THE TREE DOCTOR (aka The Plant Doctor)

Fire Blight

Fire Blight Management for 2001

David L. Roberts, Ph.D.
Senior Academic Specialist
Michigan State University

The year 2000 will probably rank as one of the most prominent disease years in recent history. The abundant moist weather averaged previous dry years and contributed to many cases of disease epiphytotics (epidemics) around Michigan. Although many diseases such as anthracnose, powdery mildew, and scab reached epidemic levels in 2000, fire blight was more devastating on plants in 2000 and than in any year in recent memory. Unlike many diseases that may reach epidemic proportions, fire blight may be lethal on its host plants. I was informed of the destruction of many landscape plants due to severe fire blight infections in 2000.

Understanding Fire Blight:

Fire blight is caused by the bacterium, Erwinia amylovora. The disease was named after Erwin F. Smith who first discovered the disease in the late 1800’s. In fact, fire blight was one of the first diseases to be attributed to bacteria. Bacteria diseases are not nearly as common as fungal diseases on our landscape plants. Bacteria are highly prolific, often reproducing (doubling) in populations every half hour or so to several hours under optimal conditions. These organisms are extremely small, measuring 1/25,000-1/50,000 inches.

Under conducive conditions in the spring, bacteria populations increase quickly. Sometimes the bacterial populations attain such levels that amber droplets “ooze” form on overwintering “holdover” cankers or other infection areas. Each amber droplet may contain millions of bacterial cells. These bacterial cells may be picked up by bees or rain splashed to near by branches. Bacteria may be spread by virtually any mechanical means including our pruning tools. Fire blight infections can be identified during the winter by blackened, sunken cankers. Often, these cankers surround lateral branch spurs.

Host Range and Symptoms:

Fire blight is usually identified as a problem on fruit trees or ornamental fruit trees. Certainly, apple, crabapple and pear are common host plants of the bacterial incitant. But many members of the Rosaceae family of plants may be infected, and I think we often forget about these other landscape and nursery host plants. Some of these plants include cotoneaster, pyracantha, mountain ash, hawthorn and spirea….and many of these were infected in 2000. Primary symptoms on most of these plants are blackened blossoms, blackened stems, and blackened, sunken cankers. The name “fire blight” originated from the symptoms that appear as if the plants were scorched by fire.

Fire Blight Management:

Recommendations often suggest pruning infected twigs from plants when any infection is observed. This may be a valid technique when infection levels are at reasonably low levels in the spring after infections manifest themselves. However, many strikes (infections) on plants may result in further and more devastating infections if humans “fiddle” with infected plants when bacterial populations are highest and bacteria are most active during warm periods. Hence, winter or dormant pruning is an advisable technique. When pruning strictly for fire blight, as opposed to general pruning of non-infected plants, it is probably wise to use a sanitizing agent between cuts and certainly between trees or shrubs. Because the bacterium may exhibit limited systemic activity, always prune at least 6-8 inches below any visible signs of infection.

Antibiotics, primarily streptomycin derivatives, may be registered and available in your area. Their use is primarily advised during blossoming when bees and other insects may transmit bacterial cells between blossoms and shoots. Their use may be warranted where economics dictate a benefit.

One aspect of culture of our nursery and landscape plants which runs counter to what we normally like to do is the avoidance of high levels of fertilizer. High rates of fertilizer, especially nitrogen, tends to increase succulent growth which promotes bacterial disease incidence in some plants. Fire blight is one disease promoted by abundant/excessive fertilizer.