Progress 06/01/15 to 11/30/18
Outputs
Target Audience: Blueberry producers and shippers in the southeastern United States Cooperative Extension agents in blueberry-producing counties in the Southeast Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?A PhD student at the University of Georgia has worked on these objectives and was trained as a direct result of the project. The student graduated in 2018 with his dissertation titled "Symptomology, epidemiology, and real-time PCR-based detection of Exobasidium leaf and fruit spot of blueberry caused by Exobasidium maculosum". How have the results been disseminated to communities of interest?Presentations 2015 Southern Association of Agricultural Scientists Horticultural Sciences Meeting, Atlanta, GA (30 Jan - 3 Feb). Presented "New blueberry disease issues in Georgia" in the blueberry session. 2015 Bladen County (NC) Blueberry Production meeting (17 Feb). Presented "Blueberry disease update and outlook for the 2015 season." (85 attendees). 2015 Florida Spring Blueberry Short Course; Tampa, FL (17 Feb). Presented "New disease issues in Georgia that may impact Florida growers." 2015 Berry Marketing and Production Conference; Virginia State University, Petersburg, VA, (13 Mar). Presented "Blueberry and blackberry disease management." 2015 North Carolina Blueberry Field Day, Castle Hayne, NC (24 May). Presented "Identification of mummy berry, stunt, stem canker, blueberry red ring spot, Exobasidium and other diseases." 2015 Southeastern Professional Fruitworkers Conference, Montgomery, AL (6-8 Oct). Presented "Management of Exobasidium leaf and fruit spot of blueberry." 2015 Alabama Fruit and Vegetable Association Conference, Clanton, AL (19-20 Nov). Presented "Emerging disease-management issues in fruit production." Brannen, P., Scherm, H., Allen, R. 2016. Management of Exobasidium leaf and fruit spot disease of blueberry in the southeastern United States. Page 45 in: Abstracts of the XI International Vaccinium Symposium, Orlando, FL, 10-14 April 2016. IFAS, University of Florida, Gainesville. Brannen, P.M. 2016. Southeastern Regional Blueberry Conference; Savannah, GA (8-9 Jan). Presented "Georgia blueberry pathology update." Brannen, P.M. 2016 IPM App Meeting; Clemson, SC (11 Oct). (Blueberry Section Organizer) Exobasidium added to app. App was published in 2016. Brannen, P.M. 2018. The current status of disease management challenges for the southeastern blueberry industry. Southeast Regional Fruit & Vegetable Conference, Savannah, GA (11-14 Jan.). Cline, W.O. and Bloodworth, B.K. 2016. Update on Exobasidium leaf and fruit spot NC. Blueberry Council Annual Open House, Fayetteville, NC, 01/12/2016 Cline, W.O. and Bloodworth, B.K. 2016. Blueberry disease update and crop outlook for 2016 Bladen County Blueberry Meeting, Elizabethtown, NC, 02/16/2016 Cline, W.O. 2016. Commercial Blueberry Production. Foothills Specialty Crop Grower's Round Table. Pilot Mountain, NC, 03/04/2016 Cline, W.O. 2017. Blueberry disease update and observations in North Carolina. Great Lakes Fruit and Vegetable Expo, Grand Rapids, MI (6 Dec.). Cline, W.O. 2018. Blueberry diseases. NC Blueberry Council 52nd Annual Open House, Fayetteville, NC (10-11 Jan.). Cline, W.O. 2018. Blueberry disease update. Michigan Blueberry Growers Spring Horticulture Day, Hudsonville, MI (14 Mar.). Ingram, R., Scherm, H., and Allen, R. 2016. Symptomology and epidemiology of Exobasidium leaf and fruit spot of blueberry. Page 173 in: Abstracts of the XI International Vaccinium Symposium, Orlando, FL, 10-14 April 2016. IFAS, University of Florida, Gainesville. Ingram, R., and Scherm, H. 2016. Symptomatology and epidemiology of Exobasidium leaf and fruit spot of blueberry. (Abstr.) Phytopathology 106:S4.142. Ingram, R., Allen, R., and Scherm, H. 2018. Impacts of late-dormant applications on epiphytic levels ofExobasidium maculosum, causal agent of Exobasidium leaf and fruit spot of blueberry. Southeast Regional Fruit & Vegetable Conference, Savannah, GA (11-14 Jan.). Trainings: County agent trainings 2016 SRSFC Agent Training for Blueberries, Blackberries, and Muscadines, Shelby, NC (12-15 Sept.) (35 attendees) 2017 Blueberry Nutrition, Disease, and Farm Management Practices Training, Alma, GA (19 Jan.) Producer trainings 2015 Bacon County (GA) Blueberry Meeting, Alma, GA (7 Jan.) (150 attendees) 2015 Clinch County (GA) Blueberry Disease Update, Homerville, GA (9 Nov.) (55 attendees) 2016 Bacon County Blueberry Meeting, Alma, GA (6 Jan.) (150 attendees) 2016 Ware County Blueberry Meeting, Manor, GA (28 Oct.) (75 attendees) 2017 Bacon County Blueberry Meeting, Alma, GA (4 Jan.) What do you plan to do during the next reporting period to accomplish the goals?The project has been completed. Future research will employ a population genomics approach to shed light on the cause(s) for the sudden emergence of Exobasidium leaf and fruit spot. Additionally, work is currently underway to determine the potential contribution of arthropods to the dissemination of primary inoculum in late winter/ early spring.
