Progress 01/01/12 to 12/31/12
OUTPUTS: Objective 1: Work has been completed and results are being prepared for publication. This project sought to develop an integrated system for disease, insect, and nutrient management in organic blueberries centered around foliar applications of fish-derived products. Three on-farm trials were conducted on rabbiteye and southern highbush blueberries. Four to six foliar sprays of four fish product formulations (Omega Grow, Organic Gem, Organocide, and SeaCide) were made during summer and early fall at each site and compared with an untreated check, an organic standard fungicide (Serenade Max), and one or two additional biofungicides. The intensity of foliar diseases was assessed periodically during the fall. Similarly, leaf beetle-induced damage was assessed, a visual plant vigor rating was conducted, and defoliation was quantified. Samples of spring leaves from each plot were submitted for tissue nutrient analysis. - Objective 2: Our work on emerging systemic diseases has been refocused on bacterial leaf scorch (BLS), caused by Xylella fastidiosa, given the greater economic importance of this disease compared with BRRV. The BLS pathogen infects the xylem of the plant, obstructing water movement and ultimately causing death of the plant. It is unknown whether early-stage infections in affected plantings can be pruned out by removing shoots that are starting to show symptoms, or whether more seriously affected plantings can be cured by mowing plants to ground level. Whether these tactics will be successful depends on where the bacterium is located in plants of different severity classes. Three southern highbush blueberry fields naturally infected with BLS were sampled to determine the distribution of X. fastidiosa across the plant. In each field, asymptomatic plants as well as plants each with light, moderate, or severe symptoms were selected in the fall. A representative main stem from each plant was dissected into sections corresponding to different age groups, i.e., fall growth, spring growth, last year's growth, and lower trunk sections of different a classes. In addition, root segments were sampled similarly. Xylem sap was extracted from each segment, and concentrations of X. fastidiosa in sap were determined using qPCR with species-specific primers. - Objective 3: We determined the fine-scale genetic structure of populations of M. fructicola in individual peach tree canopies to better understand fine-scale pathogen diversity and to complement previous work on spatio-temporal disease development at the canopy level. Across 3 years, six trees were monitored for disease development season-long, tagging each individual symptom (blossom blight, green fruit rot, pre-harvest fruit rot, and twig canker) and mapping it in three dimensions using a magnetic digitizer. Trees had between 244 and 861 fruit total, with a final fruit rot incidence of 11.1 to 58.2%. The pathogen was isolated from each of the mapped symptoms, and all 694 single-spored isolates were evaluated with 13 SSR markers developed previously under this objective. - Objective 4: Work on this objective has been completed and the results published (Plant Disease 94:1213-1218, 2010). PARTICIPANTS: Harald Scherm, Professor; Phil Brannen, Plant Pathology Extension Specialist; Rock Christiano, postdoc; Moukaram Tertuliano, postdoc; Sydney Everhart, PhD student; Renee Holland, MS student. TARGET AUDIENCES: Peach and blueberry producers, shippers, and packers in the southeastern U.S. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Objective 1: Septoria leaf spot and leaf rust were suppressed with fish products across the trials. There was no clear "winner" among the four formulations, with Omega Grow, Organocide, and SeaCide being most effective in different trials. Effects on leaf beetle damage were inconsistent, with one trial showing damage reduction by SeaCide and another showing no such effects. Effects on plant vigor and leaf retention also were variable, with Organocide and SeaCide improving both parameters at a low-vigor site but not a higher-vigor site. Thus, improvements in plant growth seem limited to conditions with weaker growth potential. None of the treatments translated into a