DRAFT: This module has unpublished changes.

Fire Blight or Fire Fright

The Epidemiological, Agricultural, and Socioeconomic Impact of Fire Blight in Rosaceae

 


Found here, this is a necrotic canker formed on an apple tree, caused by E. amylovora.

 

Found here, these are pears afflicted by the pathogen.

 

Found here, this is the damage caused to the leaves and fruit by E. amylovora.

 

Fire Blight or Fire Fright:

The Epidemiological, Agricultural, and Socioeconomic Impact of Fire Blight in Rosaceae

       Fruits of the Rosaceae family, such as apples, pears, plums, and strawberries, are all well-known edible fruits that are commonly found around the world in nearly every supermarket. Unfortunately, there is a pathogen caused by the bacterium Erwinia amylovora known as fire blight that is ravaging fields and orchards in more than forty countries, recently becoming an even more potent threat (Taylor et al., 2003). E. amylovora is most detrimental to pear and apple trees, as well as the ornamental shrubs also found within the Rosaceae family. Farmers and greenhouse operators have an extremely difficult time preventing and containing the disease, causing it to become one of the most intensively studied bacterial diseases of plants (Schroth et al., 1974). The impact can be felt by both producers and consumers; in the end, everyone loses money to the scourge. By studying the epidemiology, control, and genetic resistance to the pathogen, there is hope to minimize the detrimental socioeconomic impact.

       E. amylovora is a gram negative bacterium that infects the plant through flower blossoms and wounds, and can be spread by insects and birds, the rain, the air, and through general damage to the bark (Granatstein et al., 2011). Once the pathogen is inside the tree, the bacteria proliferate and spread throughout via the plant’s vascular system. If the bacteria moves into the roots, it can kill the entire tree. Fire blight symptoms result in extended necrosis (death or degeneration of infected tissues) as a consequence of the massive intracellular colonization (F. Sparla et al., 2004). The presence of large intercellular spaces in the stigma provides a nutrient-filled environment for the reproduction and proliferation of bacterial cells(Kim et al., 2012). The death of woody tissue leads to the formation of necrotic cankers (lesions), where bacteria can eventually pass through the winter (F. Sparla et al., 2004). Once an infection is identified, growers typically prune out the diseased wood, which is typically very labor intensive and expensive (Granatstein et al., 2011). Control of rosaceous plants is also limited to the use of antibiotics and copper-based compounds, both of which are simply preventatives and are typically strictly regulated (Jensen et al., 2012). Current production methods have shifted towards high-density plantings on dwarfing rootstocks, resulting in greater yields per acre. However, since the dwarfing rootstocks are highly susceptible to fire blight, greater disease problems arise.

       Insects, wind, and rain are typically accountable for a narrow spread of the pathogen, while birds, contaminated wood, and the sale of infected plants probably were the cause for it to become widespread; the bacteria has spread world-wide over the past few hundred years (Hildebrand et al., 2000). Piercing and sucking insects, such as bees, can spread the bacterium into healthy, succulent twigs or shoots. The bacterium survive within bees for up to 2 days after contact. Some aphids, leafhoppers, and borers were thought to be important vectors for spreading fire blight through twigs due to their abundance in greenhouses, nurseries, and orchards. Non-pollinating insects have been found to carry only a small quantity of the bacteria, but their high numbers add to the distribution of E. amylovora and the destruction of apple and pear orchards. The survival of the pathogen in calyxes of discarded fruit within orchards has been shown not to contaminate susceptible hosts, even when placed in close proximity to the healthy fruit (Taylor et al., 2003). The bacteria survive on the surface of fruit only for short periods of time because they are exposed to adverse environmental conditions, meaning that fire blight probably doesn’t spread through commercial activity, as there is no evidence to suggest that these populations would provide residence for a new fire blight infection.

       Planting resistant cultivars (grouping of plant, e.g. Gala apples) seems to be an important measure available to manage the disease. Therefore, classical breeding is currently developing new resistant cultivars by introgressing (moving a gene from one species into another, then repeated backcrossing with one of the parent species) resistance from accessions of wild Malus species into new breeding material (Broggini et al., 2014). However, the fruit quality of such new selections may differ substantially, yet beneficially, from most commercially grown cultivars and may not readily be accepted by consumers. A solution could be to improve a popular cultivar by adding to it the resistance gene isolated from a donor genotype belonging to the native gene pool of apple. Apples and pears are grown as grafted trees, so the resistance of both the scion (fruiting top-part of the tree) and rootstock must be considered. Over the past few decades, commercial apple and pear growers have shifted to using dwarfing rootstocks that increase productivity and quality, as well as general pest and disease control efficacy (Granatstein 2012).

