Development of Biological Control Strategies for Integrated Management of Pre- and Postharvest Diseases of Apple in Pennsylvania
Open Access
- Author:
- Poleatewich, Anissa Mahon
- Graduate Program:
- Plant Pathology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 08, 2010
- Committee Members:
- Paul Anthony Backman, Dissertation Advisor/Co-Advisor
Paul Anthony Backman, Committee Chair/Co-Chair
Maria Del Mar Jimenez Gasco, Committee Member
Beth Krueger Gugino, Committee Member
Henry K Ngugi, Committee Member
Richard P Marini, Committee Member - Keywords:
- Bacillus
Venturia inaequalis
Biological Control
Malus domestica - Abstract:
- Disease prevention is an essential practice in producing quality fruit. Chemical pesticides are widely used for controlling pre- and postharvest diseases of apple and development of fungicide resistance in pathogen populations has resulted in fewer fungicides available for disease control. Additionally, registration withdrawal and restrictions on postharvest fungicide applications continue to plague the industry. The objective of this research was to develop biological controls for foliar and fruit diseases that can be implemented into existing conventional, reduced risk and potentially organic apple production systems to reduce the use of fungicides needed to maintain a quality product. Specific objectives were to (1) determine the effectiveness of novel indigenous endospore-forming bacterial isolates collected from apple and vegetables to suppress pre-harvest and postharvest diseases of apple; (2) evaluate application strategies for theses isolates to optimize suppression of multiple diseases; (3) evaluate growth and survival of these isolates on several apple cultivars; and (4) examine interactions between plant, pathogen, and the biological control agent in order to formulate hypotheses regarding the mechanism(s) of disease suppression. Healthy leaves and fruit were collected from abandoned, organic, and conventionally managed apple orchards in Adam’s County, PA in 2006 and 2007. Endophytic and epiphytic bacteria were isolated and screened for the ability to form endospores, colonize apple leaves and fruit and the ability to hydrolyze chitin. A total of 75 endospore-forming bacterial isolates were collected from apple and 35% were chitonlytic. Isolates were further screened for the ability to reduce pre-harvest diseases; apple scab (Venturia inaequalis), cedar-apple rust (Gymnosporangium juniper-virginianae), flyspeck (Zygophiala jamaicensis), and fire blight (Erwinia amylovora) in growth chamber, greenhouse and field experiments. Isolates were also screened for the ability to reduce severity of the postharvest diseases bitter rot (Colletotrichum acutatum) and blue mold (Pencillium expansum). Following preliminary screens, six isolates were selected for field experimentation. Four isolates were tested in a two-year field study on biological control of pre- and postharvest diseases at the Penn State Fruit Research and Extension Center in Biglerville, PA. Bacteria were applied to ‘Golden Delicious’ and ‘Rome Beauty’ trees in May or in May +June. Foliar apple scab severity was assessed weekly using a 7-point rating scale to estimate the percent leaf area scabbed. Bacillus megaterium isolate A3-6, B. mycoides isolate A1-1 and B. cereus isolate FLS-5 applied in May or in May+June significantly reduced apple scab severity on ‘Rome Beauty’ and ‘Golden Delicious’ leaves and fruit. Fruit harvested from this experiment were challenged with the bitter rot pathogen, C. acutatum. When applied as postharvest treatments, these isolates significantly reduced mean bitter rot lesion size on both 'Golden Delicious' and 'Rome Beauty' fruit. A pre-harvest + postharvest application of B. megaterium isolate A3-6 resulted in the greatest suppression of bitter rot with an average of 45% and 95% reduction in lesion size compared to no-bacteria controls on ‘Golden Delicious’ and ‘Rome Beauty’ fruit respectively. While a synergistic effect was not observed in treatments combining pre- and postharvest application strategies, there was an additive effect as pre-harvest applications of the biocontrol bacteria were able to reduce foliar and fruit scab. Futher, these pre-harvest effects persisted postharvest by suppressing bitter rot, but were enhanced by the addition of a postharvest application of the same isolate. Five isolates were tested in a field experiment conducted at the PSU Department of Horticulture research farm in Rock Springs, PA to evaluate integration of bacteria