higher flower bud set. Consistently across the trials, application of Organic Gem resulted in considerably higher Na levels in leaf tissue of treated plants, although this did not appear to impact plant growth negatively. In two of three trials, increased concentrations of P were observed following application of some fish products, providing a nutritional benefit. Overall, although fish products are no "silver bullets" for organic blueberries, they contribute to leaf disease suppression and foliar nutrition. - Objective 2: The bacterium was not detected in tops and roots of asymptomatic plants, but was detected at low levels in middle and base stem sections of such plants. In plants with light symptoms, X, fastidiosa was readily detected in all stem sections (top, middle, and base) as well as in roots, indicating that the bacterium spreads quickly as symptoms become apparent. Bacterial titers were again highest in middle and base stem sections. In plants with moderate and severe symptoms, titers were highest in middle and base stem sections as well as in roots, indicating that the pathogen accumulates in the roots over time. Because X, fastidiosa is already present in lower sections of the plant when symptoms become apparent, selective pruning is not a suitable management practice for removal of BLS infections. Similarly, because of the presence of the bacterium in the roots in moderately and severely affected plants, mowing of such plantings will be ineffective in eliminating the disease. - Objective 3: Each M. fructicola population had 65 to 173 isolates per tree, and these populations showed high genetic (average uh = 0.529) and genotypic diversity (average D = 0.928). The percentage of unique multilocus haplotypes in trees was greater for blossom blight isolates (78.2%) than for fruit rot isolates (51.3%), indicating a greater contribution of clonal reproduction during the pre-harvest epidemic. Spatial genetic structure was observed among fruit rot isolates, with all populations showing positive autocorrelation up to ~1.1 m, which was positively correlated with the size of the clonal groups within those trees. Despite high levels of within-tree pathogen diversity, the relative contribution of locally available inoculum combined with short-distance dispersal is likely the main factor in generating the observed fine-scale spatial patterns within trees. - Objective 4: Work on this objective has been completed and the results published (Plant Disease 94:1213-1218, 2010).
- Brannen, P.M., Scherm, H., and Holland, R. 2013. UGA Department of Plant Pathology research results in additional cultural management recommendations for bacterial leaf scorch (Xylella fastidiosa). Dixie Blueberry News 13(2):10-12.
- Brannen, P., and Scherm, H. 2011. Exobasidium fruit and leaf spot: A newly developing issue on Georgia blueberries. Dixie Blueberry News 11(4):4-8.
- Brannen, P., Scherm, H., Cline, B., and Ingram, D. 2011. Exobasidium fruit and leaf spot: An emerging concern on blueberries in the Southeast. Small Fruit News 11(4):1-3.
- Brannen, P., Scherm, H., Deom, M., Srinivasan, R., and Harmon, P. 2011. Blueberry necrotic ring blotch disorder widespread and severe in 2011. Dixie Blueberry News 11(6):21-25.
- Everhart, S.E., Askew, A., Seymour, L., and Scherm, H. 2012. Spatio-temporal patterns of pre-harvest brown rot epidemics in individual peach tree canopies. European Journal of Plant Pathology doi 10.1007/s10658-012-0113-3
- Everhart, S,E., Askew, A., Seymour, L., Glenn, T.C., and Scherm, H. 2012. Spatial patterns of brown rot epidemics and development of microsatellite markers for analyzing fine-scale genetic structure of Monilinia fructicola populations within peach tree canopies. Plant Health Progress doi:10.1094/PHP-2012-0723-04-RS
- Everhart, S.E., Askew, A., Seymour, L., Holb, I.J., and Scherm, H. 2011. Characterization of three-dimensional spatial aggregation and association patterns of brown rot symptoms within intensively mapped sour cherry trees. Annals of Botany 108:1195-1202. DOI:10.1093/aob/mcr029
- Scherm, H., Savelle, A.T., Tertuliano, M., and Krewer, G. 2011. Fish product trials for leaf disease management in organic blueberries in Georgia, 2009-2010. Dixie Blueberry News 11(4):10-23.
