Fire Blight of Apple (Erwinia amylovora) is a dangerous bacterial disease that affects not only apples and pears but also many other species, attacking around 160 plant species across 30 genera. In addition to apples and pears, the disease affects quince, hawthorn, medlar, and several other stone fruit and ornamental species. Due to its prevalence in our production, Fire Blight can cause significant economic damage. It is widespread in almost all European countries.
Fire Blight of Apple (Erwinia amylovora) is considered a relatively recent disease of apples. It was introduced from America to Europe in 1957 and was first identified in England. In Croatia, it was observed in 1995 in Nuštar and the surrounding area of Osijek, subsequently spreading throughout eastern Slavonia. Upon its appearance, all measures prescribed for quarantine diseases were implemented. Infected trees were removed and burned, but despite these efforts, the disease was not eradicated. According to the Croatian Plant Protection Service (HZPSS), Fire Blight is now widespread in the Osijek-Baranja, Vukovar-Srijem, Požega-Slavonia, Brod-Posavina, Virovitica-Podravina, Međimurje, Koprivnica-Križevci, and Varaždin counties, with a tendency for further spread. Therefore, it is crucial to conduct forecasting of Fire Blight occurrence in practice to ensure timely preventive measures and to slow its spread beyond quarantine areas, thereby reducing damage in orchards.
Alongside apple scab, it is the most important disease of apples worldwide.
Disease attacks
Leaves, flowers, shoots, branches, trunk.
Causative agent
Symptoms of the disease
This disease is called fire blight in English, because the infected parts of the fruit tree resemble being scorched by fire or flame.
The diseased tree is recognized by its dry leaves, dried shoots, and fruits. The bacterium, which is the causative agent, enters the tree through the flower. Signs of infection are identifiable on the flowers, characterized by a change in color from brown to black, and eventually the flowers dry up. Unfortunately, bees, essential for pollination, are often vectors of this disease. The bacterium further spreads from the flower through the flower stalk to the leaves and shoots. Infected shoots turn brown, while initial symptoms on the leaves include a color change in the petiole and midrib to brown or black. In apples, the color of the leaves and shoots changes to dark brown. Eventually, the leaves become necrotic and dry, but may hang on the branches well into winter. Young fruits on infected trees shrivel, dry up, turn black, and become mummified, also hanging on the branches like dried leaves. Typical symptoms also include the appearance of infected one-year-old shoots that darken, with their tips bending and curling like the handle of an umbrella.
The bacterium further spreads through the tissue, infecting all other organs, so signs of infection can be found on thicker branches and the trunk. An infected tree is recognizable by the reddish-brown color of the bark, where cankers develop that eventually crack and cause the bark to peel off. Under favorable conditions, typical drops of whitish exudate, known as ooze, also appear on these spots and on the fruits, which later turn brown. The exudate is a slimy mass containing the bacteria.
Disease biology
The bacterium, the causative agent of fire blight, overwinters in the marginal tissues of cankers formed by infections from the previous year. It can also overwinter in buds as latent infections. The disease begins its development in spring with increased humidity, rain, or heavy dew, and temperatures above 18°C. Symptoms appear after 5 to 30 days, depending on the temperature. The bacterium can spread from cankers by wind and rain to infect new, unaffected trees. Droplets of bacterial ooze appear on the edges of cankers during fruit tree blossoming. Rain, especially bees, ants, and fruit flies, spread primary infection from cankers to open blossoms. Subsequent infection then spreads from flower to flower via rain or insect pollinators attracted by the ooze droplets. The optimal temperature range for the development of fire blight on susceptible plant organs is 18 to 30°C.
During the summer months, this disease often spreads secondarily from infected trees nearby. Infection can occur through natural openings or wounds caused by pests, strong winds, or hail. In this case, the most common vectors are insects that feed on sap (such as aphids, flies, and cicadas), as well as rain, wind, pruning and grafting tools. Secondary infections can lead to the decline of the entire plant.
Protection against fire blight
It is necessary to monitor and inspect the orchard regularly, especially if infection was observed in the previous year.
On infected trees, leaves often remain on the branches until spring. Branches with retained leaves should be vigorously shaken to dislodge them, collected, and removed from the orchard. If there are infected fruits, they should also be removed from the orchard and burned. Special attention should be paid to changes on the branches and trunk. Additionally, a yellowish-amber or brown discharge, known as exudate, may be observed.
During green pruning, any branches showing signs of infection should be pruned at least 30 cm below the transition from diseased to healthy apple tissue. The cutting site should be treated with a 3% copper solution. Infected branches, fruits, and where possible, leaves should be collected in bags and burned. It is essential to disinfect pruning tools (shears or saw) with 70% alcohol, formaldehyde, or a disinfectant like Varikin after each cut. All pruned plant parts should not be discarded on the ground but placed in plastic bags or tractor trailers covered with plastic sheeting. All pruned material, along with the plastic sheeting, should be burned.
In case the trunk is infected, it is recommended to remove the entire fruit tree.
Chemical protection measures are carried out preventively at the beginning of vegetation using copper-based fungicides (similarly used for controlling apple scab) and during flowering with the application of fosetyl. The use of copper fungicides is also recommended in the fall, immediately after leaf drop.
Among apple varieties, there are differences in susceptibility to fire blight. Highly susceptible rootstocks mentioned by authors include M9, M26, and O3. Varieties highly susceptible to fire blight include Braeburn, Fuji, Pink Lady, Gala, Jonathan, Yellow Transparent. Moderately susceptible varieties include Red Delicious, Liberty, Prima, Redfree, Priscilla. Varieties considered susceptible include Granny Smith, Golden Delicious, Jonagold, Sharon, and others.
