Progress 07/01/02 to 06/30/08
Outputs
OUTPUTS: Armillaria root rot of peach: Fatty acid methyl ester (FAME) analysis was applied to cultures of Armillaria tabescens, A. mellea, and A. gallica isolated from stone fruits and other woody hosts in order to better characterize fatty acid composition of these species and to aid in species identification and discrimination. To explore the potential for biological control of Armillaria root rot of peach, isolates of A. tabescens and A. mellea were experimentally challenged with the wood-colonizing saprophytes Phanerochaete velutina, Hypholoma fasciculare, Schizophyllum commune, Ganoderma lucidum, and Xylaria hypoxylon on peach root segments in laboratory microcosms. A long-term field trial to evaluate the effects of rootstocks (Nemaguard and SL0040), fumigants (MeBr, MeI, and 1,3-D + C35), and the biocontrol agent Trichoderma harzianum T-22 on peach tree longevity was initiated on an Armillaria-infested site in spring of 2003. - Bacterial spot of peach: Work on this disease, caused by Xanthomonas arboricola pv. pruni (Xap), has focused on quantifying the role of different host tissues as overwintering niches for the pathogen and on identifying alternatives to the antibiotic oxytetracycline for disease control during the cover sprays. Xap-specific primers were incorporated into a BIO-PCR protocol to detect the pathogen in symptomless plant tissue. Tissue samples were collected from orchards with a history of the disease in fall after leaf drop, in winter during the dormant phase, and in early spring at bud swell. The samples included leaf scars, terminal buds, floral buds, vegetative buds, current-season bark, previous-growth bark, and cankers. Based on a previous report of significant synergism between copper (a commonly used bactericide) and allyl sulfur compounds from garlic against the model organism Saccharomyces cerevisiae, we investigated the effects of mixtures of copper with garlic extract, allyl disulfide, or diallyl disulfide against Xap in vitro and in the field. - Leaf spots of blueberry: A 2-year survey of fungal diseases and abiotic disorders on leaves of rabbiteye and southern highbush blueberry was completed to better characterize the foliar pathogen complex. Over a period of 3 years, temporal progress of Septoria leaf spot and the dynamics of disease-induced defoliation as well as associated reductions in flower bud set and return yield were quantified. Experimental units consisted of individual 20-cm shoot segments selected early in the season and monitored regularly for growth and leaf emergence (in the spring), number of Septoria leaf spot lesions on individual leaves (spring through fall), defoliation (summer and fall), flower bud set (in the winter), and return yield (in the following summer). Survival analysis was applied to quantify defoliation progress in relation to leaf spot severity. Rain-dispersed pycnidiospore inoculum was monitored with funnel traps to determine periods of inoculum availability. Field trials to evaluate several biofungicides for the control of foliar diseases were conducted to support the fledgling organic blueberry industry in the Southeast. PARTICIPANTS: Dr. Phillip M. Brannen, Fruit Pathologist (Extension), Univ. of Georgia; Dr. Thomas Beckman, USDA-ARS, Byron, GA; Ms. Amy T. Savelle, Research Professional, Univ. of Georgia; Mr. Jeff M. Cook, County Extension Coordinator, Taylor County, GA; Dr. Peter S. Ojiambo, former graduate student, Univ. of Georgia; Dr. Kerik D. Cox, former graduate student, Univ. of Georgia; Georgia Peach Council, Byron, GA; South Carolina Peach Council, Columbia, SC TARGET AUDIENCES: Southeastern peach producers; southeastern blueberry producers; southeastern extension personnel PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
Armillaria root rot of peach: The three Armillaria species could be distinguished from each other and from other lignicolous basidiomycetes on the basis of FAME profiles using stepwise discriminant analysis. In laboratory biocontrol studies, the saprophytic antagonists reduced both external and internal colonization of roots by Armillaria, whereby G. lucidum, S. commune, and X. hypoxylon had the most pronounced effects. Reduction of colonization underneath the bark is particularly important as this occurs in the pathogen's primary niche where it colonizes the root system and gains access to the sapwood. The rootstock-fumigation-biocontrol field trial against Armillaria root rot is still ongoing. As expected based on the slow development of the disease, tree