Fire blight is a highly infectious bacterial disease that usually affects the members of the Rosaceae family, especially apples, pears, quince, and other ornamental plants.
The disease is characterized by a burnt appearance on infected parts of the plant. Its origin is in North America, though the disease has spread to become a global problem that has extensively damaged orchards in Europe and Asia and numerous other places across the world. It leads to devastating losses in fruit yields; enhanced management costs; and eventual mortality of the tree, thus causing colossal loss in terms of economy in the fruit-growing industry.
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Causal Organism of Fire Blight
The disease is caused by Erwinia amylovora, a rod-shaped gram-negative bacterium that belongs to the family Enterobacteriaceae.
The bacteria possess peritrichous flagella and secrete a highly viscous exo-polysaccharide known as “levan,” which is one of the components of the ooze that exudes from infected tissues. This polysaccharide helps to create a protective biofilm that covers the bacterium, permitting the bacterium’s survival and adhesion to host tissues. The bacteria also secrete several virulence factors that aid in infection.
The pathogen can survive on or inside the infected plant tissues besides surviving on different surfaces of plants, thus making it highly adaptable to many different environmental conditions.
Symptoms of Fire Blight
The symptoms of fire blight disease according to infected plant parts are as follows-
Blossom Blight
It is one of the earliest symptoms which includes the wilting and browning of blossoms, which appear first water-soaked. As the disease advances, wilted blossoms become brown or black and look burnt. Infected flowers exude bacterial ooze, characteristic of fire blight, and serve as inoculum for later infections.
Shoot Blight
The infected shoots exhibit a characteristic “Shepherd’s Crook” appearance due to the bending of the tips, which wilt and die. Infected leaves often appear scorched, being black or dark brown. On inflorescences and the shoots that emerge along with rain splash, blowing rain, or insect vectors, there could be bacterial ooze on stems and leaves.
Cankers formation
As the bacteria move on towards the branches, they cause sunken discolored cankers on the bark that are usually brown or black.
These cankers can girdle branches and stems in such a manner that it causes bigger plant structures to die back. Cankers can survive overwintering with the presence of the bacteria and can cause reinfection during the next growing season.
Fruit Blight
The infected fruits will develop dark, consisting of water-soaked spots that darken to black color. They may release oozing bacterial ooze and later shrink and fall off the tree before maturity, causing a considerable loss in the crop.
Rootstock Blight
The bacteria can move into the rootstock of young trees, especially when grafted onto susceptible rootstocks. Here, decline and possible death can occur quickly, as a whole tree. Infections of rootstock are lethal in nature since the bacteria can completely girdle and kill the plant from the base.
Disease Cycle of Fire Blight
The various phases of the disease cycle of fire blight are as follows-
Overwintering
The bacteria overwinter in blighted branches and at the edge of cankers. During the spring, when temperatures often reach 65°F, the bacteria multiply rapidly. The masses of bacteria are discharged through cracks and bark pores to the bark surface, where they form a sweet, gummy exudate called bacterial ooze.
Insects such as aphids, ants, bees, beetles, and flies, are attracted to this ooze, carry the bacteria on their bodies, and transfer the bacteria to opening blossoms. Bacterial ooze splashed by rain can also spread the pathogen.
Primary infection/ blossom infection
Most primary infections occur on the blossoms as the bacteria invade the plant through natural openings called nectaries. The bacterium develops within the floral tissues before it begins to invade the shoots and branches. When the bacteria invade and kill the cambial tissue of the branch, all flowers, leaves, and fruit above the girdled area die.
Infection also can take place through natural openings in leaves (stomata), branches (lenticels), pruning wounds, insect feeding and ovipositing, and hail.
Secondary infection/spread
Insects, particularly bees, serve as a significant carrier in the dispersal of Erwinia amylovora as they transfer bacterial ooze from infected to uninfected blossoms. Other insects, such as flies, ants, and wasps, may also contribute to the disease spread through the transfer of ooze. Wind and rain splash ooze onto nearby plants, which enables the secondary infection cycles. Once established within the host, the bacteria can spread within the vascular system, and infect other tissues with cankers and shoot blight. During severe conditions, bacteria often move down to the rootstock, particularly in young or grafted trees on susceptible rootstocks, causing systemic infections.
Environmental factors and epidemiology
The environmental factors affecting disease are as follows-
Temperature– Fire blight thrives in warm temperatures. The optimum temperature required for bacterial growth has been reported between 24–27°C. The majority of outbreaks take place during spring and early summer when the temperature is high.
Humidity– Relatively high humidity (more than 60%) is required for the disease. Heavy rain and irrigation generate humid conditions and tissue saturation by moisture, where bacteria multiply and ooze is spread. Splash from rain or irrigation aids in the transfer of bacteria to new tissues and plants and helps the spread of disease.