- Tertuliano, M., Srinivasan, R., and Scherm, H. 2012. Settling behavior of the glassy-winged sharpshooter, Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae), vector of Xylella fastidiosa, on different southern highbush blueberry cultivars. Entomologia Experimentalis et Applicata, DOI: 10.1111/j.1570-7458.2012.01228.x
- Tertuliano, M., Krewer, G., Smith, J.E., Plattner, K., Clark, J., Jacobs, J., Andrews, E., Stanaland, D., Andersen, P., Liburd, O., Fonsah, E.G., and Scherm, H. 2012. Growing organic rabbiteye blueberries in Georgia, USA: Results of two multi-year field studies. International Journal of Fruit Science 12:205-215. DOI: 10.1080/15538362.2011.619348
Progress 01/01/10 to 12/31/10
OUTPUTS: Objective 1: Four foliar sprays of four fish formulations were made between late August and early October in an organic planting of Star southern highbush blueberry: (1) Omega Grow, 2%; (2) Organic Gem, 2%; (3) SeaCide, 1%; and (4) Organocide, 2%. Furthermore, (5) Sporan, a plant-derived biofungicide; (6) KeyPlex 350 OR, a micronutrient-based resistance inducer; (7) an organic standard of Serenade Max; and (8) an untreated check were included. In mid-October, Septoria leaf spot severity was evaluated by estimating percent leaf area covered with spots, and leaf beetle-induced damage was assessed on a 0 (no damage) to 4 (severe damage) scale. A plant vigor rating was conducted using a whole-plant scale from 1 (poor) to 5 (excellent), and defoliation was determined. Non-abscised leave samples from each plot were submitted for tissue nutrient analysis. In early February of 2011, flower bud set was determined to assess treatment effects on return yield potential. Objective 2: We quantified vegetative transmission of BRRV compared with two other novel systemic diseases of blueberry, bacterial leaf scorch (BLS; caused by Xylella fastidiosa) and Blueberry necrotic ringblotch-associated virus (BNRaV). In June and August of 2010, softwood cuttings were taken in blocks of Star southern highbush blueberry that were either asymptomatic or affected by BRRV, BLS, or BNRaV. Cuttings were placed in 36-cell flats with milled pine bark and placed under intermittent mist. After ~2 months, assessments were made for plant survival, percent rooting, and root volume. In addition, 10 to 20 cuttings per treatment and replicate were transplanted into 6" pots for further monitoring of growth and symptom development. The remaining cuttings were frozen for PCR determination of disease transmission. Objective 3: Forty M. fructicola microsatellite primers developed as part as this study and 12 SSR markers published previously for Sclerotinia spp. were screened against a set of seven isolates. Twelve primer sets yielded amplification in all isolates, whereas five required re-design to amplify in all isolates. Remaining were 35 primer pairs that did not amplify sufficiently (including the markers for Sclerotinia spp.) or showed the locus was monomorphic. To assess polymorphisms, DNA was purified from 47 isolates of M. fructicola from three distinct geographic locations, from fruit or blossoms, isolated from 1982-2010. Results showed that 16 of the primer sets consistently amplified a polymorphic microsatellite region, but one was excessively monomorphic and will not be included for further use. Objective 4: Work on this objective has been completed and the results published (Plant Disease 94:1213-1218, 2010). Briefly, we documented the dynamics of oxytetracycline (OTC) residues on attached peach seedling leaves in relation to temperature, natural sunlight, and simulated rain. We further evaluated the potential of three UV protectants (lignin, titanium dioxide, and oxybenzone) and one sticker-extender (Nu Film-17) to prolong OTC longevity on the leaf surface as a potential means of improving disease control. PARTICIPANTS: UGA Investigators: Harald Scherm, Professor of Plant Pathology; Moukaram Tertuliano, postdoc; Rock Christiano, postdoc; Renee Holland, MS student; Sydney Everhart, PhD student. Collaborators: Chuck Reilly, USDA-ARS, Byron, GA; Bill Miller, UGA Crop and Soil Sciences; Gerard Krewer, UGA Horticulture; Phil Brannen, UGA Plant Pathology. Partner organizations that provided partial support: Organic Farming Research Foundation; Southern SARE; Southern Region Small Fruit Consortium; Georgia Blueberry Commission; Alma Pak; MBG Marketing; SunnyRidge Farm. Training or professional development: Two postdocs and two graduate students (one each MS and PhD) are being trained as