A recent caller from southern Illinois reports that fire blight is again present on ornamental pears. Other susceptible hosts include apple, crabapple, edible pear, cotoneaster, hawthorn, firethorn, and mountain-ash. Resistant varieties are available; in fact, many people have purchased cultivars of Callery Pear over the past decade, believing they were getting plants resistant to fire blight. Resistance is a relative term, as was dis cussed in issue no. 2. Many of the Callery cultivars marketed as resistant have been showing some infection. In some cases, severe infection has occurred. Possibly the cultivars were not tested in a wide geographic area under high disease pressure; possibly the bacterium has changed. The fact is that some Callery Pears “resistant” to fire blight may become infected. Be able to distinguish this disease from frost or other environmental stress. Details are in Report on Plant Disease, no. 801, available in U of I Extension offices or on the Web at Extension’s VISTA site.
This disease occurs in warm, wet weather. Symptoms rapidly progress. Look for water-soaked or wilted new growth that quickly turns brown to black and remains attached to the stem. Stem tips often curl over in a characteristic “shepherd’s crook.” Dark cankers develop in the wood as the bacterium moves down the shoots or flowers.
Flowers are the primary site of infection; and warm, wet weather is required. The causal bacterium may spread by wind, water, equipment, and animals. As long as warm, wet conditions continue during bloom, the bacterium can continue to infect. It is also known to cause infection directly through wounds made during a hail storm.
If fire blight has been a problem in past years, there are some chemicals that may be used as protectants, meaning they must be applied before infection to be effective. Are chemicals useful once symptoms have appeared? If the plant is still in bloom, sprays slow further spread; if the plant is done blooming, chemicals are of little benefit. However, as the pathogen can infect hail wounds, sprays within 24 hours of a hail storm help prevent infection by the fire blight organism. Chemical options are listed in the 2003 Commercial Landscape & Turfgrass Pest Management Handbook and the Home, Yard, & Garden Pest Guide.
It is important to remove infected wood to prevent spread of this bacterium. Pruning must be done in dry weather and cuts made 8 to 10 inches below the last sign of disease. Recent research reports that disinfecting tools between cuts might not be required if pruning is done in dry weather. Over-fertilization should be avoided. The most important action to help prevent fire blight is to prune trees so as to improve air movement through the tree, allowing rapid drying of tissue, making infection less likely.