       Genetic analysis in apples is arduous due to its largely self-incompatible nature and substantial genome (Jensen et al., 2012). Apples cannot self-pollinate, and require assistance to reproduce. In 2010, the genome for the Golden Delicious apple was sequenced and found to contain a whopping 57,000 genes (Palmer et al., 2010). The clonal propagation of apples provides an opportunity for genetic analysis of rootstock-regulated phenotypes (Jensen et al. 2012). Conventional breeding of E. amylovora requires extended periods of time and leads to genetic recombination, hampering the selection of high-quality commercial cultivars (Reynoird et al., 1999). Genetic transformation offers an alternative method for the introduction of genes conferring resistance into specific varieties of vegetative propagated species. Attacin E is a lytic protein reported to be active against the pathogen in transgenic apples. The integration of the attacin E gene (attacin is italicized when referring to the gene, not italicized when referring to the protein) leads to a significant reduction of symptoms in comparison with a susceptible control, but to my knowledge there is no information currently available about the activity of attacin E against plant pathogens. The first transgenic plants expressing the attacin E gene were obtained from apple rootstock and displayed enhanced resistance to fire blight. Inoculation of adult trees is desirable to determine whether attacin also increases resistance of blossoms, since infection of flowers is common in nature (Reynoird et al., 1999). Bacterial-inducible promoters could be used to obtain strong and rapid transcription levels at the site of infection and to avoid accumulation of the transgenic protein under non-inductive conditions. Pear engineering with antimicrobial peptide genes also raises the questions of potential toxicity or allergenic risks when transgenic fruits are consumed.

       The fire blight disease is a worldwide problem for pome fruit growers because all popular apple cultivars are susceptible to the disease. Recent epidemics caused relevant damages in different Western European countries, demonstrating the increasing economic importance of fire blight in this part of the world (Broggini et al., 2014). In Switzerland, $9 million of losses due to fire blight were calculated between 1997 and 2000, whereas in 2007 a $54 million loss arose from the disease. In 2013, approximately 600 hectares of apple orchards and surrounding areas were infected in a small region in Eastern Germany. The life of a modern apple planting is generally 15-20 years, compared with a 30-80+ year life for pear orchards. Replanting an orchard is a very costly process: it amounts to between $12-22,000 or more per acre to remove the old trees, rectify the soil, plant new trees and install support systems (Granatstein et al., 2011). At present it is not economically sustainable for organic growers to assume the market risk of planting acreage of a fire blight resistant variety with no consumer acceptance or recognition. Growers are concerned about the loss of the antibiotics oxytetracycline and streptomycin for control of fire blight in organic apples and pears (Granatstein 2012). The evidence for the availability of effective suitable replacement controls is not clear. The deficit of oxytetracycline will disproportionately affect Washington State growers, as they use it but not streptomycin. Since Washington is the prime producer of organic apples and pears in the US, the loss of this material could dramatically impact domestic production and supply.

Ninety percent of the antibiotics used on plants in the United States is sprayed on pear and apple trees for the management of fire blight (Stockwell et al., 2008). Reduction in the number of antibiotic sprays for control may significantly reduce the selection pressure for the development of antibiotic resistant mutants of E. amylovora, as well as minimize exposure of the environment and orchard workers to antibiotics. In the northwestern states, streptomycin provided excellent antibiotic control of fire blight until resistant mutants of the pathogen arose. Oxytetracycline, also known as Mycoshield, is now sprayed as an alternative antibiotic.  The durability of inhibitory activity of oxvtetracycline is similar to that of streptomycin, but oxytetracycline is considerably less effective than when the antibiotics are targeted toward sensitive strains. The annual cost of antibiotic loss for Washington State growers alone could range from $8.77-16.54 million per year. This would account for an estimated 80-85% of the national economic impact (Granatstein 2012).

       Fire blight is a serious disease of the Rosaceae family that is negatively affecting the economy: both the consumer and the producer feel the impact caused by the blight. Apple and pear prices are slowly being driven up as the number of available, arable orchards is dwindling. By conducting a vast array of experiments, the optimal route of transgenic resistance will be found. Common acceptance of genetically modified fruits will lead to a hastening of socioeconomic advancement, which will, in effect, lead to a solution for this terrible pathogen.

