As spring approaches, fruit tree growers will need to be on the lookout for different types of tree-killing bacteria.

Mark Sutphin, an extension agent for the Virginia Cooperative Extension, said one bacteria causes fire blight.

According to the Virginia Cooperative Extension Pest Management Guide, which can be found online at, fire blight is caused by the bacterium Erwinia amylovora and is considered one of the most destructive diseases of apple and pear trees in the United States.

“Pears are most susceptible, but apples and other related species can also get it,” Sutphin said.

Fire blight gets its name from an apple or pear tree being so severely infected by the disease that it has the appearance of being scorched or burned by fire.

Fire blight usually appears in the spring as a blight on a tree blossom, leaf and twig. Infected blossoms suddenly wilt and soon turn light to dark brown. The fire blight bacteria may move down the twig and into branches and limbs, where the infection becomes established. These infected branches and limbs may become entirely girdled with the infection, which spreads upward and downward.

“It can also be spread through open wounds on stems and branches during pruning or hail and storm damage,” Sutphin said.

Trees with fire blight may also develop reddish water-soaked lesions on its bark. On warm days, these lesions may leak orange-brown liquid.

Fire blight control, like most bacterial diseases, can be difficult and expensive. It is recommended that if fire blight is discovered on a tree, to cut off the infected branches of the tree and burn them to prevent the infection from spreading.

The extension recommends avoiding excessive use of nitrogenous fertilizers and excessive pruning of trees. It is also recommended to avoid any pruning during the blossom period and immediately thereafter.

Fruit tree farmers need to be also on watch for large populations of “sucking” insects, like mites and maggots, that are present in trees during the bloom period. These insects spread bacteria to blossoms and open wounds on trees. It’s recommended to use effective insecticides following the “bloom" period.

Streptomycin sulfate, an antibiotic, is the most effective material for fire blight control. The first application should be completed just before the center blossoms of the tree begin to open. Additional applications should be made at five-day intervals until all petals have fallen and there should only be two or three sprays. The extension warns to not spray too much as antibiotics are usually locally systemic and over spraying may cause foliage chlorosis and reduce fruit set.

Products containing streptomycin sulfate can be ordered online.

Contact Donald Lambert at