losses to date have been too low to assess treatment efficacy. We expect mortality to increase rapidly over the next few years, and assessments will continue beyond the life of this project. - Bacterial spot of peach: BIO-PCR testing of overwintering niches of Xap indicated that the incidence of positive detections decreased during the winter, and that it was greatest in cankers and vegetative buds and lowest in bark and floral buds. Analysis of inhibition of Xap exposed to mixtures of copper with garlic extract, allyl disulfide, or diallyl disulfide showed significant synergistic activity for virtually all concentrations tested. This suggests potential for better disease control with copper-allyl sulfur mixtures than with copper alone. - Leaf spots of blueberry: Disease surveys showed that the foliar disease complex on blueberry is more diverse than previously appreciated. Septoria and Gloeosporium leaf spots were most prevalent and most severe, followed by Pestalotia leaf spot, Phyllosticta leaf spot, and leaf rust. More than 70% of samples, especially from rabbiteye cultivars, showed symptoms of non-pathogenic leaf damage. Among eight cultivars, Powderblue was the least affected whereas Star had high foliar disease levels. Epidemiological studies showed that onset of Septoria leaf spot occurred between late April and mid-June, and average disease severity increased sigmoidally until late September, after which it declined due to the abscission of severely affected leaves. Pycnidiospore inoculum was present and leaves became infected season-long. Disease severity, defoliation, flower bud set, and return yield were interrelated: severely affected leaves abscised earlier in the fall than those with low disease severity, and shoots with severely diseased leaves and/or high levels of defoliation had reduced flower bud set and low return yields. The results form the basis for identifying disease levels that can be tolerated during specific periods of crop development without negatively impacting flower bud set and return yield. The trials on foliar disease management in organic blueberries with biofungicides showed that fish emulsion-based products provided better suppression of Septoria leaf spot than the current organic standard. The potential added value of these products as insect repellents and foliar fertilizers is being explored.
Publications
- 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.
- Thornton, H. A., Savelle, A. T., and Scherm, H. 2008. Evaluating a diverse panel of biocontrol agents against infection of blueberry flowers by Monilinia vaccinii-corymbosi. Biocontrol Science and Technology 18:391-407.
- Tarnowski, T. L. B., Savelle, A. T., and Scherm, H. 2008. Activity of fungicides against Monilinia vaccinii-corymbosi in blueberry flowers treated at different phenological stages. Plant Disease 92:961-965.
- Haralson, J. C., Brannen, P. M., Scherm, H., and NeSmith, D. S. 2008. Evaluation of fungicides for the control of Cylindrocladium root rot in blueberry cuttings, 2007. Plant Disease Management Reports 2:SMF038.
|
Progress 01/01/07 to 12/31/07
Outputs
Peach - Because of nearly complete crop loss in the field due to a late spring freeze, we focused our peach bacterial spot work in 2007 on laboratory research. Based on a previous report of significant synergism between copper (a commonly used bactericide) and allyl sulfur compounds from garlic against the model organism Saccharomyces cerevisiae, we investigated the effects of mixtures of copper with garlic extract, allyl disulfide, or diallyl disulfide against the bacterial spot organism Xanthomonas arboricola pv. pruni in vitro. The compounds were mixed at different concentrations, applied to suspensions of the bacterium, and the degree of synergism was calculated using the Gowing equation or the killing curve method. Blueberry - Our blueberry research this past year has focused on organic disease management in support of the small but rapidly expanding organic blueberry industry in Georgia. One trial was carried out in the spring against mummy berry disease and
three in the summer and fall against foliar diseases (primarily Septoria leaf spot). All were on-farm trials conducted in collaboration with blueberry producers and their county extension agents. No data was collected from the mummy berry trial because of complete crop loss due to a late spring freeze. Results of the foliar disease trials were shared with producers at the Southeast Fruit and Vegetable Conference in Savannah, GA, and at the Blueberry Open House in Clinton, NC.