Impacts
What was accomplished under these goals?
Objective 1: Results of multi-year field trials (2014-2016) involving tissue plating on semi-selective medium consistently showed that E. maculosum overwinters on the surface of dormant blueberry tissues such as bark, leaf buds, and flower buds. Green bark was shown to produce the most consistent isolation results during the dormant season (typically at or above 1,000 colony-forming unites per g of tissue), indicating that pathogen monitoring studies should focus on that tissue. Additional data collected in winter and spring of 2017 documented that epiphytic pathogen population densities on green bark in March (the presumed infection window) correlate significantly with subsequent disease development. Thus, the critical role of E. maculosum surface inoculum in initiating primary infection in the Exobasidium leaf and fruit spot pathosystem has been documented. These findings also provide a mechanistic explanation for the high efficacy of dormant-season lime sulfur applications against Exobasidium leaf and fruit spot. Objective 2: Based on disease monitoring in the field, the use of spore trapping, and the weekly exposure of trap plants during the 2015 and 2016 growing seasons we showed that both leaf and fruit infection are initiated prior to the presence of sporulating leaf lesions producing basidiospores and prior to the detection of airborne basidiospores with an Andersen spore sampler. Thus, it was concluded that initial infection of leaves and fruit is not initiated by basidiospores; instead, primary infection in the early spring is most likely due to splash dispersal of epiphytic yeast-conidia of the pathogen. Disease progress curves strongly suggested that disease progression on leaves is monocyclic. The trap plant studies showed that young leaves at the mouse-ear stage were most susceptible to infection and that leaf infection periods were associated with rainfall variables such as the number of days per week with ≥1.0 mm of rain or cumulative weekly rainfall. The first symptoms on young, green fruit were observed soon after petal fall (requiring removal of the waxy fruit layer to visualize lesions), and visible disease progress on fruit was delayed by 1 to 3 weeks relative to that on leaves. Fruit infection of field plants and trap plants occurred before airborne propagules were detected by spore-trapping and before sporulating leaf lesions were present in the field. Collectively, these studies showed that fruit infections are initiated by the same initial inoculum as leaf infections, but it was not possible to conclusively exclude the possibility of a contribution of basidiospore inoculum from leaf lesions to disease progress on later developing fruit. Objective 3: Given the epiphytic nature of E. maculosum on blueberry tissues during the fall and winter, it is not surprising that pest management practices applied during these periods may impact surface population of the pathogen as well as subsequent disease development. Field trials were initiated in two 'Premier' rabbiteye blueberry plantings during fall of 2015 to determine the added value of fall or dormant-season applications of agrichemicals in reducing oversummering or overwintering inoculum, respectively, of E. maculosum. Fall fungicide applications consisted of two sprays (in September and October) of Captan or lime sulfur, both of which are known to be effective against Exobasidium leaf and fruit spot when applied during the period when the epidemic is active in the spring. The rationale behind these two treatments was to determine whether typical fungicide sprays for fungal leaf spot control (such as Septoria or anthracnose) can add value against Exobasidium by suppressing oversummering yeast cells. Another (single) fall treatment consisted of late-August application of stylet oil, mimicking a schedule for bud mite control. The rationale here was to determine whether oil coverage of the plant during the oversummering phase can suppress E. maculosum cells on the plant surface. Subsequent dormant-season applications (late January 2016) consisted of a single application of lime sulfur, which has been shown to control the disease effectively in previous trials; this served as a standard treatment for comparison. Other dormant applications utilized Dormex (hydrogen cyanamide), which is used commercially to induce bud break and may have activity against overwintering surface inoculum of E. maculosum due to its caustic activity; and of dormant oil, which is used commercially against scale insects but may suppress overwintering cells of the pathogen through oil coverage of the plant surface. Sufficient disease for treatment evaluation developed subsequently in one of the two trials. Fall applications of stylet oil and Captan had no effect on leaf or fruit spot severity in this trial. In contrast, fall applications of lime sulfur, the dormant application of Dormex, and the dormant application of lime sulfur had the lowest leaf and fruit spot levels, significantly lower than the untreated control and statistically equivalent to each other. Interestingly, leaf and fruit spot severity in the late dormant superior oil treatment was twice as high as in the untreated check. The finding of decreased Exdobasidium levels following a dormant hydrogen cyanamide application, and