Bacterium causing the disease: Erwinia amylovora (Burrill) Winslow

Introduction - Fire blight is one of the most destructive diseases of apple and pear. Outbreaks are sporadic in most parts of the Northeast, but can cause extensive tree damage when they do occur. Epidemics develop quickly, destroying blossoms, vegetative shoots, major limbs and, sometimes, whole trees. The necessary intensity of control programs will vary considerably for different plantings and in different years, depending on individual orchard factors (such variety, rootstock, age of planting) and weather conditions.

Symptoms - Symptoms of fire blight are often very characteristic and not easily confused with other diseases or conditions.

 

Apple flowers (fig. 1) and fruit (fig. 2)infected with Erwinia amylovora.

Blossom blight starts in spring when flowers become infected. Infected flowers first appear water soaked, then they begin to shrivel, wilt and eventually turn brown to black (fig.1). Individual flowers or entire clusters may be affected in a given cluster. If the infected blossoms do not fall, and they typically do not, the bacteria will progress into the flower stem where a similar progression of symptoms takes place. The bacteria typically progress into young spur growth, leaves, and developing fruit. The bacteria travel along the midveins and the leaves soon wilt, shrivel, and turn black. Infected leaves cling to the infected stem and will often remain attached throughout the season. Young fruit will often become infected from bacterial invasion through the fruit spur. Infected fruit appear black and shriveled and will remain attached to the tree (fig. 2 & 3). The blossom blight phase of fire blight includes shoot death that develops as a result of bacterial invasion from the flower clusters.

 

Fire blight infected pear fruit with bacterial ooze (fig. 3) and "shepard's crook" symptom on apple shoots (fig. 4).

Shoot blight develops from secondary infections that originate on young terminal shoots, including suckers and water sprouts. Shoot blight usually develops in late spring or early summer. Infected shoots may at first become oily in appearance and turn dark green. Shoot blight can progress very rapidly under favorable conditions, moving 15-30 cm over the course of a few days. Blighted shoots will often form the characteristic "shepard's crook" at their tip (fig. 4). When infection is severe, the appearance of blighted shoots gives the impression that the tree has been scorched by fire, hence the name fire blight (fig. 5).

 

Shoot blight on pear (fig. 5) and canker blight on apple trunk (fig. 6).

Canker blight is also referred to as "limb", "trunk", or "body" blight depending on where the infection occurs. Cankers form as result of the bacteria traveling systemically into the woody tissue of the tree (fig.6 & 7). Cankers form as result of the bacteria traveling systemically into the woody tissue of the tree. The cankers appear sunken and dark and, when the outer bark is cut away, the underlying tissue appears water soaked. The water soaked tissue will be redder in color in young, active cankers, eventually turning a darker brown as the canker ages.

Trauma blight is a term used to describe infections that occur when blight is initiated at leaf or bark injuries resulting from hail or severe windstorms. This type of infection can often be confused with shoot blight because of the similarity between symptoms. Often, the only diagnostic that helps to determine whether a shoot is suffering from shoot blight or trauma blight depends upon the origin of the infection. If the infection appears to have originated from the tip of the shoot it is assumed that one is dealing with shoot blight. Of course, a rampant infection following a hail storm will be attributed largely to trauma infections.

Rootstock blight occurs when bacteria from infected blossoms or shoots moves internally through symptomless trunks and infects roots (fig. 7). Trees on M.26 and M.9 are often, but not always killed when the roots become infected. Infected rootstock turns dark brown to black depending on the severity of the infection and susceptibility of the rootstock and a stark contrast between the at graft union is often noticeable.

 

Rootstock blight on apple (fig.7) and bacterial ooze from canker on apple (fig. 8).

Signs (visible presence of the pathogen) - The fire blight bacterium is too small to be seen without the aid of a microscope. However, in the spring liquid masses of the bacterium can sometimes be seen oozing from infected tissues (fig. 3 & 8).

Disease Cycle - Fire blight is caused by the bacterium Erwinia amylovora. E. amylovora overwinters in cankers on infected limbs. Cankers become active in early spring as temperatures warm and buds begin to develop. Active cankers produce a yellowish to white bacterial ooze that can appear several weeks prior to bloom. During this period, insects (mainly flies) disseminate the bacteria throughout the orchard. During bloom, pollinating insects rapidly move the pathogen from flower to flower initiating the blossom blight phase of the disease. Flowers can become infected within minutes after a rain or heavy dew when the average daily temperatures are equal to 60° F or greater. Flower receptacles and young fruits are resistant after petal fall. Early symptoms of blossom blight can be expected 5 to 30 days after infection depending upon daily temperatures.

Disease cycle of Erwinia amylovora - E. Gotham, Cornell University.