Impacts
Peach - Analysis of inhibition of X. arboricola pv. pruni exposed to mixtures of copper with garlic extract, allyl disulfide, or diallyl disulfide showed significant synergistic activity for virtually all concentrations tested. Thus, the antibacterial activity of the mixture was greater than the sum of its parts. This suggests potential for better control of bacterial spot with copper-allyl sulfur mixtures than with copper alone. The efficacy of these mixtures in the greenhouse and field will be evaluated in 2008. Blueberry - Results of the trials on foliar disease management in blueberries with biofungicides showed that two fish oil-based products provided better suppression of Septoria leaf spot than the current organic standard, a Bacillus subtilis-based product. The fish oils also performed better than another commercial biofungicide containing Bacillus pumilus. One of the trials was conducted on a certified organic farm, and the producer has adopted a fish
oil-based spray schedule against foliar diseases based on these results. Further research to evaluate fish oils and emulsions for blueberry disease control are warranted, especially in light of the potential added value of these products as foliar fertilizers.
Publications
- 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:SMF037.
- Scherm, H., Savelle, A. T., Boozer, R. T., and Foshee, W. G. 2008. Seasonal dynamics of conidial production potential of Fusicladium carpophilum in southeastern peach orchards. Plant Disease 92:47-50.
- Ojiambo, P.S., Scherm, H. and Brannen, P.M. 2007. Temporal dynamics of Septoria leaf spot of blueberry and its relationship to defoliation and yield. Online. Plant Health Progress doi:10.1094/PHP-2007-0726-05-RS.
- Scherm, H., Savelle, A.T., and Law, S.E. 2007. Effect of electrostatic spray parameters on the viability of two bacterial biocontrol agents and their deposition on blueberry flower stigmas. Biocontrol Science and Technology 17:285-293.
|
Progress 01/01/06 to 12/31/06
Outputs
I. Peach - Armillaria root rot: The laboratory research to evaluate the genetic diversity of Armillaria isolates, to develop a fatty acid-based species identification method, and to evaluate saprobic lignicolous fungi as competitive antagonists to prevent colonization of peach roots by Armillaria have been completed and the results published. The long-term field trial to evaluate the effects of rootstocks (Nemaguard and SL0040), fumigants, and the biocontrol agent Trichoderma harzianum T-22 on peach tree longevity (established on an Armillaria-infested site in spring of 2003) is still ongoing. As expected based on the slow development of Armillaria root rot, tree losses to date have been too low to assess treatment efficacy. We expect Armillaria-related mortality to increase gradually over the next 3 to 5 years. II. Peach - bacterial spot: Work on this disease, caused by Xanthomonas arboricola pv. pruni (Xap), has focused on quantifying the role of different host
tissues as overwintering niches for the pathogen and on identifying alternatives to the antibiotic oxytetracycline (Mycoshield) for disease control during the cover sprays. Xap-specific primers have been incorporated into a BIO-PCR protocol for detecting the pathogen in symptomless host tissue. Host tissue samples were collected from orchards with a history of the disease in fall after leaf drop, in winter during the dormant phase, and in early spring at bud swell. The samples included leaf scars, terminal buds, floral buds, vegetative buds, current-season bark, previous-growth bark, and cankers when present. PCR results indicated that the incidence of positive detections decreased during the winter, and that it was greatest in cankers and vegetative buds and lowest in bark and floral buds. III. Blueberry - leaf spots: Research on the epidemiology of Septoria leaf spot on rabbiteye blueberry has been completed and the results have been published. Based on the data collected during
this multi-year study, a hierarchical sampling plan for optimal assessment of the disease was developed. Variance components were calculated for disease severity to partition total variation into variation among leaves per shoot, shoots per bush, and bushes within the field. In all cases, leaves per shoot and shoots per bush accounted for >90% of the total variation. Based on the variance components and linear cost functions (which considered the time required to assess each leaf and select new shoots and bushes for assessment), the optimum sample size for assessing disease severity as number of spots per leaf (with an allowable variation of 20% around the mean) was 75 leaves, 1 each selected from 3 shoots per bush on 25 bushes. For disease severity expressed as percent necrotic leaf area, the corresponding values were 144 leaves, 2 each sampled from 3 shoots per bush on 24 bushes. With an allowable variation of 10% around the mean, a sample of 27 shoots from 9 bushes was the optimum
sample size for assessing defoliation.
