both private and public sectors to encourage production and an adequate supply of these parasitoids into the future. This quarter 30,793 wasps of the previously established strain of T. radiata from Taiwan and South Vietnam were produced in the colony at SWFREC, increasing the 2011 total to 22,393. These wasps were used for research, to maintain the colony and to release in treated and untreated plots of commercial experimental blocks in Collier, Lee and Hendry counties. This quarter, 17,108 wasps were released, increasing the number released in 2011 to 142,078. Additional field releases this quarter of 20,900 wasps from the Pakistan colony established at DPI, Gainesville increased the total number of Pakistani wasps released this year to 79,600. Wasps from the Pakistan colony were released at the SWFREC (Collier County). Fewer nymphs were available in the groves to assess parasitism rates this time of the year due to scarce flush and already suppressed populations of psyllid through effects of biological and chemical control during growing season. Incidence of parasitism based on adult emergence from feral nymphs in a 2 year old block of citrus at SWFREC during October averaged 21.5% (N=523) compared to a release block of a neighboring conventional grove which averaged less than 1% (N=555). In the same conventional grove in December, parasitism averaged 6.5% (N=62) in the blocks where releases were made compared to 5% (N=43) in the blocks without releases, probably indicating movement from the release blocks. Parasitism averaged 8% (N=63) in blocks that were untreated or treated only with nutritional sprays compared to 2.4% (N=42) in the blocks treated with insecticides for psyllid control. The recovered parasitoids were preserved in 95% ETOH and sent to Dr. Evan Braswell of USDA APHIS, Edinburg, TX, to characterized using molecular markers. We have been assisting OrangeCo in their efforts to mass rear T. radiata and observed their production and release system on two occasions this quarter. We also planned with them collaborative experiments to be undertaken this spring to determine the effects of release rates on psyllid populations which will be conducted this spring. The research will be undertaken with the assistance of a self-financed master’s student Ziyi Zhang AKA “Mike” with whom we had committee meeting Orange Co. chaired by Dr. Norm Leppla. Another master’s student, Xulin Chen, whose assistantship is being paid for by this project, has been taking courses in in Gainesville since August and has already undertaken a study of the use of CO2 for anesthetizing T. radiata to assist with manipulations during wasp rearing. Findings on the production, field release and evaluation of the parasitic wasp T. radiata against ACP in Florida were presented at the annual meeting of the Entomological Society of America held in Reno, Nevada, and at the Subregional Workshop “Biological Control in Huanglongbing” Recommendations for Central America held in San Carlos, Costa Rica. We also initiated two new colonies of the biparental colony of Diaphoencyrtus aligarhensis from Pakistan at SWFREC.
This is a one-year project. The overall objectives are to rapidly screen and evaluate chemical compounds for the control of citrus HLB using a graft-based screening method. In this quarter from Oct. 2011 to Jan. 2012, the research was focused on the chemical treatments of the second round in Sep. 2010 and the qPCR test in the first-round treatments in May, 2011. In the second round of the contest, twenty files were received on Sep. 15, 2011. Based on the rank of an expert panel and the action modes of the compounds, 16 compounds from 32 of these files listed (ranked higher than 0.17) were purchased and tested. Some compounds such as peptides in File 9932744_044 and lead compounds in File 9932744_066 and 9932744_070 could not be purchased, so they could not be tested. The grafting of the bud sticks treated with the above compounds have been done in December 6, 2011. To date, the grafted scions grew normally. In the first round of the contest, leaf samples of scions and inoculated plants treated with 22 compounds were taken for determining the Las bacterial titers by qPCR. The scion survival rate (%), the pathogen transmission efficiency (%) and the scion infection percentage (%) were also recorded. The primary results indicated that more than 60% of the scions survived and grew when treated with all forty compounds in the first round, except actidione (cycloheximide) at 50 mg/L (0%) and benzyl isothiocyanate at 50 mg/L (54.2%), which were retested at the lower concentrations of 25 mg/L. Ampicilin (Amp) and Rifampicin (Rim) were effective in reducing Las bacteria to undetectable levels in the HLB-affected citrus; Rifamycin (Rif) and Carbenicillin (Carb) were highly effective and lowered both bacterial titers and transmission rates. Nine tested compounds partly suppressed the Las bacterium, including Kanamycin (Kana), Colistin (Col), polymixin B (PMB), Hygromycin (Hyg), Isonicotinic acid hydrazide (INH), Rifaximin (Rix), Cycloserine (Cys), Quinoline (QUI) and Vanomycin (Van). More than 80% of the inoculated plants treated with the other nine compounds, including kasumin and gentamycin, were infected by Las bacterium and had high Las bacterial titers. The other grafted material will be tested soon.
