As you drive through neighborhoods in mid-February, you may be delighted by the large trees clothed in puffy cloud looking blossoms of white. Common throughout Marin, ornamental pear trees, specifically ‘Bradford’ pears, put on a spectacular show. In the mid-1960’s landscapers and municipal planners planted lots of them – they grew fast, took any kind of soil without complaint, and were believed to be pest-resistant and disease-free. Sadly, this beautiful tree is susceptible to fire blight, a bacterial disease that causes branches to look like they’ve been torched or scorched.
In addition to pear trees, fire blight can infect fruiting pear and apples, crabapples, pyracantha, and quince. Infections may appear scattered throughout the crown of the plant and exhibit a sudden wilting, shriveling, and blackening of shoots, flowers, and fruit, giving them the telltale “scorched” look. It’s usually seen first on the plant’s blossoms and stems, turning them brown or black and possibly distorting their form. Infected spurs and twigs display a darker, water-soaked appearance; they turn brown to black, then wilt and die. The disease can spread from the point of entry (blooms, stems, and branches, limbs, trunk, or roots) and may severely disfigure or kill individual limbs or entire susceptible varieties.
Mild temperatures (65 – 85°F) with some intermittent rain provides perfect conditions for the disease to develop. As trees resume growth in the spring, the bacteria become active. It’s spread by insects – aphids, flies, leafhoppers, and honeybees, pruning tools, wind and splashing rains. As long as the warm, wet conditions exist, the bacterium can continue to spread and infect new sites.
The most effective means of managing the disease is to plant varieties that are not susceptible. Most pear tree varieties, including Asian pears (with the exception of Shinko) and red pear varieties, are very susceptible to fire blight.
To manage this disease:
- Keep the plant vigorous and healthy through good cultural care.
- Minimize excess nitrogen fertilizer and heavy pruning.
- If possible, don’t irrigate during the blooming season.
- Watch plants throughout the year and promptly remove and destroy any portions that appear infected.
- Prune out diseased branches, cutting at least 8 to 12 inches away from the visible injury or canker, or until you see healthy tissue.
- Disinfect any tools, between each cut, to prevent spreading the pathogen.