Impacts
Peach and blueberry are the first and second most important fruit crops in the southeastern United States, and the economic viability of both industries is threatened by emerging diseases. In peach, Armillaria root rot has developed into the leading cause of premature tree death, and bacterial spot is the foliar and fruit disease that is most difficult to control. Since widely planted peach cultivars lack host resistance to the two diseases and since chemical management tools are unavailable (Armillaria) or highly problematic (bacterial spot), alternative management options need to be developed. In blueberry, foliar diseases have become prevalent as production intensity has increased. Foremost among these is Septoria leaf spot, which leads to premature defoliation and associated reductions in flower bud set, return bloom, and yield. This project developed threshold-based decision aids for managing the disease utilizing basic epidemiological information in conjunction
with quantitative yield loss data.
Publications
- Cline, W.O., Brannen, P.M., and Scherm, H. 2006. Blueberry disease management in the southeastern United States. Acta Horticulturae 715:489-492.
- Cox, K.D., Scherm, H., and Riley, M.B. 2006. Characterization of Armillaria spp. from peach orchards in the southeastern United States using fatty acid methyl ester profiling. Mycological Research 110:414-422.
- Cox, K.D., and Scherm, H. 2006. Interaction dynamics between saprobic lignicolous fungi and Armillaria in controlled environments: Exploring the potential for competitive exclusion of Armillaria on peach. Biological Control 37:291-300.
- Ojiambo, P.S., and Scherm, H. 2006. Optimum sample size for determining disease severity and defoliation associated with Septoria leaf spot of blueberry. Plant Disease 90:1209-1213.
- Scherm, H., Savelle, A.T., Cook, M.J. IV, Reilly, C.C., and Hotchkiss, M.W. 2006. Evaluation of alternatives to oxytetracycline for control of bacterial spot of peach during the cover sprays, 2005. Fungicide and Nematicide Tests 61:STF10.
|
Progress 01/01/05 to 12/31/05
Outputs
1) Peach - Armillaria root rot: The laboratory research to evaluate the genetic diversity of Armillaria isolates, to develop a fatty acid-based species identification method, and to evaluate saprobic lignicolous fungi as competitive antagonists to prevent colonization of peach roots by Armillaria have been completed and the results published. The long-term field trial to evaluate the effects of rootstocks (Nemaguard and SL0040), fumigants, and the biocontrol agent Trichoderma harzianum T-22 on peach tree longevity (established on an Armillaria-infested site in spring of 2003) is still ongoing. Tree losses to date have been minimal and unrelated to Armillaria root rot. We expect Armillaria-related mortality in this long-term trial to commence shortly and increase gradually over the next 3 to 5 years in the check treatments. 2) Peach - bacterial spot: Work on this emerging disease, caused by Xanthomonas arboricola pv. pruni (Xap), has focused on quantifying the role of
different host tissues as overwintering niches for the pathogen and on identifying alternatives to the antibiotic oxytetracycline for disease control during the cover sprays. Xap-specific primers have been incorporated into a BIO-PCR protocol for detecting the pathogen in symptom-less host tissue. Host tissue samples were collected from two peach orchards with a history of the disease in fall after leaf drop, in winter during the dormant phase, and in early spring at bud swell. The samples included leaf scars, terminal buds, floral buds, vegetative buds, current-season bark, previous-growth bark, and cankers when present. Samples were extracted and subjected to enrichment as well as extraction and storage of bacterial DNA. Testing by PCR for presence and incidence of the pathogen has not yet been completed. 3) Blueberry - leaf spots: Research on the epidemiology of Septoria leaf spot on rabbiteye blueberry has been completed and the results have been published. Disease onset generally
occurred between late April and mid-June, and average disease severity increased sigmoidally until late September, after which it declined due to the abscission of severely affected leaves. Disease severity was highest on early-emerging leaves and on those located on shoots closer to the ground. Pycnidiospore inoculum was present throughout the season, and leaves became infected season-long. Disease severity, defoliation, flower bud set, and return yield were interrelated: severely affected leaves abscised earlier in the fall than those with low disease severity, and shoots with severely diseased leaves and/or high levels of defoliation had reduced flower bud set. Furthermore, such shoots consistently had low return yields the following year. Disease management trials carried out in collaboration with Dr. P. M. Brannen showed that Septoria leaf spot can be controlled with after-harvest applications of dipotassium phosphite at a fraction of the cost of conventional fungicides. In
addition, the use of these materials will help to extend the life of the QoI fungicides through resistance management.