Our research on use of double stranded RNA (dsRNA) targeting specific psyllid genes has previously shown that, when dsRNA targeting either a psyllid cathepsin or a psyllid vacuolar ATPase gene are fed in artificial diets to the Asian citrus psyllid, an increase in psyllid mortality is realized. Associated with the elevated mortality, we observed that mRNA transcripts corresponding to the dsRNA being fed were reduced, strongly supporting an RNAi-induced mortality. This mortality was observed within a range of fed dsRNA concentrations between 3 ng/uL and 45 ng/uL present in the diet. Based on known accumulation levels of plant mRNAs in their cells, these concentrations are presumed to be physiologically relevant to what could be produced in transgenic citrus phloem. Therefore, we worked in association with the lab of Dr. Bill Dawson at the University of Florida (CREC) to construct plants infected with a recombinant CTV vector containing these psyllid sequences that produce dsRNA within citrus phloem. We have analyzed these plants and shown that dsRNAs of each of these sequences are present. These plants have been propagated and are being used for psyllid feeding studies. In leaf feeding assays, where psyllids are allowed to feed on excised leaves, leaves from plants containing the CTV-cathepsin construct showed greater mortality to adult psyllids than leaves from control plants. These experiments are currently being replicated to determine statistical significance. We are also pursuing research on what appears to be general toxicity of higher levels of dsRNA to psyllids when it is fed in artificial diets. It is hypothesized that the toxicity is due either to cryptic homology of the control dsRNAs to critical psyllid genes, or the result of a general yet uncharacterized dsRNA-induced toxicity This phenomenon has not been identified in insects and may be an important phonemenon that could be utilized in developing optimal psyllid control strategies based on dsRNA-mediated psyllid toxicity.
Progress on a new special regulatory trap for capturing and preserving citrus psyllids in situ has been advanced enough to develop some prototypes for further field testing and refinement. Laboratory bioassays that examined psyllid behavior in response to physical details on the surface of the traps is ongoing. Our premise is that eventually some types of chemical attractants will be identified and deployed with trap visual cues optimized in yellow to enhance the numbers responding to the vicinity of traps. We have to maximize trap efficiency by exploiting psyllid behavior. We are testing various trap physical modifications to optimize trap capture rates using lab and field experiments. We are also evaluating various structural designs of the trap to simplify mass production. So far we have developed a working prototype that is ready to use with chemical lures and other psyllid attractants as they come available.
Progress on a new special trap for capturing and preserving citrus psyllids, their associates and DNA in situ is now advanced enough to compare to conventional yellow sticky traps and combine with other behavioral tools such as chemical attractants. Using trial and error methods as well as behavioral observations under laboratory and field conditions, we have investigated Asian citrus psyllid behavior in response to trap physical parameters. Our premise is that eventually some effective chemical attractants will be identified and deployed with trap visual cues optimized in yellow to enhance the numbers responding to the vicinity of traps. However, the trap capture efficiency will depend on psyllid behavior on the trap relative to their willingness to orient, land and enter the trap. We have been concentrating on testing and improving trap components to optimize these behaviors to improve trap capture efficiency. We have also submitted the required paperwork concerning the technology to the University of Florida Technology and Licensing Office. We have located a commercial company that will manufacture the new traps for further research.