of increased disease levels following a dormant oil application, is novel. These results may help explain the variability in Exobasidium risk observed among blueberry plantings that otherwise receive similar disease management programs. The beneficial effect of hydrogen cyanamide against Exobasidium leaf and fruit spot is likely due to the compound's caustic activity, which may kill overwintering surface inoculum of E. maculosum upon contact. Despite the beneficial activity of hydrogen cyanamide, forgoing conventional fungicide applications against the disease and relying on hydrogen cyanamide alone would seem too risky at this time. The significantly increased levels of leaf and fruit spot following dormant oil application are of concern. The oil may act by protecting overwintering surface inoculum of E. maculosum from desiccation. In general, dormant oil is an important component of integrated pest management, and we are not suggesting based on these new findings that growers discontinue this practice. However, it seems critical that dormant oil be followed by an effective fungicide program against Exobasidium leaf and fruit spot, in line with what is currently recommended in the Southeast Regional Blueberry Integrated Management Guide. In 2018, an additional field trial on two rabbiteye blueberry cultivars was conducted by Oliver et al. to determine whether lime sulfur applied either before or after dormant oil could negate the negative effects of the oil in terms of exacerbating the disease. Hence, two dormant applications were made 2 weeks apart on 19 January and 1 February 2018, and the resulting leaf and fruit disease incidence was assessed in late May and early June, respectively. In both cultivars, dormant oil alone had the highest disease levels (in some cases significantly higher than the untreated control), whereas lime sulfur alone always had disease levels significantly lower than the control. Of relevance to growers likely to be applying both oil and lime sulfur in the late dormant period, applications of the two products 2 weeks apart (in either order) resulted in weaker Exobasidium leaf and fruit spot control than lime sulfur alone, with disease levels not being significantly different from the untreated control in both cultivars. Thus, lime sulfur applied either before or after dormant oil does not negate the negative effects of the oil on exacerbating Exobasidium leaf and fruit spot.
Publications
- Type:
Other
Status:
Published
Year Published:
2015
Citation:
Brannen, P.M. 2015. A review of blueberry disease issues from 2015 and implications for 2016. Dixie Blueberry News 15(5):5-9.
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2015
Citation:
Cline, W. O., Bloodworth, B. K., Brannen, P. M., and Mainland, C. M. 2015. Exobasidium update and control. NC Blueberry Council, Proceedings of the 49th Annual Open House, pp 46-51.
- Type:
Journal Articles
Status:
Published
Year Published:
2019
Citation:
Ingram, R.J., Ludwig, H.D., and Scherm, H. 2019. Epidemiology of Exobasidium leaf and fruit spot of rabbiteye blueberry: Pathogen overwintering, primary infection and disease progression on leaves and fruit. Plant Disease 103 https://doi.org/10.1094/PDIS-09-18-1534-RE
|
Progress 06/01/17 to 05/31/18
Outputs
Target Audience: Blueberry producers and shippers in the southeastern United States Cooperative Extension agents in blueberry-producing counties in the Southeast Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?A PhD student at the University of Georgia has worked on these objectives and was trained as a direct result of the project. The student is graduating in summer semester 2018 with his dissertation titled "Symptomology, epidemiology, and real-time PCR-based detection of Exobasidium leaf and fruit spot of blueberry caused by Exobasidium maculosum". How have the results been disseminated to communities of interest?Primarily via presentations at local and regional producer meetings in Georgia, North Carolina, and at the national level. An Extension Circular about the biology and management of Exobasidium leaf and fruit spot is currently in press and will be reported during the next cyvle. Brannen, P.M. 2018. The current status of disease management challenges for the southeastern blueberry industry. Southeast Regional Fruit & Vegetable Conference, Savannah, GA (11-14 Jan.). Cline, W.O. 2017. Blueberry disease update and observations in North Carolina. Great Lakes Fruit and Vegetable Expo, Grand Rapids, MI (6 Dec.). Cline, W.O. 2018. Blueberry diseases. NC Blueberry Council 52nd Annual Open House, Fayetteville, NC (10-11 Jan.). Cline, W.O. 2018. Blueberry disease update. Michigan Blueberry Growers Spring Horticulture Day, Hudsonville, MI (14 Mar.). Ingram, R., Allen, R., and Scherm, H. 2018. Impacts of late-dormant applications on epiphytic levels ofExobasidium maculosum, causal agent of Exobasidium leaf and fruit spot of blueberry. Southeast Regional Fruit & Vegetable Conference, Savannah, GA (11-14 Jan.). What do you plan to do during the next reporting period to accomplish the goals?The project will be completed by summarizing and analyzing the remaining data and by completing reports and peer-reviewed publications. We will continue using grower meetings, agent trainings, and industry newsletter to communicate the results with stakeholders.
