Fire blight is an infection that affects pear and other fruit trees, especially in warm, wet conditions. A tree can't be cured once it's infected, but fire blight sprays can prevent trees from contracting this withering disease. Sprays are designed to leave the fruit edible and not harm the tree. Different sprays are more effective at different stages of the tree's blossoming cycle. Antibacterials, copper, growth regulators and biological pesticides are all used to prevent fire blight. Each one has different effects on the pear tree, and some should be used in conjunction with other preventive methods.
Fire blight is caused by the bacteria Erwinia amylovora. Antibacterial sprays can be effective in preventing fire blight infections if applied at the right stage in the tree's development. The most common antibacterial spray for pear trees is streptomycin 17WP. Streptomycin is used to protect the tree in the blossom stage. After the blossoms have fallen, streptomycin is not effective. Streptomycin is considered to be more effective than copper sprays and doesn't cause fruit russeting -- brown spots on the fruit -- to the same extent. Streptomycin needs to be reapplied every two to three days to remain effective because it breaks down in sunlight.
Copper sprays also have bacteria-killing properties that can prevent the development of fire blight. Copper sprays should be applied before flower buds appear, otherwise they can cause russeting on fruit and leaves. Russeting can also occur on "Bartlett" pears if the tree is wet when it's sprayed. Copper sprays are sold under brands including Clean Crop Copper Spray and Guardsman copper oxychloride. Bordeaux spray is usually mixed on-site, and is used as a post-harvest and pre-blossoming treatment to prevent cankers from developing. Bordeaux mixture is a combination of copper sulfate, hydrated lime and water. It is corrosive, so clean spraying equipment thoroughly after using it.
Plant Growth Regulators
Growth regulators don't kill the fire blight bacteria, but they do alter the tree's biochemistry in a way that makes the pear tree a poor host for infection. Prohexadione-calcium is one of the most common growth regulators. Prohexadione-calcium is best used when shoots are 1 to 3 inches long during the blooming phase. Growth regulators should be reapplied every two to three weeks to prevent bud growth. Prohexadione-calcium isn't effective in controlling the blossom blight phase of infection. Growth regulators should only be used on hardy, full-grown trees, as the delay in growth can be detrimental to young pear trees.
Some beneficial bacteria are applied to pear trees to prevent the fire blight bacteria from infesting them. These sprays are used on active blossoms to prevent blossom blight. Each biological pesticide contains a different strain of bacteria that colonizes the pear tree blooms so that the fire blight bacteria can't take up residence. Blight Ban C9-1 and Bloomtime contain the bacteria Pantoea agglomerans. Blight Ban A506 contains Pseudomonas fluorescens. Serenade Max is derived from Bacillus subtilis. Biological pesticide sprays should be applied at the beginning of the blooming cycle. They should be used alongside other preventative treatments, including streptomycin.
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