One of the objetives of this project was to determine how long Diaphorina citri should feed on healthy citrus plants in order to inoculate Ca. Liberibacter asiaticus (Las). Here we describe transmission trials in which we varied the duration of the inoculation access period (IAP) of infective psyllids on healthy citrus seedlings and measured inoculation efficiency. Early transmission experiments with variable IAP. In the beginning of this project we established seven transmission trials of Las by D. citri in which the following IAP durations were tested: 0.5, 1.5, 6, 12, 24 and 96 h. Before the IAP treatments, healthy 3rd-4th instar nymphs were submitted to an AAP of 14 days on Las-infected plants, followed by a 14-day period on healthy citrus seedlings. In each trial, 7-10 test plants (sweet orange seedlings) were inoculated per IAP treatment, using five psyllids per test plant, except in trial I, in which only 1 insect was used per test plant. Inoculated test plants were assayed for Las infection by qPCR 12 months later. Despite the large number of test plants inoculated in all trials (57-59 plants per treatment), only four of them turned out positive for Las, for IAPs of 6 h (1 positive plant), 24 h (2 plants) and 96 h (1 plant). Therefore, transmission of Las by D. citri was very inefficient in this experiment. A possible reason for the very low transmission rates was the experimental design, with a rather long AAP (14 days) on Las-infected plants, followed by another long period on healthy intermediate plants (14 days).Thus, the insects were submitted to the IAP treatments only at 28 days after the beginning of the AAP, when they were probably too old. We used a long AAP and latency period in this early transmission experiment because at the time we did not have the information that shorter AAPs (48-96 h) could result in high acquisition efficiency (Study 1); in addition, there was no clear information in the literature on the duration of the latent period of the pathogen in the vector. Recent transmission experiments with variable PAI. Based on recent information on the minimum latent period of Las in the psyllid vector (around 10 days) (see a previous report), two new transmission trials with variable PAI were carried out with some modifications in the experimental design. Nymphs were submitted to an AAP of 7 days on Las-infected plants (source plants), followed by 7 days on healthy citrus seedlings (intermediate plants); the psyllids were then transferred to test plants for the following IAPs: 0.75, 1.5, 6, 12, 24, 48 and 96 h. Overall, the transmission efficiency was still low (4.7-11.7%) in the modified experimental design, but higher proportions of infected plants were observed, especially for trial VIII, whose PCR evaluations have already been concluded (12 months post inoculation). For trial IX, we have only one PCR evaluation and need to reevaluate the plants with 12 months after inoculation. Interestingly, the psyllids transmitted Las during the shortest IAP tested (45 min) and the estimated transmission probability did not increase significantly with longer IAPs. These results indicate that infective D. citri adults do not require long periods of exposure to healthy citrus to successfully transmit the pathogen, and the minimum IAP is shorter than 45 min.
Objective 1: To evaluate topical applications to the trunk of bactericides and Pentra-bark, a penetrant proven effective for trunk application of systemic insecticides. In the greenhouse, 0.5-1.0 cm dia trunks of Hamlin orange trees (1 yr old) were painted with a Magna-Bon (Copper sulfate pentahydrate), Cop-R-Quik (copper nitrate), a copper phosphite (CP), an experimental copper (EXP) or oxytetracycline (OTC) applied with 0.1% Pentra-Bark or left non-treated. After for 2-3 weeks, the leaves were observed for phytotoxicity. No phytotoxicity was observed, potential bactericidal activity in the plants was assayed with detached immature leaves inoculated with Xanthomonas citri subsp. citri (Xcc) as a Gram negative bacterial surrogate for non-culturable Canidatus Liberibacter asiaticus (Las). Canker disease control effect was be measured as the number and size of the canker lesions with an in vitro assay. Canker lesion no. per leaf was reduced for trees treated EXP, and Cop-R-Quik but not, OTC, CP or MB. In July, the bactericide treatments were scaled-up in a field trial by painting 1-2 yr-old nursery trees exposed to highly infected psyllid populations at USDA-ARS Picos farm in Ft. Pierce. The trees will be assayed for PCR status in Jan 2012.