Impacts
What was accomplished under these goals?
Work on Objective 1 has been completed. Results of multi-year field trials consistently showed that E. maculosum overwinters on the surface of dormant blueberry tissues such as bark, leaf buds, and flower buds. Green bark was shown to produce the most consistent isolation results during the dormant season, indicating that pathogen monitoring studies should focus on that tissue. Data collected in winter and spring of 2017 and reported previously documented that epiphytic pathogen population densities on green bark in March (the presumed infection window) correlate significantly with subsequent disease development. Thus, the critical role of E. maculosum surface inoculum in initiating primary infection in the Exobasidium leaf and fruit spot pathosystem has been documented. These findings also provide a mechanistic explanation for the high efficacy of dormant-season lime sulfur applications against Exobasidium leaf and fruit spot. Most of the studies associated with objective 2 have been completed as well. Based on disease monitoring in the field, the use of spore trapping, and the exposure of trap plants we showed that both leaf and fruit infection are initiated prior to the presence of sporulating leaf lesions producing basidiospores and prior to the detection of airborne basidiospores with an Andersen spore sampler. Thus, it was concluded that initial infection of leaves and fruit is not initiated by basidiospores; instead, primary infection in the early spring is most likely due to splash dispersal of epiphytic yeast-conidia of the pathogen. Disease progress curves strongly suggested that disease progression on leaves is monocyclic, whereas for fruit, the possibility of basidospores produced on leaf lesions causing some of the later fruit infections could not be excluded. To monitor dispersal patterns of basidiospores with greater temporal resolution, a real-time polymerase chain reaction (PCR)-based assay was developed to facilitate enumeration of E. maculosum from spore trap tapes collected with a Burkard 7-day volumetric spore sampler. The assay utilized primers EXORI-F and EXORI-R, targeting the ITS 5.8s region of the genome and resulting in a 188-bp amplicon, along with a fluorescent LNA probe EXORJI. Primer and probe design were based on the complete ITS region sequences from more than 60 isolates of E. maculosum deposited in the NCBI nucleotide database. Initial in silico testing of the primer set revealed that the only organisms with sequence complementarity >90% were within E. maculosum, and a BLAST search of the primers within the genus Exobasidium resulted in 100% coverage of all deposited E. maculosum isolates and no matches of other Exobasidium spp. Evaluation of the assay with DNA from 55 isolates of E. maculosum from cultivated or wild blueberry hosts, one isolate each of E. rostrupii and E. ferrugineae, and isolates of 14 other fungal species commonly found on blueberry or in the environment showed that the assay reliably detected 51 of the 55 E. maculosum isolates, whereby one of the four negatives was an isolate from Nova Scotia, Canada, that was previously shown to belong to a distinct genetic sub-population within E. maculosum. None of the 14 non-Exobasidium taxa produced a positive reaction. When the assay was evaluated with spore trap tape segments to which yeast conidial suspensions of E. maculosum had been added and from which DNA had been extracted by beat-beating, tape segments harboring ca. 100 cells or higher could be quantified reliably. Analysis of spore trap tapes collected in a rabbiteye blueberry planting during an epidemic of Exobasidium leaf and fruit spot between late February and early June 2016 by real-time PCR indicated patterns of basidiospore disseminations consistent with disease observations in the field and with semi-quantitative spore enumeration by an Andersen sampler at the same time. Objective 3: Given the epiphytic nature of E. maculosum on blueberry tissues during the fall and winter, it is not surprising that pest management practices applied during these periods may impact surface population of the pathogen as well as subsequent disease development. As reported previously, in multi-year field trials we showed that late dormant applications of lime sulfur and dormant oil, respectively, reduce and increase pathogen populations and the incidence of Exobasidium leaf and fruit spot. A field trial on two rabbiteye blueberry cultivars was conducted in 2018 by Oliver et al. to determine whether lime sulfur applied either before or after dormant oil could negate the negative effects of the oil in terms of exacerbating the disease. Hence, two dormant applications were made 2 weeks apart on 19 January and 1 February 2018, and the resulting leaf and fruit disease incidence was assessed in late May and early June, respectively. In both cultivars, dormant oil alone had the highest disease levels (in some cases significantly higher than the untreated control), whereas lime sulfur alone always had disease levels significantly lower than the control. Of relevance to growers likely to be applying both oil and lime sulfur in the late dormant period, applications of the two products 2 weeks apart (in either order) resulted in weaker Exobasidium leaf and fruit spot control than lime sulfur alone, with disease levels not being significantly different from the untreated control in both cultivars. Thus, lime sulfur applied either before or after dormant oil does not negate the negative effects of the oil on exacerbating Exobasidium leaf and fruit spot.