part of this project. TARGET AUDIENCES: Conventional and organic blueberry and peach producers in the southeastern U.S. Our most important outreach activity has been the close collaboration and interaction with the respective grower collaborators and their county extension agents in the on-farm trials and sampling efforts associated with objectives 1 and 2. They were active participants in the trials, received regular updates, and provided much needed feedback. In terms of reaching a large audience of organic growers and those interested in converting to organic production, our most important activity has been the participation in the organic blueberry educational sessions at the Southeast Fruit and Vegetable Conference and Trade Show in Savannah, GA, in January 2010 and 2011, where more than 100 growers specifically attended our sessions each year. A major venue for outreach to extension personnel was a presentation at the North American Blueberry Research and Extension Workers Conference (NABREW) in Kalamazoo, MI, in July 2010; this biannual conference has been a key educational forum for blueberry researchers, Extension educators and industry leaders since the 1960s. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Objective 1: Applications of Organocide reduced Septoria leaf spot severity, confirming the favorable results obtained with this product in our 2009 trial. All treatments reduced the number of leaf beetles within 1 week after treatment, indicating that the insects avoided the treated plots. However, the amount of beetle damage was not reduced significantly. In conditions of relatively good plant growth, none of the treatments improved plant vigor, leaf retention, or flower bud set. KeyPlex markedly increased Fe, Zn, and Mn levels, whereas Organic Gem resulted in considerably higher Na concentrations. These high Na levels did not appear to impact plant growth negatively. Increased P levels were observed with Organic Gem and Omega Grow. Similar beneficial increases in P concentrations associated with application of fish products were noted in our 2009 trial. Objective 2: Mortality was increased significantly in the BRRV and BLS cuttings in the summer and fall experiments, respectively. There was no effect of plant source on percent rooting, root volume, and percent cuttings with new growth. Defoliation was increased significantly in the BLS cuttings in the fall experiment, and marginally so (P = 0.0635) in the BLS and BRRV cuttings in the summer experiment. In the BRRV cuttings, 45.5 and 83.2% showed typical foliar symptoms at the time of transplanting in the summer and fall experiments, respectively. In contrast, cuttings from none of the other plant sources showed typical foliar symptoms. PCR testing of cuttings and the resulting plants for pathogen transmission is ongoing. Objective 3: This project developed 15 polymorphic microsatellite primer sets for population genetic analyses. Currently, more than 650 isolates collected from diseased fruit or blossoms from six individual trees in 2009 and 2010, representing all infections within each tree during the entire season, will be genotyped using these markers. The outcome will provide unique information about the spatio-temporal genetic structure of M. fructicola populations within individual trees and an orchard. Objective 4: Constant temperatures up to 40C in darkness did not affect OTC degradation. In contrast, OTC residue decreased rapidly in natural sunlight, declining, on average, by 43.8, 77.8, and 92.1% within 1, 2, and 4 days after application, respectively. Use of shade fabric reduced OTC degradation significantly but did not extend OTC persistence beyond 7 days. UV protectants and Nu Film-17 were ineffective in improving OTC persistence. Simulated rain at 44 mm/h drastically (by 67.2%) lowered OTC residue after 2 min, and levels were near the detection limit after 60 min of rain. In inoculation experiments with X. arboricola pv. pruni, OTC concentrations >50 ppm a.i. were sufficient to suppress bacterial spot development. By extrapolation, similar OTC residues following application of labeled OTC rates would be reached after <2 days under full sunlight, after 4 days under overcast sky, or after 2 min of a heavy rainstorm. These results help to explain the difficulties peach producers are experiencing in managing bacterial spot with OTC.
- Brannen, P.M., and Scherm, H. 2010. Recap of the 2010 mummy berry epidemic, or the "perfect storm" revisited. Dixie Blueberry News 10(3):4-6.
