Integrated Management of Frost Injury and Fire Blight
Frost injury is a serious problem on agricultural plants, most of which cannot tolerate ice formation within their tissues. Annual losses to agricultural production in the United States alone are estimated at more than $1 billion. At temperatures only slightly below freezing (-10° C and above), even frost-sensitive plants have a natural ability to super cool, thereby avoiding ice formation within their tissues. The capacity to super cool is limited, however, by the presence of ice-nucleation active (Ina+) bacteria, especially Pseudomonas syringae, a common inhabitant of aerial plant surfaces. Ina+ bacteria have an outer-membrane protein—the ice protein—that orients water molecules in an arrangement that mimics the crystalline structure of ice. The oriented water molecules do not super cool but, instead, freeze at temperatures very close to freezing (i.e., -2° C to -5° C). Once ice formation is catalyzed by Ina+ bacteria on the plant surface, ice crystals rapidly spread into the plant, injuring plant tissues irreversibly. Injury at temperatures close to freezing (i.e., -2° C to -5° C) can be avoided by reducing the number of Ina+ bacteria.
Fire blight, an important disease of pear and apple, is caused by the bacterium Erwinia amylovora. The pathogen can grow on shoots, leaves, or blossoms; enter the plant; and then can grow internally, sometimes killing the tree. At present, growers manage fire blight by spraying trees with the antibiotics streptomycin or Terramycin, which reduce the population size of E. amylovora on the plant surface. Resistance of E. amylovora to streptomycin, which previously was the most effective antibiotic, is now common in many pear-growing regions of the United States.
Frost injury on a variety of agricultural plants and fire blight of pear and apple can be suppressed by Blight-Ban®, a product composed of the bacterial-control agent Pseudomonas fluorescens A506. After even a single application of Blight-Ban, large populations of P. fluorescens A506 can develop on treated leaves or blossoms. These populations compete with E. amylovora or Ina+P. syringae for limiting nutrients on plant surfaces. The severity of fire blight and frost injury is reduced because the population size of the causal organisms is reduced through competition with the beneficial bacterium. Although A506 has been as effective as streptomycin in suppression of fire blight in many field experiments, it can also be used in concert with conventional practices for management of fire blight. It is naturally resistant to streptomycin and Terramycin, so can be combined with antibiotics in spray programs. The opportunity to integrate chemical and biological control is attractive to growers, who are more likely to adopt biological control initially if risks can be minimized by combining it with familiar and effective methods.
SOURCE: Lindow, S. E. 1985. Integrated control and role of antibiosis in biological control of fire blight and frost injury. In Biological Control on the Phylloplane, C. Windels and S. E. Lindow, eds. St. Paul, Minn.: APS Press.