with soluble silicon and chitosan for apple scab disease suppression. Application of Chitosan to 'Cortland' leaves in May and June did not reduce foliar apple scab severity compared to the non-treated control. All five of the bacterial isolates tested (B. mycoides isolate A1-1, B. megaterium isolates A3-6 and Ae-1, B. cereus isolate FLS-5 and Brevibacillus laterosporus isolate FLS-1) applied alone or in combination with chitosan in May and June significantly reduced foliar scab severity on ‘Cortland’. Further research on the concentration of chitosan as a foliar spray and combination with biocontrols is needed to determine the potential for apple scab control. Application of the potassium silicate product, AgSil alone or in combination with bacteria, to 'Cortland' leaves in May and June did not significantly reduce apple scab severity on leaves. No significant reduction in fruit scab was observed for any of the amendment or bacterial isolate treatments. The combination of bacteria with chitosan was also evaluated for suppression of the postharvest diseases bitter rot and blue mold. Application of chitosan to apple wounds alone or in combination with B. megaterium isolate A3-6 or Ae-1, B. mycoides isolate A1-1, or Brevibacillus laterosporus isolate FLS-1 significantly reduced bitter rot and blue mold lesion size on both 'Golden Delicious' and 'Rome Beauty' fruit. A synergistic effect was observed in treatments combining chitosan with B. mycoides isolate A1-1 or B. megaterium isolates A3-6 and Ae-1 on ‘Golden Delicious’ fruit with an average of 81% reduction in bitter rot lesion size compared to the no-bacteria/chitosan control. Application of chitosan with B. megaterium isolate A3-6 to wounds resulted in a 78% reduction in blue mold lesion size. Future experiments evaluating postharvest disease suppression when chitosan is applied in combination with our isolates as a fruit coating (applied as a dip or spray) will provide insights into the level of control that could be achieved in a commercial setting. The bacteria B. mycoides isolate A1-1, B. megaterium isolates A3-6 and Ae-1 and B. cereus isolate FLS-5 significantly reduced bitter rot lesion size and were able to colonize fruit wounds at room and storage temperatures. Population levels of isolates A3-6 and Ae-1 were typically log 2 lower than isolate FLS-5 at 2°C and log 1 lower at 20°C, suggesting that the mechanism of disease suppression does not involve competition for space or nutrients. Scanning electron microscopy was used to investigate the mechanism of postharvest disease suppression by these isolates. Observations of wounded tissue colonized with B. megaterium isolate A3-6 showed attachment of bacterial cells to hyphae of C. acutatum. Furthermore, sections of the hyphae where bacterial cells had attached were damaged and collapsed. Further experimentation is needed to more conclusively elucidate the mechanism of bitter rot suppression by isolate A3-6. The potential for integration of the collected bacterial isolates into existing apple management programs was evaluated. The bacteria B. megaterium isolates A3-6 and Ae-1 and B. cereus isolate FLS-5 were able to colonize apple foliage treated with sulfur, copper hydroxide or streptomycin sulfate. While B. mycoides isolate A1-1 significantly reduced fruit and foliar scab severity in two successive field trials, foliar colonization on non-treated or fungicide treated leaves was low. Although, B. cereus is a ubiquitous colonizer of foliar and soil environments, these bacteria are considered an opportunistic animal pathogens and some strains cause food poisoning. Even though only certain strains cause illness, the use of B. cereus isolate FLS-5 for biological control on apple may face regulatory issues. Based on the controlled environment, greenhouse and field experiments conducted for this dissertation, B. megaterium isolate A3-6 and B. mycoides A1-1 are the best candidates for biological control of pre- and postharvest diseases of apple. The combination of our biological control agents with fungicides or antibiotics may provide significant benefits by reducing the variability in disease management associated with biological control agents while reducing the rate of chemical used without compromising disease suppression. The disease management potential of our isolates combined with other BCAs with documented activity against the overwintering or secondary phase of apple scab represents an additional area of research to develop and optimize biological strategies for apple.