Issue 4, May 23, 2017

Fire Blight vs. Bacterial Blast

We're beginning to see symptoms of fire blight in central Illinois. The University of Kentucky Extension service has been reporting high risk of fire blight this spring, so we were expecting to see symptoms show up sooner rather than later. The Plant Clinic received the first two samples of suspect fire blight earlier this week, one pear, the other crabapple.

Fire blight is a bacterial disease that only affects plants in the Rosacea family. Pears, apples, crabapples, hawthorn, quince, and serviceberry are the most common hosts in Illinois. The pathogen, Erwinia amylovora, is favored by warm, wet weather during blossoming. Characteristic symptoms include blighted flower clusters, "shepherd's crooks" at the ends of branches, and sunken cankers along infected branches.

Pear sample with developing Sheppard's crook at tip of branch. Note dark brown discoloration along the green stem of the new branch tissue.

Pear sample with blighted blossoms and symptoms developing on leaves.

The disease decreases the aesthetic value and fruit production of infected trees, and may be lethal to highly susceptible varieties. Management consists of pruning out infected branches 4-6 inches below the cankers and sanitizing pruners between every cut. Avoid stimulating new growth (either through heavy pruning, or high nitrogen fertilizer applications) as succulent new growth is particularly susceptible to infection. An application of copper sulfate or streptomycin sulfate in early spring helps protect blossoms, which are the initial entry point for the pathogen.

Bacterial blast is caused by the bacterium Pseudomonas syringae. This disease is of minor concern as it only causes injury during freezing or near-freezing conditions. However, it produces similar symptoms to fire blight. Therefore, we recommend laboratory testing to confirm the diagnosis. Management for bacterial blast focuses on improving tree vitality, as the pathogen rarely causes lasting damage to healthy trees. (Diane Plewa)

Diane Plewa

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