- Christiano, R. S. C., Reilly, C. C., Miller, W. P., and Scherm, H. 2010. Oxytetracycline dynamics on peach leaves in relation to temperature, sunlight, and simulated rain. Plant Dis. 94:1213-1218.
- Cook, M.J. IV, Christiano, R.S.C., Savelle, A.T., Reilly, C.C., Hotchkiss, M.W., and Scherm, H. 2010. Evaluation of alternatives to oxytetracycline for control of bacterial spot of peach during the cover sprays, 2009. Plant Disease Management Reports 4:STF017.
- Scherm, H. 2010. The spring 2010 mummy berry epidemic: What happened, and what can you do now Dixie Blueberry News 10(2):5-9.
Progress 01/01/09 to 12/31/09
OUTPUTS: Objective 1: Two field trials were conducted in commercial plantings of Brightwell rabbiteye and Bluecrisp southern highbush blueberry. Six foliar applications of four different fish products (Omega Grow, Organic Gem, Organocide, and SeaCide) were made during summer and early fall and compared with an untreated check, an organic standard (Serenade Max), and a new plant oil-based fungicide (Sporan). Severity of Septoria leaf spot was determined in late August and late October, whereas severity of leaf rust was estimated in early December. Leaf beetle damage and plant vigor were assessed on a rating scale in October. At the same time, a sample of spring leaves from each plot was subjected to tissue nutrient analysis. In early December defoliation and flower bud set were determined. Objective 2: Experiments in 2009 focused on quantifying rates of vegetative transmission (via cuttings) of BRRV compared with two other novel systemic diseases of blueberry, bacterial leaf scorch (BLS, caused by Xylella fastidiosa) and Blueberry necrotic ringblotch-associated virus (BNRaV). In June, softwood cuttings were taken in four blocks of Star southern highbush blueberry that were either asymptomatic or known to be affected by BRRV, BLS, or BNRaV. Lower leaves of cuttings were removed and stems dipped in K-IBA to encourage rooting. Cuttings were placed in nursery flats with milled pine bark. Four replicate flats for each plant type (three diseases and healthy check) were prepared and arranged under intermittent mist. In mid-August, assessments were made for plant survival, percent rooting, root volume, and percent disease transmission as determined by PCR. In addition, 10 to 20 cuttings were transplanted into 6" pots for further monitoring of growth and symptom development. Objective 3: A double-enriched library of 159 colonies was sequenced and yielded 58 unique microsatellite repeats composed of at least four repetitions of a di-, tri-, tetra-, penta- or hexanucleotide repeat. Amplification primers were designed for 54 microsatellite repeats that contained suitable flanking sequence. From this, 24 microsatellite primers have been selected for screening to date. In addition, 10 polymorphic microsatellite markers previously developed for Sclerotinia spp. will be screened simultaneously. Objective 4: In previous experiments we found that oxytetracycline (OTC) residues on peach leaves are largely unaffected by temperature but decrease rapidly under the influence of sunlight and rainfall. This past year, we conducted greenhouse experiments to determine the minimum OTC residue needed to control the bacterial spot pathogen on peach foliage, and to calculate how many days after OTC application residues drop below this critical level in relation to sunlight and rain. Peach seedlings were sprayed with OTC suspensions of 0, 12.5, 25, 50, and 100 ppm in the greenhouse, followed by spray-inoculation with the pathogen and covering plants with plastic bags to create a moist chamber. Disease severity was measured on four symptomatic leaves per plant, and four independent experimental runs were conducted. PARTICIPANTS: Nothing significant to report during this reporting period. TARGET AUDIENCES: Nothing significant to report during this reporting period. