Fire blight

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, 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. It has been a major reason for a long-standing embargo on the importation of New Zealand apples to Australia. 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. In Europe it is listed as a quarantine disease, and has been spreading along Hawthorn (Crataegus) hedges planted alongside railways, motorways and main roads. 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. 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, and 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.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. 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.