Fire blight is a bacterial disease of pears, apples, quince, raspberries, and other members of the family Rosaceae. Caused by Erwinia amylovora, fire blight can wreak havoc on orchards, causing blossoms and new shoots to shrivel and die, leaves to wither, and infected fruits to blacken. The disease, which has no cure, has been known to result in the loss of entire crops, and the destruction of orchards in a single season.
Symptoms of fire blight—shriveling and blackening of shoots, branches, leaves, blossoms and fruit—are readily identifiable in part due to how widespread and severe they are.
Fire blight often makes its presence known in spring as trees begin to awake from dormancy and resume growth, as the pathogen E. amylovora favors temperatures of 75 to 85 degrees Fahrenheit. The first symptoms are often oozing cankers on the branches and trunks of infected trees. Cankers on smaller twigs and branches may be easily missed, while trunk cankers may be more obvious. Cankers are usually black or grey in color, and sometimes sunken. If the bark is removed from an oozing canker, red flecks are often found in the wood near the margins of the canker.
Trees grafted onto rootstock have a propensity of developing infections near the join, with the bark there blackening, cracking, and oozing—because of the location, these symptoms are sometimes mistaken for collar rot caused by Phytophthora. Occasionally the rootstock will be more vulnerable to the disease than the tree. In these cases, the tree may appear free of disease, while the rootstock produces cankers. In this case, infection often isn’t noticed until the tree as a whole begins to show weak growth and other signs of decline.
However, oozing cankers are a springtime symptom of trees infected the previous year. New infections usually occur through open blossoms. When flowers are infected, they become water soaked, and take on a grayish green color. The stem and base of the blossom blacken, before the blossom or fruitlet ultimately shrivels and dies.
The infection may stay confined to the area, only impacting nearby blossoms, or may spread further. When the disease spreads, smaller shoots and branches will turn brown and die, resulting in strips of dead leaves within the tree canopy. The ends of infected shoots often curl, taking on a characteristic “shepherd’s crook” shape. As branches become discolored, they may also produce droplets of white ooze which turn amber over time. When the bark of infected branches is peeled away, the wood beneath will exhibit brown or black streaks.
Fire blight’s name is derived from what is perhaps its most notable symptoms—the shriveling and blackening of leaves and fruit. The leaves of infected shoots will begin to turn black along the veins, and then blacken entirely and die. Similar symptoms develop in fruit. Immature fruit will develop water-soaked lesions, which sometimes produce ooze like that of infected branches. Oftentimes, infected fruits will eventually turn completely black and shrivel. Unlike many other diseases, infected leaves and fruits do not drop from the tree, instead clinging tightly to branches. This produces the burned, blighted appearance that is the disease’s namesake.
Fire blight is caused by the bacteria Erwinia amylovora, which is native to North America and well suited to the mild, warm weather found in many regions of the continent.
Erwinia amylovora is a devastating pathogen. While originally found only in North America, the early years of international trade allowed it to spread throughout much of the world. For the last several decades, Europe has worked diligently to try and contain its spread, while Australia, which is apparently free of the disease after deploying extensive eradication efforts 20 years ago, maintains stringent import embargos and monitoring efforts. The disease is so feared that in the 1990s, a Japanese scientist’s disclosure that fire blight had infected Japanese fruit trees blew up into a controversy that led him to take his own life.
Closer to home, E. amylovora is found in orchards throughout much of the United States and all of Canada. The bacterial pathogen is spread easily, capable of being carried by insects and birds, as well as wind and rain. Bees, flies, and beetles are often attracted by the ooze produced by overwintering cankers. They feed on the bacteria-bearing ooze, and carry traces of it to subsequent feeding sites.
The pathogen typically colonizes trees through cracks and pores in bark surfaces, as well as through blossoms which are fed upon by insects or contaminated via wind or water. Blossoms are a common colonization site because the stigmas are readily accessible and particularly rich in nutrients (infected flowers can in turn serve as a vector when fed upon by insects). Infection can also occur as a result of insects feeding on leaves and branches.
Spring seasons in which warming weather is punctuated by hailstorms or heavy rain are particularly dangerous. Injuries to bark and branches produce ample opportunities for the spread of the disease.
When the pathogen successfully makes its way into the tree through the blossom or bark, it then colonizes and spreads through the cambium tissue lying between the bark and wood. As it spreads, it kills this tissue, producing the characteristic discoloration of the cambium, and the oozing cankers in the bark above. When a cambial infection spreads so that it encircles a shoot or branch (“girdling”), it will kill everything beyond the infected area due to the disruption of water and nutrients.
The spread of fire blight can be explosive, and the results devastating.
Though now largely forgotten, in the early 1900s, California was home to a thriving pear industry which was nearly destroyed by fire blight. In 1902, it was recorded that Fresno County had 125,000 pear trees, and Kings County another 43,700 trees. By 1904, after a devastating outbreak of fire blight, Fresno County’s 125,000 pear trees were reduced in number to 1,500. The King County orchards were completely destroyed, with no trees surviving. During that same decade, two-thirds of the Bartlett pear trees in the state were killed by fire blight (with economic losses being estimated at more than $140 million in today’s dollars). In the Southern San Joaquin Valley, 95% of pear trees perished. Pear production recovered in many parts of California, quickly exceeding production before the outbreak. But in the 1930s, a fire blight outbreak in Sacramento devastated the area to such a degree that the number of pear trees in California was reduced by nearly half.
