Fire blight of apple

Fire blight of apple

    Causal oragnism: Erwinia amylovora (Burrill) Winslow
    Fire blight is a destructive bacterial disease of apples and pears that kills blossoms, shoots, limbs, and, sometimes, entire trees.

    Symptoms:
    • Blossom blight symptoms most often appear within one to two weeks after bloom and usually involve the entire blossom cluster which wilts and dies, turning brown on apple and quite black on pear.
    • When weather is favourable for pathogen development, globules of bacterial ooze can be seen on the blossoms. The spur bearing the blossom cluster also dies and the infection may spread into and kill portions of the supporting limb.
    • The tips of young infected shoots wilt, forming a very typical "shepherd's crook" symptom. Older shoots that become infected after they develop to the leaves may not show this curling symptom at the tip.
    • As the infection spreads down the shoot axis, the leaves first show dark streaks in the mid veins, that wilt and turn brown, remaining tightly attached to the shoot throughout the season.
    • As with blossom infections the pathogen often invades and kills a portion of the limb supporting the infected shoot.
    • The first symptom on water sprouts and shoots that are invaded systemically from nearby active cankers is the development of a yellow to orange discoloration of the shoot tip before wilting occurs.
    • In addition, the petioles and mid veins of the basal leaves on such sprouts usually become necrotic before those at the shoot tip.
    • Depending on the cultivar and its stage of development at the time infection a single blossom or shoot infection can result in the death of entire limb and where the central leader or trunk of the tree is invaded, a major portion of the tree can be killed in just one season.
    • In general, infections of any type that occurs between petal fall and terminal bud set usually lead to the greatest limb and tree loss. In addition, heavily structured trees tend to suffer less severe limb loss than those trained to systems for high productivity.
      Cultural Practices:
      While establishing new orchard, cultivars resistant to the disease should be planted. Applications of 5% urea to leaf litter as spray in the fall in order to hasten leaf decomposition and reduce primary inoculum.
      Fire blight of apple
      • The bacteria overwinter in bark tissues along the edges of cankers caused by infected in previous years.
      • The bacteria multiplies in the spring, the cankers exude a characteristic ooze and the bacteria are disseminated by rain and insects to vulnerable tissues especially open blossoms, tender vegetative shoot tips, and young leaves.
      • The bacteria penetrate the tree through natural openings or wounds. Secondary infection arises from ooze from fresh infections.
      Dissemination:
      • Disease is disseminated by bees and other pollinating insects and by rains.
      • Primary source of inoculum: Infected cankers
      • Secondary source of inoculum: Water splash borne bacterial cells
      • Monitoring in orchard blocks where the disease occurred during the previous season. Observe blighted limbs and shoots for removal during normal pruning operation.
      • Sometimes removal of completely infected tree is needed.
      • A very important aspect of fire blight management involves monitoring the weather for specific conditions that govern the build-up of inoculum in orchard, the blossom infection process and appearance of symptoms.
      Dormant season:
      • Fire blight overwintering cankers have either smooth or cracked margins. Both types of cankers should be removed.
      • The smooth margined cankers are harder to see, but they are also more likely to be active than rough margined cankers.
      • Active cankers may further expand in the spring causing further structural damage. They also provide inoculum for new infections.
      • One or more separate operations to prune out cankers is recommended.
      • Since cankers are hard to locate, it is always better to visit the orchard several times. Cankers are easily visible on bright and sunny days.
      • Make cuts 15-30 cm below the canker margins.
      • Sterilization of pruning tools during the dormant season is not necessarily required.
      • But however, disinfect your tools if spring pruning is extended into late spring when temperatures have warmed up and/or the budburst stage has arrived.
      • These conditions may also reactivate infectious bacteria in cankers.
      • In fact, an additional inspection for cankers around the budburst stage may reveal cankers that were missed earlier.
      Growing Season:
      • Remove fresh infections as soon as they are noticed.
      • Prune the infected branches at least 30-40 cm below the visible diseased part. It is necessary as bacteria are usually present beyond the discoloured area.
      • Dip tools in a disinfectant after each cut.
      • Flag trees that have been pruned and watch for further symptoms or development of cankers.
      • Pruned branches should be removed and burned immediately.
      • Scout for new fire blight strike after every 3 or 4 days. Frequent scouting will aid removal of new infections before they have a chance to invade the structural wood.
      • Where infections occur on shoots attached to scaffold limbs or the trunk, it is not always possible to cut back 30 cm without sacrificing the limb or even the tree.
      • An option on large trees is to scrape out discoloured inner bark using a hatchet or knife, down to clean wood and disinfect the cut surface.
      • Summer pruning (other than removal of strikes) should be avoided during a serious outbreak to avoid further spread of the disease.
      • In case of fire blight infection in the area, disinfect the tools for summer pruning.
      Disinfectants:
      • Good disinfectants for tools include Lysol Concentrated Disinfectant, Pine Sol, as well as the commercial disinfectants like Chemprocide. Prepare the mixture according to the instructions given in the label.
      • Bleach can be diluted up to 1:5 with water, and needed to be prepared fresh every day.
      • Tools can either be dipped into or sprayed with the disinfectant solution.
      • Bleach, if used can corrode metal tools and damage your clothing. Ideally, tools should be disinfected after every cut. Dilute disinfectant can also be sprayed on the bark after cutting out an infected branch.
      • Excess nitrogen in the soil causes vigorous shoot growth, making it more susceptible. Nutrient application should be balanced, preferably based on soil and leaf analysis. Application of fertilizer should also be timed to avoid a late flush of growth, because late season infections are more likely to produce cankers that allow the bacteria to overwinter.
      • Do not use overhead sprinklers while blossom is present on the tree and weather favourable for fire blight infection. Overhead irrigation may increase fire blight by splashing bacteria around from tree to tree, and also by increasing moisture and humidity levels in the canopy. Restricted irrigation may also help to slow down over-vigorous trees.
      • Control insects with sucking mouthparts such as aphids, leafhoppers and pear psylla which help in spreading fire blight disease.
      Chemical Control:
      • Sprays of Antibiotic streptomycin, fixed copper compounds (copper oxychloride), and copper sulfate (Bordeaux), as well as the biopesticides like BlightBan and Bloomtime.
      • These are the protectants and therefore must be applied before infection occurs. They will not cure diseased tissue
      Insect control:
      • The role of insects in transmission of fire blight bacteria is under investigation. It is suspected that insects that cause wounds (leafhoppers, plant bugs, pear psylla) can create places for bacteria to enter the tree and also that some summer infections (shoot blight) are probably facilitated by insects.
      Resistant cultivars:
      • Red Delicious, Liberty, Enterprise and Freedom are some of the varieties are resistant to the disease.

Last modified: Friday, 15 June 2012, 5:27 AM