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Objective 1: Two different foliar diseases, Septoria leaf spot (in the rabbiteye trial) and leaf rust (in the southern highbush trial), were suppressed successfully with fish products, whereby Omega Grow (which reduced Septoria leaf spot severity to <1/3 of the untreated plots) and SeaCide were most effective. Organocide and SeaCide were effective in maintaining plant vigor and improving leaf retention into the late fall. For example, by early December in the rabbiteye trial, the SeaCide-treated plants had retained 3X more leaves than their untreated counterparts. Leaf beetle damage differed among treatments in the southern highbush trial, with SeaCide, Serenade Max, and Sporan having lower damage scores than the untreated plots. In the southern highbush trial, applications of Organic Gem and Organocide increased leaf P levels compared with the untreated check. Objective 2: Mortality of cuttings was very low, with >93% of cuttings alive at the time of assessment. Rooting percentage was very good, although cuttings taken from plants affected by BRRV rooted at a lower percentage (82.6%) compared with all other treatments (91.7 to 98.6% rooting). Interestingly, 53.2% of cuttings from asymptomatic plants had already initiated new shoot growth, which was considerably higher than in cuttings from BRRV (28.6%), BLS (16.9%), or BNRaV (39.1%) plants. PCR assay results showed transmission percentages to cuttings of 79.2% for BRRV and 6.3% for BLS; assays for BNRaV are still underway since primers for this pathogen have become available only recently. Objective 3: Utilizing capillary electrophoresis, primer sets producing unambiguous polymorphic amplicons of the microsatellite-containing loci are currently being identified based on amplification of DNA from a robust group of seven isolates of M. fructicola collected in GA and SC between 1997 and 2009. Generation of 10 to 12 polymorphic markers for this cohort will be used subsequently for generating unique banding profiles for each isolate in population genetic analyses. Objective 4: Bacterial spot severity dropped rapidly as OTC concentration applied to the leaf surface increased between 0 and 50 ppm, whereas increasing the concentration from 50 to 100 ppm did not result in a markedly improved level of disease suppression. Thus, the minimum OTC concentration providing satisfactory suppression of bacterial spot was ~50 ppm. Assuming OTC is applied at the labeled rate of 150 ppm, this implies that about two-thirds of the initial residue can be removed before disease control would break down. In natural sunlight without rain, this level of OTC degradation is reached <2 days after application. Cloud cover can prolong the effective period for another 2 days, giving a total of 4 days of protection. In contrast, 2 min of a storm shower are sufficient to remove 2/3 of the applied OTC, resulting in concentrations of the antibiotic too low to provide sufficient protection against bacterial spot.
- Brannen, P., Scherm, H., and Chang, C.J. 2009. Update on bacterial leaf scorch (Xylella) in southern highbush blueberries and cultivar field resistance. Pages 72-77 in: Proc. Southeast Blueberry Conf., Savannah, GA, 9-11 Jan. 2009.
- Everhart, S.E., Scherm, H., Askew, A., Seymour, L., and Holb, I.J. 2009. Spatial patterns of brown rot symptoms in individual, intensively mapped cherry trees. Pages 143-145 in: Proc. 10th Intl. Epidemiol. Workshop. D.M. Gadoury, R.C. Seem, M.M. Moyer, and W.E. Fry, eds. NY State Ag. Experiment Station, Geneva, NY. ISBN 0-9676507-7-1.
- Scherm, H., and Krewer, G. 2009. Suggestions for organic blueberry cultivars in Georgia. Pages 15-22 in: Proc. Southeast Blueberry Conf., Savannah, GA, 9-11 Jan. 2009.
- Tertuliano, M., Krewer, G., Scherm, H., Smith, J.E., Clark, J., Plattner, K., Andersen, P., Liburd, O., Fonsah, E.G., Stanaland, D., Jacobs, J., Varnedore, T., and Andrews, E. 2009. Update on leaf beetle control and organic mulching experiment. Pages 23-25 in: Proc. Southeast Blueberry Conf., Savannah, GA, 9-11 Jan. 2009.