The objective is to evaluate soil-applied neo-nicotinoids and other SAR inducers on HLB disease progress in newly planted citrus trees subjected to psyllid-mediated infection or graft-inoculation. One yr-old Hamlin trees were planted in May 2009 and treated as follows: 1) untreated check (UTC), 2) foliar insecticide to control psyllids, 3) soil-applied imidacloprid/thiamethoxam (IMID/THIA) to induce SAR, 4) soil-applied IMID/THIA plus foliar insecticides, 5) graft-inoculated UTC, 6) graft-inoculated with IMID/THIA. There were 50 trees per treatment (5 blocks of 10 trees). In 2009, the effect of SAR inducers on HLB infection progress was inconclusive perhaps attributable to the interaction of IMID/THIA with psyllid control which may have an uncontrolled effect on psyllid transmission. In 2010, the SAR inducer acibenzolar-S-methyl (ASM, Actigard 50WP) which does not control psyllids was substituted in treatments 3, 4 and 6. At 24 months after treatments began, 105 trees were PCR+ (35%) in the trial. Higher number of PCR+ trees occurred in the UTC (20), the UTC with graft inoculation (22), and the IMID/THIA/ASM with graft-inoculation (28). A lower number of PCR+ trees occurred in the treatments with SAR inducers (11), foliar insecticides (12), and foliar insecticide plus SAR inducers (12). Two years after treatments were initiated, the effect of SAR on HLB disease progress has been minimal, which indicates a lack of promise for SAR inducers in HLB management. These findings were communicated to citrus growers in a Citrus Industry magazine article entitled “The ABCs of SAR” (May issue).
Under Objective 1 and 2 the following was submitted for presentation at a canker workshop in Ribeirao Preto, SP, Brazil: Soil applications of SAR inducers at various rates and application frequencies were evaluated for control of canker in a field trial of 3- and 4-yr old ‘Ray Ruby’ grapefruit trees in southeastern Florida. Reduction of foliar incidence of canker produced by one, two or four soil applications of the neonicotinoids, IMID and thiamethoxam (THIA), or ASM was compared with 11 foliar sprays of copper hydroxide and streptomycin applied at 21-day intervals. In 2008 and 2009 crop seasons, canker incidence on each set of vegetative flushes was assessed as the percentage of the total leaves with lesions. By the end of the 2008 season, despite above average rainfall and a tropical storm event, all treatments significantly reduced foliar incidence of canker on the combined Spring-Summer-Fall flushes. Sprays of copper hydroxide and streptomycin were effective for reducing canker incidence on shoot flushes produced throughout the season compared to the untreated control, whereas soil applied SAR inducers reduced foliar disease depending on rate, frequency and timing of application. Except for the treatment of four applications of ASM at 0.2 g a.i. per tree or two applications of imidacloprid, SAR inducers were ineffective for reducing foliar disease on the flushes that were present during the tropical storm. In 2009, all treatments significantly reduced the incidence of foliar canker on the combined Spring-Summer-Fall flushes but not all treatments of Spring-Summer flushes with SAR inducers were effective compared to the untreated control. Hence, depending on rate, frequency and timing of application, soil-applied SAR inducers reduced incidence of canker on foliar flushes of young grapefruit trees under epidemic conditions. While soil application of systemic neo-nicotinoid insecticides has been demonstrated in our field trials to induce SAR and provide long lasting canker control for non-bearing trees, use of these systemic insecticides at higher rates for young fruiting trees is restricted due to perceived risks for soil leaching and insecticide residues in flowers. An objective of current field research is to develop more effective suppression of Xcc and fruit infection using trunk applications of neonicotinoids as well non-insecticidal SAR inducers. A trial with 5 yr-old fruiting grapefruit trees trees showed the efficacy of trunk application as an alternative to soil application for IMID, THIA and ASM at 3.5X the label rate per season to compensate for the larger tree volume produced canker control on foliage that matched that of 11 sprays of copper. Trunk application was as effective for canker control as soil application. Under Objective 3, The integrated use of ASM, THIA and IMID soil applications was evaluated to increase and/or extend canker control in 3-yr-old grapefruit and 2-yr-old Vernia orange trees. The highest incidence of disease trees and/or leaves was in the untreated check in each trial compared with a very low incidence of canker in the integrated SAR treatments. A field trial with soil applied neonicotinoids in Parana, Brazil was evaluated. IMID (Confidor) as a soil drench and IMID (Winner) applied to trunk gave comparable in disease control activity on 2-yr old Valencia orange trees, as well as, the other neonicotinoids tested, THIA and Clothianidin. Clothianidin (Belay) is now registered for use on non-bearing citrus in Florida, hence all of neonicotinoids registered for non-bearing citrus in Florida have been shown to have SAR-inducing activity against canker as well.