Inoculum produced from infected blossoms is further spread by wind, rain, and insects. Shoot tip infections are likely to occur when shoots are actively growing and daily temperatures average 60° F (16° C) or more. In years when blossom infections do not occur, the primary sources of inoculum for the shoot blight phase are the overwintering cankers. Particularly, young water shoots near infected cankers become infected as the bacteria move into them systemically from the canker margins. In the absence of blossom infections, the development of shoot blight infections is often localized around areas with overwintering cankers.

Rootstock blight is associated primarily with the highly susceptible rootstocks such as M.26 and M.9. On these trees, just a few blossom or shoot infections on the scion cultivar can supply bacteria that move systemically into the rootstock where a canker may develop and girdle the tree. Trees affected by rootstock blight generally show symptoms of decline and early death by mid to late season. Sometimes symptoms may not be apparent until the following spring.

Although mature shoot and limb tissues are generally resistant to infection by E. amylovora, injuries caused by hail, late frosts of 28° F (-2° C) or lower, and high winds that damage the foliage breach the normal defense mechanisms in mature tissues. Instances of fire blight that originate with infections at sites of injury are called trauma blight and may affect even normally resistant cultivars like 'Delicious'.

Control Measures - Fire blight is best controlled using an integrated approach that combines: (a) horticultural practices that minimize tree susceptibility and disease spread; (b) reduction in the amount of inoculum in the orchard; and (c) well-timed sprays of bactericides to protect against infection under specific sets of conditions.

Horticultural practices. The most effective horticultural practice for minimizing fire blight outbreaks is to avoid highly susceptible cultivars and rootstocks. Highly susceptible apple cultivars include Crispin (Mutsu), Fuji, Gala, Idared, Jonathan, Monroe, Paulared, Rhode Island Greening, Rome Beauty, 20 Ounce, and Wayne. Such cultivars on highly susceptible rootstocks (Mark, M.9, M.26) are particularly dangerous combinations, since one bad outbreak can lead to substantial tree death within the orchard. Most popular pear cultivars are highly susceptible to fire blight, although Seckel is somewhat less so.

Shoot blight is most common on young succulent growth therefore, pruning systems and nitrogen fertilization practices that avoid excessive and prolonged shoot growth are important for limiting shoot blight severity. Advancement of disease into the supporting framework of the tree can be minimized by pruning out blighted shoots as soon as they appear in the early summer . This practice is particularly important on young or dwarf trees, where infected shoots may be only a short distance from the trunk or major scaffold limbs. Cuts should be made at least 8-12 inches (20-30 cm) below the margin of visible infection. Sterilizing pruning shears with alcohol or household bleach between each cut is commonly recommended, although this practice is often impractical and of limited value.

Good control of insects with piercing and sucking mouthparts (aphids, leafhoppers, pear psylla) can be important to slow the spread of shoot blight infections.

Inoculum reduction. Pruning out cankered limbs and branches during the dormant season is an effective way the primary sources of inoculum. Application of a copper fungicide/bactericide at or shortly after green tip will further reduce the number of new fire blight bacteria produced from overwintering cankers. In orchards with a history of fire blight, the yellow-orange shoots characteristic of canker blight infections should be scouted for and pruned out 1-2 weeks after petal fall; this is particularly useful when blossom blight is well-controlled and canker blight infections are thus the main source of inoculum for disease spread during the summer. Pruning out new shoot blight infections as they appear can also help limit disease spread, but will be most effective if practiced rigorously during the first few weeks after bloom; pruning will do little to slow disease spread if delayed until a large number of infections are visible.

Bactericide sprays. Most serious fire blight epidemics begin with infection during bloom. Certain antibiotics can effectively protect against blossom infections when applied shortly before or immediately after they occur; various prediction systems have been developed to help determine when such sprays are most important (e.g., MARYBLYT and Cougarblight). Most systems are based on the principles that: (a) a certain number of heat units, usually in excess of 65°F (18.3°C), must accumulate during bloom before a threshold level of inoculum has been reached; and (b) rain is necessary after this point to wash the bacteria to their infection sites. Thus, antibiotics should be applied just before (or after) a rain if the inoculum threshold has been reached.

Routine use of antibiotics to prevent shoot blight spread during the summer is not effective or recommended. However, applications to protect new wounds immediately following a hail storm can be very beneficial. Apogee (PROHEXADIONE CALCIUM) is a growth regulator that has demonstrated potential for managing shoot blight infection in experimental trials conducted in New York, Michigan, and Virginia. Apogee is ineffective for control of the blossom blight phase of the disease and is registered only for apples, not for pears. Apogee works by shutting down the growth of a tree and, therefore, is used primarily to control overly vigorous trees and reduce the need for seasonal pruning. Apogee has value in fire blight management because when trees stop growing, they become relatively resistant to new blight infections and further expansion of established infections is arrested. As always, check current recommendations.

Fire Blight, CCE/NYS IPM Disease Identification Sheet #D3(revised), 1994.

Additional Information

Pest Management Guidelines for Commercial Tree-Fruit Production - Information on control practices and general apple production in New York.

Tree Fruit and Berry Pathology Pome Fruit Links!