Publications
- Type:
Journal Articles
Status:
Published
Year Published:
2017
Citation:
Brannen, P., Scherm, H., and Allen, R.M. 2017. Management of Exobasidium leaf and fruit spot of blueberry. Acta Hortic. 1180:215-220.
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2017
Citation:
Cline, W.O., and Bloodworth, B.K. 2017. Exobasidium leaf and fruit spot and other diseases. Proceedings of the 50th Annual NC Blueberry Meeting, pages 2-10.
- Type:
Journal Articles
Status:
Published
Year Published:
2017
Citation:
Ingram, R.J., Allen, R.M., and Scherm, H. 2017. Symptomology and epidemiology of Exobasidium leaf and fruit spot of blueberry. Acta Hortic. 1180:205-213.
|
Progress 06/01/16 to 05/31/17
Outputs
Target Audience: Blueberry producers and shippers in the southeastern United States Cooperative Extension agents in blueberry-producing counties in the Southeast Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?One PhD student at the University of Georgia is working on these objectives and is trained as a direct result of the project. How have the results been disseminated to communities of interest?Presentations Brannen, P.M. 2016. Leaf scorch, caused by the bacterium Xylella fastidiosa, and other diseases of potential importance to maturing blueberry systems in Mexico. VI Simposio Nacional y V Internacional de Bacterias Fitopatógenas y II Simposio de bacterias benéficas de las plantas, Guadalajara, Jalisco, Mexico (21-24 Sep). Brannen, P.M. 2017. The current status of disease management for the southeastern blueberry industry. Southeast Regional Fruit and Vegetable Conference, Savannah, GA (6-7 Jan). Cline, W.O. and Bloodworth, B.K. 2017 Blueberry Disease Update. Bladen County Blueberry Meeting, 13 Feb 2017, Elizabethtown, NC. Cline, W.O. and Bloodworth, B.K. 2017 Current Trends in Blueberry Disease Management in North Carolina. NC Blueberry Open House, 11 January 2017, Fayetteville, NC. Scherm, H., Allen, R.M., Ingram, R.J., and Brannen, P.M. 2017. Non-target effects of horticultural and pest management practices in fall and winter on development of Exobasidium leaf and fruit spot of blueberry. American Phytopathological Society annual meeting, San Antonio, TX (5-10 Aug). Scherm, H., and Ingram, R.J. 2017. Recent discoveries about the biology of Exobasidium and how they impact disease management. Southeast Regional Fruit and Vegetable Conference, Savannah, GA (6-7 Jan). County agent trainings 1/19/17 Blueberry Nutrition, Disease, and Farm Management Practices Training (Alma, GA) Producer trainings 1/4/17 Bacon County Blueberry Meeting (Alma, GA) What do you plan to do during the next reporting period to accomplish the goals?The focus next year will be on Objectives 2 and 3. A qPCR method for quantifying E. maculosum propagules on spore trap tapes is currently being developed to aid in quantifying inoculum levels in the air. This methodology will be applied to spore trap data collected during the previous two growing seasons to confirm that the initial inoculum is not airborne. The field study on the effects of dormant lime sulfur, hydrogen cyanamide, and oil on epiphytic populations of E. maculosum and subsequent disease levels will be repeated. The graduate student working on the project will complete his PhD dissertation and submit peer-reviewed journal papers for publication. We will continue using grower meetings, agent trainings, and industry newsletter to communicate the results with stakeholders.