Progress 01/01/08 to 12/31/08
OUTPUTS: Objective 1: Blueberries in Georgia and other southeastern states are subject to attack by several foliar diseases which can cause premature defoliation and subsequent yield loss. A test to evaluate six biofungicides approved for use in organic production was carried out in a mature, certified organic rabbiteye blueberry planting in Appling County, GA. The products included five fish and/or plant extracts, Omega Grow (fish oil), SeaCide (fish and cottonseed oils) + Therm X70 wetting agent, Organic Gem (liquid fish-based fertilizer), Organocide (fish and sesame oils), Sporan EC (rosemary, clove, and thyme oils), as well as Serenade MAX (Bacillus subtilis QST 713) as an organic standard. The materials were applied on 14 Aug, 28 Aug, 10 Sep, and 25 Sep 2008. On 6 Nov, ten spring shoots were collected randomly from the two center bushes in each plot, and four leaves from the midsection of each shoot were assessed for disease severity. At the same time, the most prevalent pathogen species were noted, a defoliation assessment was made, and the number of flower buds per shoot was counted. Objective 2: A BRRV survey was carried out between late Aug and early Oct 2008 in the blueberry production belt in southeastern Georgia. The entire survey encompassed a total of 8 counties, 45 farms, 167 fields, and 26 different southern highbush cultivars. In four quadrants within each field the BRRV incidence was estimated visually. Select samples assessed as positive or negative in the field were confirmed with a BRRV-specific PCR assay. Objective 3: During the 2009 growing season, individual peach trees (different cultivars with varying maturity dates) were monitored intensively over time for brown rot symptoms, including previous year's cankers and current year's blossom blight, twig blight, green fruit rot, and mature fruit rot. Each individual symptom was digitized with a motion tracking device to give its exact coordinates within the tree canopy, and M. fructicola isolates were collected and stored for future determination of small-scale genetic structure within canopies. Development of microsatellite markers for population genetic analyses is currently underway. Objective 4: Oxytetracycline (OTC), a member of the tetracycline antibiotics, is used as a foliar spray to control Xanthomonas arboricola pv. pruni on peach and Erwinia amylovora on apple and pear. Applications need to be made at close intervals in order to be effective. We studied the dynamics of OTC on peach seedlings whose foliage was treated with 300 ppm of an agricultural OTC in relation to constant temperature, natural sunlight, and simulated rainfall. We further evaluated the potential of three UV protectants (lignin, titanium oxide, and oxybenzone) and one sticker-extender (Nu Film-17) to prolong OTC longevity on the leaf surface. OTC residue was determined by HPLC (C18 reverse-phase column) with UV/Vis detector. PARTICIPANTS: INDIVIDUALS: Harald Scherm, University of Georgia, Athens (PD; overall project coordination and participation in all research); Amy T. Savelle, University of Georgia, Athens (research technician; participation in all research). PARTNER ORGANIZATIONS: USDA-ARS Fruit and Tree Nut Research Lab, Byron, GA (provided access to experimental peach plantings and HPLC for OTC quantification). COLLABORATORS: Phillip M. Brannen, Extension Fruit Pathologist, University of Georgia, Athens (coordination of disease survey under objective 2; project-related outreach). TRAINING/ PROFESSIONAL DEVELOPMENT: Sydney Everhart, University of Georgia, Athens (PhD student; conducted dissertation research under objective 3). TARGET AUDIENCES: The primary target audience for this project are peach and blueberry growers throughout the southeastern United States. Knowledge transfer occurred in educational sessions at the Southeast Fruit and Vegetable Conference (Jan. 2008 and 2009) in separate presentations in the peach, blueberry, and organic sessions; at the Middle-Georgia Peach Update (Feb. 2008); at the 23rd Carolina Sustainable Agriculture Conference (Nov. 2008); and in frequent farm visits and discussions with individual peach and blueberry producers. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Objective 1: Phyllosticta and Septoria leaf spots were the most prevalent diseases in the organic leaf spot trial. We initially intended to assess disease severity on the second flush of leaves formed during the spray period in summer and fall, but since shoot growth during this period was very limited, we assessed disease on leaves from the spring growth flush instead. Since disease progressed slowly on these leaves, treatment effects on disease severity were limited, with only Organocide being significantly better than the untreated check. There were no significant treatment effects on defoliation, although there was a trend toward reduced defoliation in Organocide-treated plots. None of the treatments affected the number of flower buds per shoot. Objective 2: BRRV was present in 7 of the 8 counties surveyed. The prevalence of BRRV at the farm level was 42.2%, i.e., 19 out of 45 farms had the disease in at least one of their fields. At the field level, 25 of 167 fields (14.9%) had some BRRV. In most of the symptomatic fields, only few plants were affected. In addition to symptoms typical of BRRV, a pronounced necrotic blotching on leaves was encountered during the survey. Affected leaves consistently tested negative for BRRV by PCR. However, subsequent isolation of dsRNA by Dr. Robert Martin, USDA-ARS small fruits virologist in Corvallis, OR, strongly suggests presence of another virus. This new disorder, which has not been fully characterized but is distinct from BRRV, will be referred to as "necrotic ringblotch" henceforth. The disorder was more prevalent than BRRV, with 55.6% of all farms and 28.7% of all fields included in the survey showing symptoms. Among the surveyed cultivars, Star, Millennia, and Emerald had the highest BRRV prevalence, whereas Rebel and Windsor were BRRV-free. For necrotic ringblotch, Star, O'Neal, and FL 86-19 had the highest prevalence, whereas Emerald, Millennia, Rebel, and Windsor were disease-free. Objective 3: The main output of this objective thus far has been the development of statistical algorithms to analyze three-dimensional spatial patterns (of digitized data points corresponding to individual brown rot symptoms or collected pathogen isolates) within tree canopies. These algorithms are based on an extension of Diggle's G and Ripley's K statistics and should be applicable to other problems where spatial patterns need to be quantified in three dimensions. Objective 4: In darkness, constant temperatures up to 40C did not affect OTC degradation. In contrast, OTC residue decreased rapidly in natural sunlight, declining, on average, by 43.8, 77.8, and 92.1% within 1, 2, and 4 days after application, respectively; 7 days after application, OTC levels were near the detection limit. Use of shade fabric with 25 and 75% sunlight transmittance, simulating overcast sky, reduced OTC degradation significantly. UV protectants were ineffective in improving OTC persistence in outdoor conditions. Simulated rainfall at 44 mm/h drastically lowered OTC residue after 2 min, and levels were near the detection limited after 60 min. OTC is highly photosensitive and possesses a low retention capacity on the leaf surface.
- Scherm, H., and Krewer, G. 2008. Disease management in organic rabbiteye blueberries. International Journal of Fruit Science 8:69-80.
- Plattner, K., Fonsah, E.G., Escalante, C., Krewer, G., Scherm, H., Andersen, P.C., Liburd, O., and Tertuliano, M. 2008. Economics of organic blueberry establishment in Georgia. Journal of Food Distribution Research 39:111-115.
- Amiri, A., Scherm, H., Brannen, P. M., and Schnabel, G. 2008. Laboratory evaluation of three rapid, agar-based assays to assess fungicide sensitivity in Monilinia fructicola. Plant Disease 92:415-420.
- Holb, I.J., and Scherm, H. 2008. Quantitative relationships between different injury factors and development of brown rot caused by Monilinia fructigena in integrated and organic apple orchards. Phytopathology 98:79-86.
- Scherm, H., Savelle, A. T., Brannen, P. M., and Krewer, G. 2008. Occurrence and prevalence of foliar diseases on blueberry in Georgia. Plant Health Progress doi:10.1094/PHP-2008-0421-01-RS.
- Scherm, H., Savelle, A. T., Krewer, G., Tertuliano, M., and Clark, J. R. 2008. Control of Septoria leaf spot of blueberry with biofungicides, 2007. Plant Disease Management Reports 2:STF037.