Under Objective 1 (define rates and formulations of copper sprays for more effective control) and Objective 2 (establish the period of fruit susceptibility, residual activity and phytotoxicity of copper) The following has been accepted for publication in the proceedings of the Florida State Horticultural Society: Protection of ‘Hamlin’ orange fruit from infection by Xanthomonas citri subsp. citri, the cause of citrus canker, is necessary to reduce premature fruit drop. The objective was to evaluate copper formulations for control of fruit infection and drop in 6- to 8 year-old ‘Hamlin’ trees. Copper sprays were applied at 21-day intervals after fruit reached 0.5 to 1.0 cm (0.25 to 0.50 inches) diameter. The period of susceptibility of fruit to canker infection and fruit drop was established by increasing the number of applications through the fruit growth period. Separate treatments ended at each 21-day interval so that there were four to seven applications per season. In 2008, early season infection occurred during rains before copper sprays commenced in late April. Subsequently, five sprays of copper formulations at rates exceeding 0.5 kg /ha (1.1 lb/acre) metallic copper significantly reduced incidence of lesions on fruit. Early season fruit disease and cumulative fruit drop were highly correlated among copper treatments (r = 0.95). Although a tropical storm in early August promoted disease on fruit late in the season, late season fruit disease and fruit drop were less well correlated (r = 0.78). In 2008 and 2009, additional sprays after the period of early fruit susceptibility did not further reduce canker incidence or fruit drop. In 2009, copper sprays were initiated before significant spring rainfall and the incidence of fruit disease and fruit drop were lower and the correlation of early season fruit disease was less positively correlated with fruit drop (r = 0.52) compared to 2008. In 2010, disease on fruit and premature drop were not significantly different from the untreated checks although fruit disease and early season infection were still significantly correlated (r = 0.70). Overall, there was little difference in efficacy among copper formulations, although control was reduced for treatments with copper sulfate pentahydrate at lower rates of metallic copper. In each season, copper treatments controlled fruit drop by ~50% compared to the untreated check, however as ‘Hamlin’ trees grew from 6 to 8 years of age, canker incidence dropped due to the development of hedgerows, which reduced windblown rain penetration into the grove. Hence, fewer copper sprays will be necessary after canopy closure promotes an internal windbreak effect. Under Objective 4 (To define risk for development of bacterial resistance to copper and streptomycin in FL citrus groves) the results were submitted and accepted for publication in European Journal of Plant Pathology. Briefly summarized: Copper resistance determinants from a citrus epiphytic strain of Stenotrophomonas maltophilia (Stm) were cloned and expressed in Xcc and other Xanthomonas strains. Copper resistance genes in Xcc were determined to be present on a large (~300 kb) conjugative plasmid. Cu resistance was transferred via conjugation from two copper resistant citrus strains, Xcc and X. alfalfae subsp. citrumelonis (Xac), and two tomato pathogens, X. euvesicatoria (Xe) and X. perforans (Xp), to Xcc. PCR analysis revealed that two CuR strains from citrus, an epiphytic Xanthomonas ssp. and a strain of Stm, harbored homologs of the copper resistance genes found in CuR Xcc. The introduction of copLAB gene cluster from Stm into different xanthomonads conferred copper resistance to sensitive strains of Xcc, Xac, Xe and Xp. Under Objective 5 (rapid transfer improved canker management technology to the Florida citrus industry), the results from Objective 1 were presented at the annual meeting of the of the Florida State Horticultural Society in St. Petersburg.
(863) 956-8817
(863) 956-5894
700 Experiment Station Road
Lake Alfred, FL 33850
Email Us