Impacts
What was accomplished under these goals?
Work on Objective 1 has been completed. Dormant tissue washes from two overwintering seasons (2014/15 and 2015/16) documented consistent presence of E. maculosum surface inoculum on leaf buds, flower buds, green bark, and lignified bark in the field. In both 2016 and 2017, potted trap plants at a susceptible leaf development stage placed at weekly intervals into the experimental field in early spring became infected prior to the appearance of sporulating leaf lesions in the field, strongly suggesting that trap plant infection occurred from overwintered primary surface inoculum. Based on these findings, management recommendations against Exobasidium leaf and fruit spot (ELFS) now emphasize application of eradicant contact fungicides (such as liquid lime sulfur) during the dormant season. Under Objective 2, work this past year focused on replicating our findings from spring 2016 that small green fruit on weekly exposed trap plants become infected prior to the appearance of sporulating leaf lesions in the field, documenting that fruit infection also occurs via primary inoculum, as opposed to via inoculum produced on the leaves. Data analysis from 2017 is still in progress. In addition, spore trapping studies with a volumetric air sampler were conducted to document that both leaf and fruit infection occur prior to the appearance of basidiospores in the air above the field. A qPCR method for quantifying E. maculosum propagules on spore trap tapes is currently being developed to aid in quantifying inoculum levels in the air. We hypothesize that the volumetric spore sampler will only trap airborne basidiospores, but not splash-dispersed yeast cells (conidia). Objective 3: Previous disease management trials in 2014/15 and 2015/16 confirmed that a single dormant spray of lime sulfur, the standard for ELFS control, reduced leaf and fruit spot intensity considerably and significantly. Interestingly and unexpectedly, statistically equivalent control was achieved with a single dormant application of the growth regulator hydrogen cyanamide. Also unexpectedly, dormant oil increased leaf and fruit spot intensity considerably and significantly in these trials. An additional field trial was carried out in 2017 to elucidate the mechanism(s) behind the ELFS increase observed with dormant oil and the disease reduction following dormant application of calcium cyanamide or lime sulfur. The three compounds were applied on 18 January 2017 to six-plant plots replicated four times in an RCBD. Bark samples were collected from each plot 1 day before application and again 2, 8, and 59 days after application to determine E. maculosum epiphytic population densities by wash-plating on semi-selective medium. The final sampling date was at the time of shoot elongation and petal fall, when infection in the field would be expected to occur. Prior to application, epiphytic population densities of E. maculosum were low (averaging 8.0 X 10^3 CFU/g) and not significantly different among treatments. Following application, populations on plants treated with lime sulfur and hydrogen cyanamide declined, whereas those in the dormant oil treatment and the untreated check increased. At the final assessment date 59 days after application, populations were highest for dormant oil, lowest for lime sulfur and hydrogen cyanamide, and intermediate for untreated. During the week of 4 to 11 April 2017, potted trap plants were exposed to natural inoculum in three plots of the lime sulfur treatment and the untreated check. Following the exposure period, trap plants were incubated on a mist bench in a greenhouse for 7 days and then assessed at ambient conditions in the greenhouse for 3 weeks for development of leaf spots. Trap plants exposed in untreated check plots developed 28.7 spots per plant, whereas no spots were observed on trap plants in lime sulfur plots. Thus, the inoculum reduction associated with lime sulfur application effectively suppressed transmission of E. maculosum to trap plants in the field.
Publications
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2016
Citation:
Brannen, P.M. 2016. Leaf scorch, caused by the bacterium Xylella fastidiosa, and other diseases of potential importance to maturing blueberry systems in Mexico. VI Simposio Nacional y V Internacional de Bacterias Fitopat�genas y II Simposio de bacterias ben�ficas de las plantas. http://www.simposiumbacterias.mx/memories-list.php
- Type:
Websites
Status:
Published
Year Published:
2017
Citation:
Brannen, P.M., and Cline, W. 2017. MyIPM Blueberry App Pathology Section. Bugwood Apps, Center for Invasive Species and Ecosystem Health, University of Georgia, Tifton. https://apps.bugwood.org/apps/myipmseries/
- Type:
Book Chapters
Status:
Published
Year Published:
2017
Citation:
Burrack, H.J. and Cline, W.O. 2017. Blueberry Management Program. North Carolina Agricultural Chemicals Manual. CALS/NCSU, pgs. 197-201.
