Marianne Ophardt

Garden Tips: Did fire blight attack your pears trees? Time to cut your losses

Fire blight (Erwinia amylovora) of a pear tree.
Fire blight (Erwinia amylovora) of a pear tree. Courtesy photo

This spring has provided ideal conditions for fire blight attacks on local pear trees and other susceptible plants.

Fire blight is a bacterial disease that is often devastating to both ornamental and fruiting pears. However, it also infects other members of the rose family, including crab apples, apples, cotoneaster, mountain ash, roses, flowering quince, pyracantha, and hawthorn.

Warm weather (65 to 80 degrees) combined with rain or high humidity (over 65 percent) are the perfect conditions for fire blight (Erwinia amylovora) infections. As the weather turns warm in the spring, fire blight bacteria start to multiply in cankers where the disease has overwintered on infected plants.

The proliferating bacteria quickly become a soupy, gummy mess, making it easy to spread the disease to healthy plants via splashing water, insects, or pruning equipment.

Fire blight infections occur when bacteria is introduced into the plant via natural plant openings, such as flowers, or through wounds, such as pruning cuts. Insect vectors are a primary way that the bacteria is transmitted to healthy plants. The main culprits are bees, pollen flies, ants, and wasps, but other insects can also spread the disease.

The fire blight name is very appropriate because it aptly describes the appearance of infected twigs that wilt, turn black, and die, appearing as if scorched by fire. The blackened twig tips also often bend to form “shepherd’s crooks.” The infection usually spreads downward in the tree, causing twig and branch dieback.

Once fire blight is noticed, the main avenue gardeners have for control is pruning out infected tissues. Cuts during the growing season should be made 12 to 18 inches or more below any apparent darkened tissues, especially in young, rapidly growing trees. It is important to sterilize pruners between each cut to avoid spreading the disease.

This should be followed with a copper fungicide spray next spring to open blossoms when temperatures are above 65 degrees. Because all the flowers do not open at the same time, repeat applications need to be made at 4- to 5-day intervals until bloom is over.

When possible, plant ornamental pear cultivars that are moderately resistant to fire blight, such as Prairie Gem, Chanticleer, and Bradford. Avoid cultivars that are highly susceptible, such as Aristrocrat, Autumn Blaze, and Redspire.

Commercial orchardists have more options for control, such as using bactericide sprays. Because there is concern that their repeated use can lead to the development of resistant bacteria, researchers have been looking for biological alternatives. It is intriguing to learn what has already been developed.

One material is called Blossom Protect. It contains two strains of yeast that occur naturally in pear and apple flowers. When applied to the flowers, these organisms grow and compete with fire blight bacteria for the available resources on the surface of the flower parts. Blossom Protect has been found to be the most effective bio-control currently available. There are also two other bio-control materials, Bloomtime Biological and BlightBan, that contain strains of “good” bacteria that also compete for resources on flower part surfaces.

While these new biologicals are exciting, there is only one bio-control available to home gardeners for fire blight control. It is made from the fermentation byproducts of a strain of bacteria. It has both antibacterial and antifungal activity. It is contained in Bayer Advanced Natri Disease Control, Plant Guardian Biofungicide, and Serenade Garden Disease Control. Keep in mind that these only suppress the disease, they do not control it. Pruning and other cultural management options should also be utilized to save trees from a fire blight demise.

For more about fire blight go to:

https://pnwhandbooks.org/plantdisease/host-disease/pear-pyrus-spp-fire-blight

For control in the home landscape go to:

http://hortsense.cahnrs.wsu.edu/Search/MainMenuWithFactSheet.aspx?CategoryId=3&PlantDefId=64&ProblemId=90

Marianne C. Ophardt is a retired horticulturist for Washington State University Benton County Extension.

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