The discoloration is seen on newly infected tissue

Fire Blight of Fruit Trees and Certain Ornamentals

Causal Agent: The bacterium, Erwinia amylovora
Hosts: Apple, crabapple, pear, raspberry, blackberry, and pyracantha in Oklahoma
Symptoms: Disease symptoms first appear when trees are blooming and they include blossom, spur, and terminal blight. Infected blossoms suddenly wilt and turn a light to dark brown. The infection spreads down the pedicel and the tissue becomes water-soaked and dark green. The leaves on blighted terminals turn brown to black and usually remain firmly attached to the infected twig throughout the growing season. Blighted terminals usually take on a shephard's crook appearance. Infection through flowers will not spread as fast as infections on twigs. Affected parts of the plant (blossoms, spurs, fruit, branches, trunk, and cankers) often produce droplets of clear, milky, or amber-colored exudates or moisture. These droplets contain millions of bacteria which can initiate new infections. Insects disseminate or spread the bacteria. When honeybees or flies visit infected blossoms their bodies may become covered with the bacteria contained in the droplets and this is how the disease is spread. Rain and windblown mist may also transfer bacteria from diseased to healthy plant parts.
Control: Sanitation - The first step in a good control program starts with removal of all infected tissue as soon as it appears. During pruining activities, all cutting tools should be disinfected after each cut to prevent the spread of bacteria (use denatured alcohol or 10% bleach). Other important sanitation practices include removal of all root suckers and succulent water sprouts that might become infected and carry bacteria into the main trunk. Cankers on large limbs and trunks are best removed during the dormant period by cutting or scraping away infected tissue. Good management practices include monitoring of orchards or garden plantings from bloom period through midsummer to locate and removing oozing cankers. Plant or Orchard Management - Plants or orchards can be managed to reduce incidence of Fire Blight. Avoid using excessive amounts of nitrogen fertilizers and avoid heavy cultivating or pruning which promotes fast growth. Removal of water sprouts as they form will prevent infection from entering through these sprouts into the limbs, trunks and roots of the tree. Mowing ground cover, avoiding excess irrigation and aiming sprinkler heads low to reduce leaf wetting will reduce humidity in the grove or garden, which will help reduce incidence of Fire Blight. Cultivar Selection - No known cultivar of apple, pear, pyracantha, flowering crabapple, cotoneaster, quince, hawthorne, or rose is completely resistant to Fire Blight. However, some cultivars of these plants are more resistant or tolerant of Fire Blight than others. Therefore, if Fire Blight has been a problem in the past it may be beneficial to consider planting less susceptible cultivars. Chemical Control - Several types of bacterial formulations, in conjunction with the previously described cultural practices, may be effective in prevention of the blossom blight and terminal blight stages of Fire Blight.
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