Fireblight in Crabapples & Flowering Pear


Fireblight has been active recently in eastern Kansas. Fireblight is caused by a bacterial species called Erwinia amylovora. It affects many plants in the rose family including apple, crabapple, pea (including ornamentals), hawthorn, quince, and cotoneaster.

The pathogen survives the winter in cankers. In the spring the disease is most active during wet weather when temperatures are about 65-85 F. During wet spring weather the bacteria ooze out of cankers and are spread by rainsplash, wind and insects to blossoms. This causes “blossom blight.” The pathogen can also infect succulent new leaves and shoots, especially tissue that has been wounded by pruning or hail. The diseased shoots become brown (on apple) or black (on pear) and often have a curled top, referred to as a shepherd’s crook (see photo). Infected shoots can also produce sticky, orange ooze.

The bacterium can move systemically within the infected tree. In
commercial apple plantings the bacterium is most damaging when it moves down into the rootstock, eventually killing the entire tree.

There are several cultural practices to help manage fire blight. The
pathogen is most infective on lush, new growth. Avoid fertilization,
especially later in the season. When the shoots harden off and stop growing they are much less susceptible to infection.

Diseased areas can be pruned out in the summer, but do not prune in wet weather. Make the cuts at least 12-18 inches below the lowest visibly diseased tissue. Disinfect tools between each cut with 70 ethanol or 10% bleach (if you use bleach be sure to clean and oil your tools after use to avoid damage).

Pruning in winter is often a simpler, easier method. Remove dead shoots and cankers. As with summer pruning make the cut below visible diseased tissue, at least 8-10 inches to be safe, and sanitize tools between cuts.

Chemical controls are available but are most effective at bloom so this year we have passed the window of chemical control. In addition, it can be difficult to get the timing and applications correct, so I would encourage home fruit growers to concentrate on the cultural practices.

Commercial apple growers should follow the recommendations in the Midwest Tree Fruit Spray Guide. The content is available online for free at this address: http://www.extension.iastate.edu/publications/pm1282.pdf Or, contact me (Kennelly@ksu.edu) for ordering information. In addition to antibiotics, commercial apple growers can use a growth regulator called Apogee. Apogee should be applied in the spring when shoots are 1 to 3 inches long, and again a few weeks later. Apogee reduces succulent shoot growth and there have been reports that it also stimulates natural defenses in the tree but the mode of action remains unclear. The Apogee label is available here: http://www.greenbook.net/docs/Label/L50185.pdf

Articles reprinted from the K-State Research & Extension Horticulture Newsletter

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