Dieback of flowering pears and crabapples is often due to the disease fire blight. This is a bacterial disease that can infect many different types of plants.
While most often seen on ornamental pear and crabapple in Nebraska, other susceptible plants include apple, pear, apricot, hawthorn, serviceberry, cotoneaster, Spirea and raspberry.
Fire blight infections can kill blossoms, new shoots, branches, and entire trees. Infections of blossoms and new shoots require warm spring temperatures. During cool springs, these infections are fairly rare.
When infected, flower blossoms turn black and appear scorched. This can be confused with freeze injury. However, blighted blossoms remain attached to the tree through the season and sometimes into winter.
If new shoots are infected, the growth will bend over forming a shepherd’s crook. Leaves on infected shoots eventually turn brown and wilted and remain attached to the tree through summer and into winter.
In Nebraska, fire blight is usually first noticed when a clump of leaves turns brown and branches begin to die. If infection levels are high, a tree can appear scorched by fire, hence the name fire blight.
Infected branches and twigs may be darker in color and bark may appear sunken and cracked in places. These areas are called branch cankers. If the bark is scraped from an infected branch, a reddish brown discoloration may be seen on the inner wood.
On warm wet days during summer, a honey colored sticky liquid or bacterial ooze may be seen on infected plant parts. Severely infected branches can release enough bacterial ooze for liquid to be seen running down the branch.
The bacteria overwinters in infected branches. When conditions are warm and moist, bacterial ooze is spread by insects, wind, or pruning tools. Once any plant part is infected, the infection continues moving down the branch. If it moves into the trunk and roots, trees will die.
The best control of fire blight is planting resistant plants or cultivars and using correct watering and fertilization. Most important is to avoid overwatering and using too much nitrogen fertilizer.
Trees with lush, succulent growth are more susceptible because soft shoots are easily wounded and thus more easily infected. For example, trees grown in an irrigated and fertilized lawn are more susceptible. It is important to plant resistant trees in these locations.
Correct pruning reduces the risk of fire blight. Maintaining an open crown by pruning branches growing towards the trees center, closely parallel to one another, or crisscrossing another branch helps promote air circulation and leaf drying.
Removing infected branches can stop the spread of infection. The best time of year to do this is in February or March when the tree and bacteria are dormant. Prune infected branches at a point that is at least eight inches below the infection. Between cuts, disinfect pruning tools with a 10 per cent bleach solution or an anti-bacterial cleaner such as Lysol or Listerine.
Applications of a copper fungicide can help control fire blight when applied to dormant trees during late winter. Bordeaux mixture, a combination of copper sulfate and lime, or fixed copper fungicides can be used.
Source: University of MN Extension