Hundreds of millions of dollars are spent around the world every year on fire blight control measures, and for excellent reason, as history shows.
There is no cure for fire blight—management of the disease instead depends on growth management, pruning of diseased tissue, careful sanitation practices, and appropriate nutrition.
Growers should avoid practices which encourage rapid growth, such as excessive nitrogen fertilization and heavy pruning. Rapid growth generally results in the excessive production of twigs and tissue which is weak and vulnerable to wounds, allowing for transmission of the disease. Because the disease favors moist conditions, avoid irrigation of trees during bloom. Remove weeds and any other low-lying plants which can foster humid conditions in orchards.
If there is a history of fire blight outbreaks, apply appropriate protective blossom sprays during bloom. This will not treat entrenched infections, but will help mitigate disease spread.
Be extremely wary of pinching off the flowers of newly planted trees. Because infection often occurs through blossoms, pinching off an infected blossom can contaminate the hands of a worker with the fire blight pathogen, leading to the infection of flowers which are subsequently pinched off. Apply a streptomycin spray first to kill any bacteria on flower surfaces beforehand. Do not pinch flowers in wet conditions.
When outbreaks of fire blight occur, careful removal of infected tissue is critical. Note that where possible, pruning of diseased wood should only be performed during the winter or summer, when temperatures are more inhospitable to the pathogen. However, with species (pears) and cultivars (Fuji, Gala, Golden Delicious, Granny Smith, etc.) which are particularly vulnerable, it may be necessary to prune the moment that symptoms are noticed. But regardless of the season, only prune in dry weather.
Identify twigs and branches that show cankers or other symptoms of fire blight, and then select a cutting site that is as far back as possible. For instance, a branch with fire blight should not be trimmed where its base meets a larger branch, but instead the larger branch to which it is attached should be trimmed below where the infected branch joins it. Failing to cut far enough back (“short cutting”) can leave diseased tissue in place and allow the disease to reoccur.
Between each branch pruning, shears should be dipped in a 10% bleach or 70% alcohol solution to sanitize them and prevent transmission of bacteria.
If a fire blight canker is visible on a major limb or on the trunk of a tree, it may be possible to remove the infected wood. If possible, wait until winter when trees and the pathogen are dormant. Scrape away the outer and inner bark around the point of infection. Be on the lookout for narrow lines of infected tissue which can extend well beyond the canker. Remove all infected tissue and the surrounding 6 to 8 inches of healthy tissue. If a limb has been girdled by infection, the entire limb must be removed below the point of infection, as the limb will inevitably succumb and die. Treat all prune wounds with a solution of 70% alcohol. The remaining stubs often form small cankers, which can be pruned away during winter.
If a tree is so badly infected that diseased tissue cannot be completely removed, the tree must be removed. Likewise, with trees grafted on rootstock, if the rootstock exhibits cankers, the trees must be removed. Cases of fire blight where the infection moves into the root system are terminal.
If an outbreak of fire blight is extremely severe, you may be forced into a situation where you must prioritize what trees to prune. Focus on younger trees, as it’s much easier to remove all diseased tissues, whereas pruning an orchard of large, decades-old trees may be cost prohibitive. With larger trees, it may be necessary to allow the infection to play out, and then decide in the fall whether the trees can be saved. But before making such a challenging and risky decision, we recommend consulting with an expert, as there is always the risk of disease spread if badly infected trees are allowed to remain in place.
After pruning has been performed and all diseased branches and wood have been removed, be extremely cautious with this debris. Immediately removing the debris may result in the inadvertent contamination of adjacent orchards. Instead, the Michigan State University Extension recommends leaving prunings in the middles of orchard rows and allowing them to try completely. Only remove the debris once all the tissue within pruned branches and wood scrapings has browned.
Lastly, as always, ensuring balanced nutrition is key. As we noted above, it is critical that growers avoid excessive use of nitrogen fertilizers, as this stresses plants and results in plant tissue that is structurally fragile and vulnerable to forming the wounds E. amylovora invades. However, a balanced nutritional regime will improve tissue integrity and prime the immune systems of plants. A program we have previously recommended for use in controlling fire blight in apples and pears—note that you should consult with your local PCA or advisor as needed before instituting this program—is as follows:
Fill a spray tank at least three-quarters full of water and start agitation before adding the following:
- Fusion 360 Foliar FG-31 – 3 gallons
- Integrity 9.5% Calcium – 2 quarts
- Keel 0-0-1 – 1 quart
- Integrity Z-422 – 1 pint
- Manex – 1 pint
- Agrimycin (oxytetracycline) – 1 pound
- Silicone Surfactant – 4 ounces
Once mixed, apply at a volume that delivers a fine mist. If disease pressures are high, repeat at a 3 to 5- day interval. Otherwise, repeat every 5 to 7 days. Rotate Agrimycin with tetracycline to manage antibiotic resistance. Be careful to supplement with additional Integrity 9.5% Calcium, Integrity Improved 5% Magnesium, and KMend potassium injected through lines. Minimize the use of nitrogen during this susceptible period.