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Fire blight, also written fireblight, is a contagious disease affecting , , and some other members of the family Rosaceae. It is a serious concern to apple and pear producers. Under optimal conditions, it can destroy an entire orchard in a single growing season.
The causal pathogen is Erwinia amylovora, Type strain NCPPB 683 (dead link 3 December 2019) a Gram-negative bacterium in the genus Erwinia, order Enterobacterales. It is a short rod with rounded ends and many peritrichous flagellae. are the most susceptible, but , loquat, , , Crataegus, cotoneaster, Pyracantha, raspberry and some other rosaceous plants are also vulnerable. The disease is believed to be indigenous to North America, from where it spread to most of the rest of the world.
Fire blight is not believed to be present in Australia though it might possibly exist there. It has been a major reason for a long-standing embargo on the importation of New Zealand apples to Australia. In Europe it is listed as a quarantine disease, and has been spreading along hawthorn ( Crataegus) hedges planted alongside railways, motorways and main roads.
It is believed that the pathogen was first introduced into Northern Europe in the 1950s through fruit containers, contaminated with bacterial ooze, imported from the USA. During the 1950s-1960s, E. amylovora spread through much of Northern Europe. Initially large areas of Germany and France seemed untouched by fireblight, but the disease, and E. amylovora, were discovered in the later 1990s in Germany. In the 1980s the bacterium was found in isolated regions in the Eastern Mediterranean and from the years 1995-1996 cases of fireblight began to be reported in countries such as Hungary, Romania, Northern Italy and Northern Spain.
Other than through the flowers, the bacterium can enter the plant through the stomata. Also highly susceptible to infection are lesions such as punctures caused by plant-sucking insects and tears caused by a variety of means, including infected cultivating tools. A few minutes of heavy hail can spread the disease throughout an entire orchard and growers normally do not wait until symptoms appear but begin control measures within a few hours.
Once the bacterium gains access to the xylem or cortical parenchyma of the plant, it causes blackened, necrotic , which may also produce a viscous exudate. This bacteria-laden exudate can be distributed to other parts of the same plant or to susceptible areas of different plants by rain, birds or insects, causing secondary . The disease spreads most quickly during hot, wet weather and is dormant in the winter when temperatures drop.
The pathogen spreads through the tree from the point of infection via the plant's Vascular tissue system, eventually reaching the and/or grafting junction of the plant. Once the plant's roots are affected, the death of the plant often results. Over-pruning and too much fertilization (especially with nitrogen) can lead to water sprout and other midsummer growth that leave the tree more susceptible.
Unfortunately, while chemicals and meticulous pruning can keep an infected tree productive, there is no known comprehensive cure for fire blight; the best that can be done is to prevent its spread by measures such as avoidance of overhead water systems, as falling water can spread the disease and the careful pruning of tainted stems or branches. Great attention must be paid to any gardening tools that have been exposed to the causitive microorganisms. These tools should be disinfected in an alcohol solution containing three parts denatured alcohol to one part water. Diluted household bleach (one part bleach to nine parts water) can likewise be utilized. Of course, implements should afterwards be dried and oiled to forestall corrosion.
The fly Delia platura has been observed visiting fire blight wounds to feed and can successfully transmit fire blight to already damaged apple shoots. Fire blight exopolysaccharide also served as the adhesive to attach propagated cells to D. platura. D. platura shed fire blight at a constant rate - and did not suffer from doing so - for at least five days.
Methods to predict the likelihood of an outbreak so that control measures can be best targeted, were introduced from the 1980s following the work of Eve Billings at East Malling Research Station, UK. These were based on temperature and rainfall, and have been developed further by Billings and others.
E. amylovora generally needs to be destroyed externally, before it enters plant tissues because once it enters the host, it spreads during the endophytic phase of pathogenesis. Once this happens external control methods become ineffective. The application of copper and antibiotics to the plant externally is the most effective method of prevention. Currently it has been noted that E. amylovora has developed a resistance to the antibiotic streptomycin, as do most bacteria able to transfer preferential genes horizontally from species to species.
New research conducted by John C. Wise out of Michigan State University shows that E. amylovora can be controlled with relative efficacy through tree trunk injection of either streptomycin, potassium phosphites (PH), or acibenzolar-S-methyl (ASM). PH and ASM both work through gene inductions of PR-1, PR-2, and PR-8 in the leafy material. Oxytetracycline Hydrochloride (OTC) was also tested and found to greatly reduce the activity of the bacteria within the tree. These new control methods are still being researched and have not been approved for fruit crop production by the EPA.
Phytosanitary measures have been employed as the best sanitary measures against E. amylovora dispersal. High risk countries are encouraged not to import plants susceptible to the pathogen into their territory because, once the bacteria become established in an area it is nearly impossible to eradicate the disease. Nurseries and orchards in such regions are placed on strict phytosanitary surveillance measures and well-monitored. Imported and infected crops are destroyed as soon as they are noticed since the bacteria spreads very rapidly and eradication methods are usually costly and inefficient.
Current fire blight strategies depend upon phytosanitary measures to lessen inoculum in the plantation and the utilization of splash medicines to forestall contamination, particularly blossom infections. Decreasing essential inoculum in the plantation by removing remainder holdover cankers during winter pruning is a set up as a basic method of control fire blight disease.
Slowing the growth rate of the tree will also slow the development of cankers. This may be achieved through reduced watering and fertilising. Controlling insects which cause tree wounds will also decrease secondary infection.
Cultural control options include selecting resistant cultivars, however most commercially successful apple cultivars lack fire blight resistance. Breeders have developed fire blight resistant rootstocks, but resistance is not conferred to the grafted scion. (1999). 9781879906426, University of California, Agriculture and Natural Resources. ISBN 9781879906426
Prohexadione calcium (BASF brand name Apogee in the United States) is a plant growth inhibitor which is recommended for shoot blight. Since fire blight relies on gibberellin-dependent growth for much of its own life cycle, prohexadione's gibberellin synthesis inhibition effect also suppresses blight. Not effective in blossom blight.
A relatively small number of apple cultivars are responsible for an enormous proportion of yearly apple production. Food sellers and shoppers prize these cultivars for their appearance, quality, flavour, and storability, while cultivators additionally esteem their orchard attributes and guaranteed market due to this popularity. To maintain the desirable qualities of a cultivar while at the same time changing its disease resistance through ordinary breeding techniques is for all intents and purposes impossible due to the apple's heterozygosity, self-incongruence, and long growth span. Hereditary designing offers an appealing option since it can be faster, resistance qualities can be acquired from numerous sources, the statement of local apple qualities can be altered, and the attractive characteristics of the changed cultivar or rootstock can be safeguarded.
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