 

Works Cited

Broginni, Giovanni A.L., Thomas Wöhner, Johannes Fahrentrapp, Thomas D. Kost, Henryk Flachowsky, Andreas Peil, Maria-Viola Hanke, Klaus Richter, Andrea Patocchi, and CasareGessler. "Engineering fire blight resistance into the apple cultivar 'Gala' using the FB_MR5CC-NBS-LRR resistance gene of Malus x robusta 5." Plant Biotechnology Journal 12 (2014): 728-733. Print.

 

Granatstein, David, Tim Smith, and Greg Peck. "The Role of Tree Genetics in Controlling Fire Blight in Apples and Pears." Organic Tree Fruit Industry Work Group. N.p., n.d. Web. 27 Sept. 2014.                     

 

Granatstein, David. "Economic impact of loss of oxytetracycline for control of fire blight on organicapples and pears." Washington State University. N.p., n.d. Web. 27 Sept. 2014.

 

Hildebrand, M., E. Dickler, and K. Geider. "Occurrence of Erwinia amylovora on Insects in a Fire Blight Orchard." Journal of Phytopathology 148 (2000): 251-256. Print.

 

Jensen, Philip J., Noemi Halbrendt, Timothy W. McNellis, Gennaro Fazio, Izabela Makalowska, Naomi Altman, Craig Praul, Siela N. Maximova, Henry K. Ngugi, Robert M. Crassweller, and James W. Travis. "Rootstock-regulated gene expression patterns associated with fire blight resistance in apple." BioMed Central Genomics 13.9 (2012): 1-17. Print.

 

Palmer, M, K Stormo, A Mraz, Q Tao, T Koepke, D Jiwan, S Schaeffer, V Krishnan, C Wu, V Chu, S T King, J Vick, R Bogden, Y Lespinasse, A C Allan, V Bus, C-E Durel, A Gutin, R EBumgarner, S E Gardiner, M Skolnick, M Egholm, Y V D Peer, D Ederle, F Salamini, R P Hellens, B Lazzari, S Toppo, D Chagné, R N Crowhurst, A P Gleave, E Lavezzo, J A Fawcett, S Proost, A Stella, P Rouzé, L Sterck, R Viola, R Velasco, P Baldi, S Castelletti, M Cavaiuolo, G Coppola, F Costa, V Cova, A Vecchietti, A D Ri, V Goremykin, M Pindo, S Salvi, D Pruss, A Zharkikh, J Affourtit, A Dhingra, A Cestaro, A Kalyanaraman, M M Kater, P Fontana, S K Bhatnagar, M Troggio, M Komjanc, S Longhi, P Magnago, J Lanchbury, T Macalma, J T Mitchell, J Reid, S Masiero, B Wardell, C Kodira, Z Chen, B Desany, N Gutin, L M Fitzgerald, G Eldredge, G Malacarne, M Malnoy, D Micheletti, P Lasserre, M Moretto, M Perazzolli, A Si-Ammour, S Vezzulli, E Zini, F Niazi, and A Stormo. "The genome of the domesticated apple (Malus ×domestica Borkh.)." Nature Genetics 42.10 (2010): 833-839. Print.

 

Reynoird, J.P., F. Mourgues, J. Norelli, H.S. Aldwinckle, M.N. Brisset, and E. Chevreau. "First evidence for improved resistance to fire blight in transgenic pear expressing the attacin E gene from Hyalophora cecropia." Plant Science 149 (1999): 23-31. Print.

 

Schroth, M.N., S.V. Thomson, D.C. Hildebrand, and W.J. Moller. "Epidemiology and Control of Fire Blight." Annual Reviews of Phytopathology 12 (1974): 389-412. Print.

 

Sparla, Francesca, Lorenza Rotino, Maria Chiara Valgimigli, Paolo Pupillo, and Paolo Trost. "Systemic resistance induced by benzothiadiazole in pear inoculated with the agent of fire blight (Erwinia amylovora)." Scientia Horticulture 101 (2004): 269-279. Print.

 

Stockwell, V.O., T.N. Temple, and K.B. Johnson. "Integrated Control of Fire Blight with Antagonists and Oxytetracycline." 1W on Fire Blight 1 (2008): 383-387. Print.

 

Taylor, R.K., C.N. Hale, F.A. Gunson, and J.W. Marshall. "Survival of the fire blight pathogen, Erwinia amylovora, in calyxes of apple fruit discarded in an orchard." Crop Protection 22 (2003): 603-608. Print.

DRAFT: This module has unpublished changes.