Fire blight, caused by the bacterium Erwinia amylovora, is a sporadic disease of apples and pears. The first commercial biological control product for fire blight, Blight Ban (Plant Health Technologies, Boise, Idaho), went on the market in 1995. This product uses a beneficial bacterium, Pseudomonas fluorescens Pf-A506, which competes with the fire blight bacterium for nutrients on blossoms, keeping the numbers of Erwinia low enough to avoid severe infection. To be effective, the biocontrol organisms have to be applied early so they can build up a large population before the pathogen arrives.
Because biological controls are living organisms, they don't always perform consistently over a wide range of conditions. When sprayed on trees, Pf-A506 often establishes well on pear or apple blossoms, but sometimes it dies off rapidly and does not persist long enough to suppress fire blight. Research is underway to understand the genes that govern how bacteria respond to environmental stresses like desiccation so eventually beneficial bacteria can be genetically manipulated to help them survive under adverse conditions.
Pf-A506 is now being tested in combination with other biocontrol organisms, including Erwinia herbicola C9-1. Although this bacterium is related to the pathogen that causes fire blight, it is a different species and does not hurt fruit trees. Instead C9-1 produces antibiotics that inhibit the growth of E. amylovora. Plant Health Technologies hopes to use C9-1 once registration with the U.S. EPA is complete.
New screening techniques are speeding up the search for additional effective biocontrols. An assay method that uses live blossoms plucked from greenhouse-grown crab apple trees is a big improvement over previous methods that used sliced pear fruit-which is very different from the flower tissue the slices are meant to represent. One promising bacterial strain discovered in these assays is targeted for further testing.
Stelljes, K.B. and D. Senft. 1998. Fire blight control, nature's way. Agricultural Research/Jan. 1998, p. 14-16.
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