Fire blight

Erwinia amylovora
Scientific classification
Domain:	Bacteria
Kingdom:	Eubacteria
Phylum:	Proteobacteria
Class:	Gammaproteobacteria
Order:	Enterobacteriales
Family:	Enterobacteriaceae
Genus:	Erwinia
Species:	E. amylovora
Binomial name
Erwinia amylovora (Burrill 1882) Winslow et al. 1920 Type strain = NCPPB 683

Fire blight, also written fireblight, is a contagious disease affecting apples, pears, 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,^[1] a Gram-negative bacterium in the family Enterobacteriaceae. Pears are the most susceptible, but apples, loquat, crabapples, quinces, hawthorn, 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.^[2] It has been a major reason for a long-standing embargo on the importation of New Zealand apples to Australia.^[3] Japan was likewise believed to be without the disease, but it was discovered in pears grown in northern Japan. Japanese authorities are, however, still denying its existence, and the Japanese scientist who discovered it is believed to have committed suicide after his name was leaked to affected farmers.^[4] In Europe it is listed as a quarantine disease, and has been spreading along Hawthorn (Crataegus) hedges planted alongside railways, motorways and main roads.

History

In the early 1800s, E. amylovara was the first bacterium that could be used in experiments to demonstrate that it did indeed cause disease in plants. It is accepted that this destructive crop bacterium had initially originated in North America. Today, E. amylovara can currently be found in all the provinces of Canada, as well as in some parts of the United States of America; states include Alabama, California, Colorado, Connecticut, Georgia, Illinois, Maine, Maryland, Massachusetts, Michigan, New York, North Carolina, Ohio, Oregon, Pennsylvania, Texas, Utah, Virginia, Washington, West Virginia and Wisconsin. Other American countries of its occurrence include but are not limited to Mexico and Bermuda. On the African continent, E. amylovora has been confirmed in Egypt.

It is believed that the pathogen was first introduced into Northern Europe through bacterial ooze from fruit containers in the 1950s, imported from Northern America. During the 1950s-1960's, E. amylovora had spread through much of Northern Europe, yet leaving large areas of Germany and France seemingly untouched by the disease of which the bacteria cause a devastating disease known as "fireblight". This was short lived, as E. amylovora made its presence known when it was discovered in the later 1990s in Germany. Nonetheless by the 1980s the E. amylovora bacteria had been found in the Eastern Mediterranean, although its appearance in this region is thought to be an isolated appearance and not as a result of local transmission. Finally from the years 1995-1996 cases of fireblight had begun to be reported in countries such as Hungary, Romania, Northern Italy and Northern Spain.

Symptoms

Fire blight on a pear tree caused by Erwinia amylovora

Tissues affected by the symptoms of Erwinia amylovora include blossoms, fruits, shoots, and branches of apple (Pomoideae), pear, and many other rosaceous plants. All symptoms are above ground and are typically easy to recognize. Symptoms on blossoms include water soaking of the floral receptacle, ovary, and peduncles. This results in a dull, gray-green appearance at 1–2 weeks after petal fall, and eventually tissues will shrivel and turn black. The base of the blossom and young fruit show similar symptoms as infection spreads. Opaque white or amber colored droplets of bacterial ooze can be seen on the infected tissue when the environment is high in humidity. Shoots show similar symptoms but develop much more rapidly. A “Shepherd's Crook,” can be seen when the tip of the shoot wilts, diseased shoot leaves typically have blackening along the mid-vein and then die. In number, diseased shoots give the tree a blighted appearance. Initial infection of blossoms and shoots can spread to larger tree limbs. Branches will darken and become water soaked. Advanced infection develops cracks in bark and a sunken surface. Wood under the bark will become streaked with black discoloration. Immature fruit forms water-soaked lesions and later turned black. Bacterial ooze can be found on these lesions. Severe infections result in fruit turning entirely black and shriveling.^[5] A primary inoculum of this disease is typically from cankers formed the season before. The factors that determine whether or not cankers become active are not well known, but it is thought that cankers found on larger tree limbs are more likely to become active. It is also thought that age may be a factor.^[6]

Dissemination

Gala apple branch with “scorched” leaves after a severe fire blight infection.

Honeybees and other insects, birds, rain and wind can transmit the bacterium to susceptible tissue. Injured tissue is also highly susceptible to infection, including punctures and tears caused by plant-sucking or biting insects. Hailstorms can infect an entire orchard in a few minutes, and growers do not wait until symptoms appear, normally beginning control measures within a few hours.