Impacts
Peach and blueberry are the first and second most important fruit crops in the southeastern United States, and the economic viability of both industries is threatened by emerging diseases. In peach, Armillaria root rot has developed into the leading cause of premature tree death, and this project will explore the potential for biocontrol of the disease using saprophytic root-colonizing fungi. In blueberry, foliar diseases have become prevalent as production intensity has increased. Foremost among these is Septoria leaf spot, which can lead to premature defoliation and associated reductions in flower bud set, return bloom, and yield. This project will develop threshold-based decision aids for managing the disease utilizing basic epidemiological information in conjunction with quantitative yield loss data.
Publications
- Cox, K.D., Scherm, H., and Serman, N. 2005. Ground-penetrating radar to detect and quantify residual root fragments following peach orchard clearing. HortTechnology 15:600-607.
- Ojiambo, P.S., and Scherm, H. 2005. Temporal progress of Septoria leaf spot on rabbiteye blueberry (Vaccinium ashei). Plant Disease 89:1090-1096.
- Ojiambo, P.S., Scherm, H., and Brannen, P.M. 2006. Septoria leaf spot reduces flower bud set and yield potential of rabbiteye and southern highbush blueberries. Plant Disease 90:51-57.
|
Progress 01/01/04 to 12/31/04
Outputs
Armillaria: All laboratory and microcosm experiments on interaction dynamics between Armillaria and saprophytic lignicolous fungi have been completed. To evaluate the concept of competitive exclusion of Armillaria from dead or dying trees by such fungi in field conditions, a test was established in a declining peach planting at the USDA-ARS research station in Byron, GA, this past year. Pathway herbicide (picloram + 2,4-D) was applied to fresh cuts made in the lower trunk area of each tree in April, and the treatment was reapplied to surviving trees in September. In November, trees were inoculated with either Ganoderma lucidum, Xylaria hypoxylon, or Hypholoma fasciculare. These saprophytes had been grown on poplar dowel rods, and two colonized dowel rods were inserted into predrilled holes on the sides of the trunk above the soil level. Control treatments consisted of uncolonized dowel rods. In spring of 2006, trees will be cut and the root systems excavated and
assessed for colonization by the saprophytes and/or Armillaria. - Septoria leaf spot: After 3 years of field experimentation, a comprehensive picture of the epidemiology of the disease has emerged. Disease onset is generally observed between late April and mid-June, and disease severity increases rapidly, reaching a maximum by mid- to late September and decreasing thereafter due to premature defoliation of severely affected leaves. Disease severity is highest on early-emerging leaves and on leaves located closer to the ground. Pycnidiospore inoculum is present throughout the season, and leaves are susceptible to infection season-long, resulting in a polycyclic epidemic. Disease severity, defoliation, flower bud set, and return yield are interrelated: leaves with high disease severity at harvest abscise earlier in the fall than leaves with low disease severity, and shoots with severely diseased leaves or high levels of defoliation have a reduced flower bud set potential and low return
yields. Return yield potential drops markedly and significantly as end-of-season disease severity the previous fall exceeds about 50 to 60 spots per leaf on a susceptible rabbiteye blueberry cultivar. Based on typical disease progress curves derived for Septoria leaf spot, a final disease severity of 60 spots per leaf corresponds to a disease level of about 5 to 10 spots per leaf at harvest in early summer -- a value we propose as an action threshold for the first fungicide application against the disease.