Hail, wind, and storms produce physical damage to the surfaces of plants by which the bacteria become implanted. Heavy periods of rainfall followed by warm days are especially conducive to the occurrence of fire blight.
Management Strategies for Fire Blight
Management of fire blight should be integrated, including cultural practices, chemical treatments, biological controls, and breeding for resistant varieties.
Cultural Practices
Pruning and sanitation– Infected shoots and cankers should be continuously pruned to limit the spread of disease. When cuts are made, disinfect pruning tools to avoid accidental bacterial spread. Pruned branches and cankers must be burned or disposed away from the orchard.
Avoiding excessive fertilizers-The application of excessive nitrogen fertilizers promotes excessive new growth, which becomes vulnerable to infection. Nitrogen application should thus be moderate so that there are fewer susceptible shoots.
Plant trees with sufficient gaps to permit air circulation. Dehydrated surfaces of a plant contain less moisture and hence do not favor bacterial activity as much.
Chemical Control
Antibiotics– Out of the many antibiotics, streptomycin is the most abundantly used in the management of fire blight. It has been demonstrated that when applied at bloom, the use of it results in a decrease in the number of bacteria on flowers, and therefore no primary infection occurs.
Copper-based Sprays– Cu compounds are some of the good preventive treatments for killing bacteria on plant surfaces. Copper is usually applied in early spring before blossom opening, though one must be cautious with it since it could lead to phytotoxicity, especially on young leaves.
Biological Control
Beneficial Bacteria- Experiments have shown that certain bacterial species such as Pseudomonas fluorescens and Bacillus subtilis interfere with Erwinia amylovora. This interference reduces the activity of the pathogen leading to the suppression of fire blight.
Yeast Preparations- Some Yeast genera are demonstrated to be resistant to E. amylovora on plant surfaces. These yeast species are not hazardous and environment-friendly. Thus they can be used as substitutes for the standard chemical spray.
Resistant Cultivars and Rootstocks
Cultivar Variety- There are varieties of apples and pears that have a partial resistance to fire blight. Infected-resistant cultivars will show less severe symptoms and damage.
Some rootstocks show resistance to fire blight example Geneva-series (Geneva 11, Geneva 16) rootstocks are resistant.
Routine Monitoring
The times when the optimal temperature and moisture happen such that they facilitate bacterial growth are what need to be monitored.
Using different disease forecasting models helps to predict the possibility of infection based on temperature, humidity, and blossom stage thus enabling immediate use of protective treatments.
Integrated use of treatments
Alternation of copper, antibiotics, and biological agents would rarely create resistance by overuse of any single group.
Fire blight is one of the most serious diseases affecting pome fruit worldwide. Fire blight presents a big problem due to its ability to overwinter in cankers, its aggressiveness, and its ability to invade most plant tissues. Integrated management utilizing cultural, chemical, biological, and genetic strategies would appear to be the best option to reduce the impacts of fire blight.
References
- Fire blight of apples and pears. (n.d.). Ohioline. https://ohioline.osu.edu/factsheet/plpath-fru-22-0
- The Editors of Encyclopaedia Britannica. (1998, July 20). Fire blight | Description, Symptoms, & Treatment. Encyclopedia Britannica. https://www.britannica.com/science/fire-blight
- Washington State University. (n.d.). https://treefruit.wsu.edu/crop-protection/disease-management/fire-blight/
- Fire blight of apple and pear. (n.d.). Fire Blight of Apple and Pear. https://www.apsnet.org/edcenter/disandpath/prokaryote/pdlessons/Pages/FireBlight.aspx
- Planet Natural. (2019, November 12). Fire blight: Symptoms, treatment, and control | Planet Natural. https://www.planetnatural.com/pest-problem-solver/plant-disease/fire-blight/
- Fire Blight : New England Tree Fruit Management Guide : UMASS Amherst. (n.d.). https://netreefruit.org/apples/diseases/fire-blight
- Agrios, G. N. (2005). PLANT DISEASES CAUSED BY PROKARYOTES: BACTERIA AND MOLLICUTES. In Elsevier eBooks (pp. 615–703). https://doi.org/10.1016/b978-0-08-047378-9.50018-x
- Fireblight. (n.d.). https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/diseases/bacterial-spots/fireblight
- Vanneste, J. L. (2000). What is fire blight? Who is Erwinia amylovora ? How to control it? In CABI Publishing eBooks (pp. 1–6). https://doi.org/10.1079/9780851992945.0001
- Fire blight. (n.d.). CT.gov – Connecticut’s Official State Website. https://portal.ct.gov/caes/fact-sheets/plant-pathology/fire-blight
- Peter, K. A., PhD. (n.d.). Fire blight in home apple and pear orchards. https://extension.psu.edu/fire-blight-in-home-apple-and-pear-orchards
