Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
5/20/02
Contacts:
Stephen Wegulo, Plant Pathology, (515) 294-5896, wegulo@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for the Week Beginning May 24, 2002
Watch Out for Fire Blight
Stephen Wegulo
Extension Assistant Scientist
Iowa State University
Fire blight is a destructive disease that attacks trees and shrubs in the rose family. It is caused by the bacterium Erwinia amylovora. The disease attacks apple, crabapple, pear, quince, mountain ash, cotoneaster, hawthorn, firethorn, serviceberry, rose, raspberry and other related plants. In Iowa, apple and pear are the most common victims, along with the occasional crabapple. Fruit-bearing twigs, branches and entire trees or shrubs can be killed.
The fire blight bacterium survives the winter in dead branches and at the edge of cankers (areas of darkened, softened bark killed by the bacterium during the previous season). During the spring, in rainy periods when temperatures exceed 65 F, bacteria can multiply rapidly.
During these mild, rainy episodes, a sweet, sticky ooze containing masses of bacteria forms on the bark surface. This cream-colored bacterial ooze attracts insects such as bees, flies, ants, beetles and aphids, which carry the bacteria to opening blossoms. Splashing rain also can spread the bacteria.
Bacteria multiply rapidly in the blossoms and then move into branches. They can also enter young branch tips through natural openings such as stomata (breathing pores on leaves), lenticels (breathing pores on branches), or wounds created by hail, insects or pruning. Leaves, flowers and fruit above the point of infection die.
Prevention
There is no cure for fire blight and no single method is adequate in controlling the disease. Therefore, a combination of practices should be used to prevent or reduce fire blight damage.
Planting resistant varieties whenever possible is the most effective prevention method. Apple, pear and crabapple varieties differ widely in level of resistance, but no variety is immune when conditions for disease development are highly favorable and the fire blight bacterium is present. Many varieties with moderate-to-high resistance to fire blight are available at nurseries and garden centers. If you don't see a mention of fire blight on the tree's label, ask the garden center personnel to look up the resistance rating in their guides. As always, planting varieties that are adapted to your growing area prevents stresses that predispose trees to attack by fire blight and other diseases.
Another good general practice is to choose a well-drained site to plant trees. Try to avoid sites with overhead irrigation, because they help spread the fire blight bacterium. Also avoid excessive nitrogen fertilization, because very succulent growth increases susceptibility to fire blight.
Symptoms
Blossom blight symptoms are first visible around petal fall (when the last flower petals drop off). Infected blossoms first appear water-soaked then rapidly wilt and turn dark brown. Usually flower parts remain attached. Flowers contain nectar, which is rich in nutrients that support rapid multiplication of bacteria. Blossoms are usually the first plant parts to be infected because they are attractive to insects, which can carry the bacteria.
The bacteria also can invade fruit, which becomes water-soaked and covered with droplets of bacterial ooze. The fruit eventually becomes leathery, turns brown on apples and black on pears, shrivels and remains attached to the branch.
Shoot blight symptoms are the most obvious and noticeable. Young twigs blacken and curl, giving the appearance of a "shepherd's crook." Leaves on dead twigs, branches and leaders remain attached on the tree and appear fire-scorched, hence the name fire blight.
Fire blight cankers of various sizes can develop and girdle twigs, branches and even trunks. The infected area becomes discolored and slightly sunken. Bacterial ooze may leak out of cankers and run down the branches and trunk during wet conditions.
The conditions most favorable for fire blight development include the
following:
temperatures between 65 and 86 F,
rain,
high relative humidity,
succulent shoot growth, and
injury due to hail, insects or pruning.
Treatment
If fire blight shows up on your trees, act right away. Spring or summer pruning can prevent spread of fire blight and reduce the risk of disease next year. Inspect your trees for fire blight symptoms every 5 to 7 days starting at petal fall. Prune twigs and limbs at least 12 inches below the visible edge of infection. Most commercial apple growers prune out fire blight damage between the beginning of January and the time that buds open in April. This pruning schedule works for a backyard grower, too.
At this time of year, prune out blighted shoots in dry weather, cutting at least 4 inches below cankers. If you have susceptible varieties, consider applying a copper-containing fungicide just before the leaf buds open in the spring. This copper spray kills the fire blight bacteria loitering on the tree's surfaces and reduces your fire blight risk once the bloom period starts. Bordeaux mixture or various liquid copper products are widely available. Make sure the one you use is labeled for fire blight management. Always follow label directions when using any pesticide.
The best strategy for managing fire blight is to combine use of resistant varieties with good cultural practices, pruning and preventive copper sprays. But it all comes back to resistance. If you buy a resistant variety, your chances of avoiding a traumatic encounter with fire blight are going to be much greater than with a susceptible variety. So make sure to check those tags before you whip out your credit card!
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ml: isugarden
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