Impacts
Peach and blueberry are the first and second most important fruit crops in the southeastern United States, and the economic viability of both industries is threatened by emerging diseases. In peach, Armillaria root rot has developed into the leading cause of premature tree death, and this project will explore the potential for biocontrol of the disease using saprophytic root-colonizing fungi. In blueberry, foliar diseases have become prevalent as production intensity has increased. Foremost among these is Septoria leaf spot, which can lead to premature defoliation and associated reductions in flower bud set, return bloom, and yield. This project will develop threshold-based decision aids for managing the disease utilizing basic epidemiological information in conjunction with quantitative yield loss data.
Publications
- Cox, K.D., Scherm, H., and Beckman, T.G. 2004. Armillaria root and crown rot. Pages 162-166 in: Southeastern Peach Grower's Handbook (D. Horton and D. Johnson, eds.). University of Georgia, College of Agricultural and Environmental Sciences, Athens.
- Ojiambo, P.S., and Scherm, H. 2005. Survival analysis of time to abscission of blueberry leaves affected by Septoria leaf spot. Phytopathology 95:108-113.
- Roloff, I., Scherm, H., and van Iersel, M.W. 2004. Photosynthesis of blueberry leaves as affected by Septoria leaf spot and abiotic leaf damage. Plant Disease 88:397-401.
|
Progress 01/01/03 to 12/31/03
Outputs
Objective 1: Armillaria tabescens and A. mellea were experimentally challenged with the wood-colonizing saprophytes Phanerochaete velutina, Hypholoma fasciculare, Schizophyllum commune, Ganoderma lucidum, and Xylaria hypoxylon on peach root segments in laboratory microcosms. Results showed significant reductions (by <30 to 100%) in both external and internal colonization of roots by Armillaria in the presence of the potential antagonists. There was no significant difference between A. tabescens and A. mellea in their response to the presence of the saprophytes. G. lucidum, S. commune, and X. hypoxylon had the most pronounced effect in preventing colonization of roots by Armillaria, both externally and internally. Reduction of colonization underneath the bark is particularly important as this occurs in the pathogen's primary niche where it colonizes the root system and gains access to the sapwood. Externally, G. lucidum and X. hypoxylon coated the entire root with a
rubbery matt of basidiome-like mycelium and with a stromatized crust, respectively. P. velutina and H. fasciculare, while being less effective in reducing root colonization by Armillaria, consistently colonized the rhizomorph initials of Armillaria on the surface or formed barrages against the mycelial sheets of the pathogen under the bark. In experiments in which roots were pre-colonized with either Armillaria or one of the antagonists before being challenge-inoculated with the opponent, Armillaria was unable to colonize root segments pre-inoculated by any of the antagonists. -- Objective 2: Research on Septoria albopunctata this past year focused on quantifying the effects of disease severity and plant attributes on the dynamics of defoliation in field plots of 'Premier' rabbiteye blueberry. Fifty shoots were selected for assessment in early spring, and all leaves on these shoots (n = 542) were monitored individually for disease progress and time of defoliation throughout the
season. Disease progress exhibited an exponential increase in disease severity up to late September, followed by a decline until the end of the assessment period in late November. Defoliation was negligible and sporadic up to late August; this was followed by more rapid and sustained levels of leaf loss, and early defoliation of severely affected leaves could explain the decline in disease severity toward the end of the season. Survival analysis revealed that older leaves (located on the lower halves of shoots) and leaves with high levels of disease (>30 spots/leaf in late August) defoliated significantly earlier than younger leaves and leaves with lower disease severity. Relative to the corresponding reference groups, mean times to defoliation were 2 weeks shorter for the older leaf group and 1 month shorter in the leaf group afflicted by high disease severity. An accelerated failure time model fitted to the data indicated that each additional leaf spot present in late August
accelerated time to defoliation (expressed using late August as a starting point) by 1%. Leaf location in upper or lower portions of the canopy had no significant effect on time to defoliation.