- Type:
Book Chapters
Status:
Published
Year Published:
2017
Citation:
Cline, W.O. and Brewer, M.T. 2017. Exobasidium leaf and fruit spot. Pages 22-23 in: Polashock, J.J., Caruso, F.L., Averill, A.L., Schilder, A.M.C. (eds.) Compendium of Blueberry, Cranberry and Lingonberry Diseases and Pests, 2nd Edition. APS Press, St. Paul, MN.
- Type:
Other
Status:
Published
Year Published:
2017
Citation:
Scherm, H., Allen, R., and Brannen, P.M. 2017. Dormant oil and hydrogen cyanamide impact development of Exobasidium leaf and fruit spot of blueberry. Mississippi Vaccinium Journal 6(2):8-11.
- Type:
Other
Status:
Published
Year Published:
2017
Citation:
Scherm, H., Allen, R., and Brannen, P. M. 2017. Dormant oil and hydrogen cyanamide impact development of Exobasidium leaf and fruit spot of blueberry. Dixie Blueberry News 17(2): 6-9.
|
Progress 06/01/15 to 05/31/16
Outputs
Target Audience: Blueberry producers and shippers in the southeastern United States Cooperative Extension agents in blueberry-producing counties in the Southeast Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?One PhD student at the University of Georgia is working on these objectives and is trained as a direct result of the project. How have the results been disseminated to communities of interest?Presentations: Brannen, P., Scherm, H., Allen, R. 2016. Management of Exobasidium leaf and fruit spot disease of blueberry in the southeastern United States. Page 45 in: Abstracts of the XI International Vaccinium Symposium, Orlando, FL, 10-14 April 2016. IFAS, University of Florida, Gainesville. Brannen, P.M. 2016. Georgia blueberry pathology update. Southeastern Regional Blueberry Conference; Savannah, GA (8-9 Jan). Cline, W.O. and Bloodworth, B.K. 2016. Update on Exobasidium leaf and fruit spot NC. Blueberry Council Annual Open House, Fayetteville, NC, 01/12/2016 Cline, W.O. and Bloodworth, B.K. 2016. Blueberry disease update and crop outlook for 2016 Bladen County Blueberry Meeting, Elizabethtown, NC, 02/16/2016 Cline, W.O. 2016. Commercial Blueberry Production. Foothills Specialty Crop Grower's Round Table. Pilot Mountain, NC, 03/04/2016 Ingram, R., Scherm, H., and Allen, R. 2016. Symptomology and epidemiology of Exobasidium leaf and fruit spot of blueberry. Page 173 in: Abstracts of the XI International Vaccinium Symposium, Orlando, FL, 10-14 April 2016. IFAS, University of Florida, Gainesville. Ingram, R., and Scherm, H. 2016. Symptomatology and epidemiology of Exobasidium leaf and fruit spot of blueberry. (Abstr.) Phytopathology 106:S4.142. Trainings: 9/12-9/15/16: SRSFC Agent Training for Blueberries, Blackberries, and Muscadines (Shelby, NC) - 35 attendees 10/28/16: Ware County Blueberry Meeting (Manor, GA) - 75 attendees 1/6/16: Bacon County Blueberry Meeting (Alma, GA) - 150 attendees What do you plan to do during the next reporting period to accomplish the goals? Continue disease monitoring and trap plant studies in the field to confirm srage of tissue susceptible and timing of leaf and fruit infection. Continue trials on non-target effects of horticulturaland pest management parctices on Exobasidium leaf and fruit spot intensity in the field. Via epiphytic population monitoring of E. maculosum, determine mechanism of trhese non-targed effects. Conduct further outreach to educate growers about the life cycle of Exbasidium and how knowledge of the life cycle impacts effectice disease management.
Impacts
What was accomplished under these goals?
