Fire Blight on Ornamental Pears

Until a few years ago, many of us assumed that most cultivars of Pyrus calleryana had reasonably good resistance to fire blight. However, over the past two seasons, fire blight damage to many cultivars has been widely reported throughout the Midwest. On the MU campus and research centers, we have observed extensive “strikes” on ‘Aristocrat’ and ‘Capital’ and moderate damage on ‘Redspire’ and ‘Cleveland Select.’ Generally, the damage was less severe than might be expected on non-resistant Pyrus like ‘Bartlett,’ but some ‘Aristocrats’ on campus have at least 18 inches of tip dieback. Surprisingly, I have not found fire blight on any of the many aging ‘Bradford’ trees on campus.

Whether the prevalence of fire blight on P. calleryana cultivars over the past two seasons is due to coincidental weather conditions or to attack by more virulent strains of the bacterium, it may be worthwhile to do some thinking about fire blight management before the 2004 season is upon us.

Fire blight, the disease caused by the bacterium Erwinia amylovora, generally infects susceptible host plants in the family Rosaceae in spring. The most common point of entry is through blossoms as bacteria move by insect activity or by splashing from an inoculum source such as a stem canker from the previous season. Spread of the disease is favored by wet conditions during bloom time. Infection can also occur through lenticels or mechanical damage to succulent stems and leaves. Strong wind, freezing temperatures and hail during early shoot growth can increase infection dramatically.

There are several principles that should be kept in mind when developing a fire blight management strategy. First, although copper fungicides and streptomycin are used by commercial apple and pear growers, these sprays are only effective when applied as a dormant spray (copper) during bloom (strep) or after petal fall, but within 24 hours following hail or wind damage. Once infection has occurred, copper and streptomycin cannot prevent further spread of the disease. Given that control of fire blight on P. calleryana with streptomycin sprays is erratic at best, it is probably not worth the risk of developing antibiotic-resistant strains of the bacterium from their use. Another important principle is that succulent tissue is more susceptible to infection than hard tissue. Thus, excessive pruning or nitrogen fertilization can increase the likelihood of infection. Paul Pecknold and John Hartman, plant pathologists from Purdue and the University of Kentucky, respectively, recommend waiting until the dormant season to prune out fire blight strikes. Pruning during the growing season may stimulate new growth, which may be susceptible to infection. Also, dormant pruning is less likely to spread the disease via pruning tools.

A search of the pesticide registration database maintained by the Missouri Dept. of Agriculture at http://www.kellysolutions.com yielded 12 products labeled for control of fire blight on ornamental pears in nurseries, all of which have copper hydroxide as the active ingredient. However, it should be emphasized that spraying any of these materials after blackened shoot tips have already appeared is a waste of time and money. To be effective, the materials would have to be applied at bloom time or within 24 hours of damage to soft tissue by wind, freezing or hail. Hopefully, conditions will be unfavorable for fire blight infection this spring, and we will not experience the same level of infection that we have for the past three seasons on our ornamental pears.

For more information on fire blight, see: http://agebb.missouri.edu/pdc/diagnosticreports/fireblight01.htm.

Chris Starbuck, Woody Ornamental Horticulture, UMC (573) 882-9630