Impacts
Peach and blueberry are the first and second most important fruit crops in the southeastern United States, and the economic viability of both industries is threatened by emerging diseases. In peach, Armillaria root rot has developed into the leading cause of premature tree death, and this project will explore the potential for biocontrol of the disease using saprophytic root-colonizing fungi. In blueberry, foliar diseases have become prevalent as production intensity has increased. Foremost among these is Septoria leaf spot, which can lead to premature defoliation and associated reductions in flower bud set, return bloom, and yield. This project will develop threshold-based decision aids for managing the disease utilizing basic epidemiological information in conjunction with quantitative yield loss data.
Publications
- Brannen, P.M., Scherm, H., and Bruorton, M.D. 2003. Fungicidal control of Septoria leaf spot of blueberry, 2002. Fungicide and Nematicide Tests 58:SMF019. DOI:10.1094/FN58
|
Progress 07/01/02 to 12/31/02
Outputs
Objective 1: Five species of potentially antagonistic fungi, most of which are typically found on peach in the Southeast, were each paired against A. tabescens and A. mellea individually in three types of laboratory experiments: on glass microscope slides, on poplar wood blocks, and on longitudinal peach root segments. On glass slides, hyphal swelling in Armillaria was induced most consistently by Hypholoma fasciculare, Xylaria hypoxylon, and Schizophyllum commune. Hyphal bursting and tip bursting, more serious adverse reactions in the target species Armillaria, were induced primarily by Phanerochaete velutina and X. hypoxylon, respectively. On wood blocks, all antagonists overgrew Armillaria about 90% of the time. The only exception was S. commune, which generally engaged in a deadlock reaction with the pathogen. Of particular interest were the antagonists H. fasciculare and P. velutina which formed mycelial cords that actively foraged toward Armillaria. When the
organisms were paired against each other on peach roots, H. fasciculare, P. velutina, and S. commune were observed to colonize rhizomorph initials of A. mellea. X. hypoxylon formed a barrier of stromatized crust over the surface of the root, while H. fasciculare and P. velutina formed barrages underneath the bark where they met with Armillaria. The latter observation is particularly interesting as the interaction occurs in the pathogen's primary niche. Reductions in Armillaria growth rate under the bark were most pronounced when challenged with S. commune and X. hypoxylon. -- Objective 2: Field studies this past year focused on yield reductions associated with Septoria leaf spot. To quantify the negative physiological effects of leaf spots in the absence of defoliation, photosynthesis was measured on affected leaves in the field during September and early October. Across two cultivars, there was an exponential decline in assimilation rates with increasing disease levels, whereby a
leaf spot severity of ca. 3% resulted in a 10% reduction in photosynthesis. In a separate field study, we quantified the effects of Septoria leaf spot during the fall on flower bud set and yield in the following year. Spring shoots with varying disease severity levels were selected and tagged, and all leaves on these shoots were monitored individually for disease progress and time of defoliation through the remainder of the fall. The number of flower buds on tagged shoots was counted in February, and berries were harvested as they matured to determine yield. Results indicated a striking pattern for bud set potential, i.e., the maximum number of buds observed on shoots within a given disease severity class, to decrease as leaf spot severity increased. A similar pattern was observed for the relationship between leaf spot severity and yield potential per shoot, which decreased by about 4 g as average disease per shoot in the previous fall increased by 10 spots per leaf.
Impacts
Armillaria root rot is a major cause of premature tree death in southern peaches. Given the lack of chemical controls and the long-term nature of resistance breeding, biocontrol is one of the few management strategies that could be developed and evaluated in the short to medium term. This project is therefore investigating the potential for competitive exclusion of Armillaria from peach roots by saprophytic asco- and basidiomycetes. In blueberries, Septoria leaf spot is a major threat to the continued growth and viability of the industry. Epidemiology and yield loss relationships are being elucidated to derive appropriate management strategies.
Publications
- Scherm, H., Brannen, P.M., Ojiambo, P.S., Savelle, A.T., Krewer, G., and Bruorton, M.D. 2003. Blueberry leaf spots: Epidemiology, losses, and control. Pages 57-66 in: Proc. SE Blueberry Conference, Savannah, GA, 9-12 January 2003.
|
|