Research conducted in 2015/16 generally confirmed results from the previous year. Dormant tissue washes from green bark, leaf buds, and flower buds between 4 February and 17 March 2016 documented consistent presence of E. maculosum surface inoculum. Population densities of the yeast phase of the pathogen were highest on green bark, ranging from 1.1 to 5.2 x 103 CFU/g. Potted trap plants at a susceptible leaf development stage placed at weekly intervals into the same field became infected during the week ending 18 March, strongly suggesting that the epiphytic surface inoculum caused primary infection. Leaf spots on plants in the field did not appear until 31 March and did not sporulate until 22 April, indicating that the trap plants must have been infected by primary (overwintering) inoculum and not from secondary inoculum from newly infected leaves in the field. Equally important, after sporulation on plants in the field commenced on 22 April, no more leaf infection on trap plants was observed, although leaves on trap plants were at a susceptible stage, suggesting that leaf infection is monocyclic. Small green fruit on trap plants became infected between 18 March and 8 April, before there were any sporulating leaf lesions on field plants. This indicates that fruit infection also occurs via primary inoculum, as opposed to via inoculum produced on the leaves. This is supported by the fact that no more trap plant fruit infection was observed after leaf lesions in the field started to sporulate on 22 April. Field trials were initiated in two 'Premier' rabbiteye blueberry planting during fall of 2015 to determine the added value of fall or dormant-season applications of agrichemicals in reducing oversummering or overwintering inoculum, respectively, of E. maculosum. Fall fungicide applications consisted of two sprays (in September and October) of Captan or lime sulfur, both of which are known to be effective against Exobasidium leaf and fruit spot when applied during the period when the epidemic is active in the spring. The rationale behind these two treatments was to determine whether typical fungicide sprays for fungal leaf spot control (such as Septoria or anthracnose) can add value against Exobasidium by suppressing oversummering yeast cells. Another (single) fall treatment consisted of late-August application of stylet oil, mimicking a schedule for bud mite control. The rationale here was to determine whether oil coverage of the plant during the oversummering phase can suppress E. maculosum cells on the plant surface. Subsequent dormant-season applications (late January 2016) consisted of a single application of lime sulfur, which has been shown to control the disease effectively in previous trials; this served as a standard treatment for comparison. Other dormant applications utilized Dormex (hydrogen cyanamide), which is used commercially to induce bud break and may have activity against overwintering surface inoculum of E. maculosum due to its caustic activity; and of dormant oil, which is used commercially against scale insects but may suppress overwintering cells of the pathogen through oil coverage of the plant surface. Sufficient disease for treatment evaluation developed subsequently in one of the two trials. Fall applications of stylet oil and Captan had no effect on leaf or fruit spot severity in this trial. In contrast, fall applications of lime sulfur, the dormant application of Dormex, and the dormant application of lime sulfur had the lowest leaf and fruit spot levels, significantly lower than the untreated control and statistically equivalent to each other. Interestingly, leaf and fruit spot severity in the late dormant superior oil treatment was twice as high as in the untreated check. The finding of decreased Exdobasidium levels following a dormant hydrogen cyanamide application, and of increased disease levels following a dormant oil application, is novel. These results may help explain the variability in Exobasidium risk observed among blueberry plantings that otherwise receive similar disease management programs. The beneficial effect of hydrogen cyanamide against Exobasidium leaf and fruit spot is likely due to the compound's caustic activity, which may kill overwintering surface inoculum of E. maculosum upon contact. Additional experiments are currently underway to test this hypothesis. Despite the beneficial activity of hydrogen cyanamide, forgoing conventional fungicide applications against the disease and relying on hydrogen cyanamide alone would seem too risky at this time. The significantly increased levels of leaf and fruit spot following dormant oil application are of concern. Further research is needed to determine whether this effect specific to the oil formulation used in this study, or whether it applies to other dormant oils as well. The oil may act by protecting overwintering surface inoculum of E. maculosum from desiccation, and this hypothesis is currently being tested experimentally in the field. In general, dormant oil is an important component of integrated pest management, and we are not suggesting based on these new findings that growers discontinue this practice. However, it seems critical that dormant oil be followed by an effective fungicide program against Exobasidium leaf and fruit spot, in line with what is currently recommended in the Southeast Regional Blueberry Integrated Management Guide. Currently we do not know the optimum time interval between a dormant oil application and a subsequent dormant spray of lime sulfur. Two fall applications of captan were not effective against Exobasidium, but when lime sulfur was used for these applications, both leaf spot severity and fruit spot incidence were reduced to a level equivalent to the dormant lime sulfur standard. Fall applications of fungicides in blueberry are commonly used to manage leaf spots and leaf rust; if it can be shown through additional research that lime sulfur applied during this application window controls leaf spots and/or rust, then the added benefit of reducing Exobasidium levels for the following year would be of considerable practical interest.
Publications
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2016
Citation:
Cline, W.O. and Bloodworth, B.K. 2016. Exobasidium leaf and fruit spot and other diseases. Proceedings of the 50th Annual Open House, NC Blueberry Council. January 12-13, 2016. Pages 2-10.
- Type:
Other
Status:
Published
Year Published:
2016
Citation:
Connelly, F. and Brannen, P.M. 2016. Mummy berry and Exobasidium leaf spot management in rabbiteye blueberry with chemical fungicides, 2015. Plant Disease Management Reports 10:SMF020.
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