By: Sheila McBride and David Appel
Fire blight causes vascular wilt in many varieties of pome trees (apples, pears, and ornamental pears) and members of the Rosaceae family (a wide range of trees, shrubs, herbs, and ornamental plants such as roses, strawberries, figs, and mulberries). With vascular wilt diseases, pathogens block the water-carrying (vascular) system of the plant, causing the leaves, stems, and branches to wither, weaken, and die.
One of the most destructive diseases of commercial apples and pears, fire blight is also a serious disease of the popular ornamental Bradford pears used in many Texas landscapes (Fig. 1). Other common Texas woody ornamentals affected by fire blight are loquat, cotoneaster, and pyracantha.
- Infected flowers become water-soaked (translucent and wilted), shrivel, and turn brown.
- Leaves progressively turn brown, develop black blotches, curl, and eventually shrivel.
- Twigs wilt from the tip downward, turning black and curling in a “shepherd’s crook,” giving them a burnt appearance (Fig. 2).
- Branches develop dark, sunken cankers that enlarge and girdle the branches. Eventually, the branch dies (Fig. 3).
Cause and Environmental Factors
The bacterium, Erwinia amylovora, causes fire blight. The pathogen
- over-winters in cankers, budscars, and branches;
- forms an ooze that attracts insects, including bees, that then spread the bacteria via the nectarthodes (openings at the base of flowers);
- also spreads by rain, which splashes onto the bacterial ooze and causes new infections; and
- infects new, tender, succulent twigs and leaves.
- During winter dormancy, use sanitation pruning to remove infected wood:
– Cut an infected branch 4 to 6 inches below the visible injury or canker.
– To avoid spreading bacteria during pruning, sanitize the pruning tool before each cut, using a 10-percent bleach solution (one part bleach to nine parts water).
– To prevent rust, dry and oil tools after using them.
- Reduce excessive succulence by avoiding extreme fertilization and watering.
- Plant moderately resistant varieties.
- Reduce new infections by spraying an antibiotic such as streptomycin sulfate (Ferti-lome® Fire Blight Spray) on flowers or shoots before the bacteria infect them. A copper sulfate fungicide (Bonide® Copper Fungicide) is also an option when applied several times while the blossoms are open. (Neither option will eliminate all new infections or those already existing in the wood.) Refer to product labels for proper rates and use.
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