Once deposited, the bacterium enters the plant through open stomata and causes blackened, necrotic lesions, 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 infections. The disease spreads most quickly during hot, wet weather and is dormant in the winter when temperatures drop. Infected plant tissue contains viable bacteria, however, and will resume production of exudate upon the return of warm weather in the following spring. This exudate is then the source for new rounds of primary infections.

The pathogen spreads through the tree from the point of infection via the plant's vascular system, eventually reaching the roots and/or graft junction of the plant. Once the plant's roots are affected, the death of the plant often results. Over pruning and over fertilization (especially with nitrogen) can lead to watersprout and other midsummer growth that leave the tree more susceptible.

E. amylovora typically makes its entry into its host xylem or cortical parenchyma. It can also enter through stomata, lenticles and hydathodes. It is dispersed by rain and or insects naturally, but this mode of dispersal is very ineffective and can only be effective for local transmission of the pathogen. Aerosols are also suspected in playing a role in its transmission due to the detection of E. amylovora in Mediterranean regions. In composition the pathogen is composed of short rods with rounded ends made motile by many peritrichous flagellae. E. amylovara is a gram negative bacterium (as stated above).

Management

Spraying plants with streptomycin or injecting plants with oxytetracycline can prevent new infections. The widespread use of streptomycin spray has led to antibiotic resistance in some areas, such as California and Washington. Certain biological controls consisting of beneficial bacteria or yeast can also prevent fire blight from infecting new trees. The only effective treatment for plants already infected is to prune off the affected branches and remove them from the area. Plants or trees should be inspected routinely for the appearance of new infections. The rest of the plant can be saved if the blighted wood is removed before the infection spreads to the roots.^[7] There is no known cure; prevention is the key.^[8]

E. amylovora needs to be destroyed externally, before it enters the cell. This is simply because once it enters the host, it spreads during the endophytic phase of pathogenesis. Once this happens external control methods become ineffective. The ideal control method is to apply copper and antibiotics to the plant externally. This is the only effective method and it is indeed preventative. Currently it has been noted that E. amylovora has developed a resistance to the antibiotic streptomycin, as do most bacteria due to their flexible ability to transfer preferential genes promoting resistance to certain antibiotics horizontally from species not even similar to it as all bacteria can.^[9]

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.

Importance

Besides the historical importance of being the first bacterium proven to be a plant pathogen, it is extremely economically important.^[5] Control and loss costs are estimated approximately $100 million a year in the USA. Specifically, in Michigan in the year 2000, $42 million in losses is estimated because of the removal of about 400,000 apple trees.^[10] Warm, humid, and wet weather in May resulted in this epidemic. While approximately $68 million is estimated in losses in Washington and northern Oregon. E. amylovora is spread all through the USA and worldwide causing severe damage although it is unlikely to cause severe damage in northern Europe. As long as E. amylovora is not introduced to Central Asia where wild apple trees still grow, it will not modify any ecosystems. Biodiversity is not impacted either, as no plant species are threatened with extinction due to this pathogen. Growing pears in Emilia-Romagna in Italy is a traditional activity for some families, and fire blight threatens this tradition that has been passed down for several generations.^[11] In southern Germany apple and pear trees have been a part of the landscape for a long time, and are difficult to protect. The decline of apple and pear trees from their landscape can be expensive to replace and could have a negative effect on tourism. In the long-run, fire blight is a very important factor of economy and society.

Pathogenesis

Pathogenicity depends on many different factors such as the production of the siderophore desferrioxamine, metalloproteases, plasmids, and histone-like proteins. However, some essential factors of pathogenicity are variations in the synthesis of extracellular polysaccharides (EPS) and the mechanism of type III secretion system and its associated proteins.^[12] EPS helps bacterial pathogens avoid plant defenses, “clog” the host’s vascular system, protect bacteria against desiccation and attach to both surfaces and one another. One EPS is amylovoran, a polymer of pentasaccharide repeating units. If a strain of E. amylovora can not produce amylovoran it is not pathogenic and can not spread in plants. Levan is another EPS, and a lack of it will slow development of symptoms. Type III secretion systems are used for exporting and delivering effector proteins into the cytosol of host plants. This system mainly consists of Hrc proteins. Motility is another major virulence factor.^[13] Since E. amylovora is not an obligate biotroph, it is able to survive outside the host which allows it to spread in many ways such as rain.

References

External links

• Type strain of Erwinia amylovora at BacDive - the Bacterial Diversity Metadatabase