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Back to Bleach to Keep Pruning Tools Clean

M. Grabowski, UMN Extension

When pruning out a branch infected with a fungal or bacterial plant pathogen there is a risk that the pathogen will stick to the pruning tool and spread to other trees and shrubs that the tools is used on. This risk is very low during the winter months, as many plant pathogens are dormant along with the trees. Pruning cuts made during the growing season carry a much higher risk of transferring plant pathogens on pruning tools from one plant to another.

Leaves & blossoms killed by fire blight. M. Grabowski UMN
This spring gardeners should be extra careful when pruning crabapple, apple or mountain ash trees that suffered from fire blight last year. Fire blight is caused by the bacteria Erwinia amylovora. This bacterial plant pathogen survives Minnesota’s winter in branch infections called cankers and emerges in warm spring weather in a sticky liquid called bacterial ooze. The fire blight bacteria are easily spread on pruning tools.

To avoid this problem, pruning tools used on infected trees anytime after bud break should be sterilized between cuts. In 1991, a researcher named Beth Teviotdale published a study showing that the best solutions for sterilizing pruning tools were a 1:5 solution of Pine-sol, Lysol, or household bleach and water. Rubbing alcohol (70% isopropyl alcohol) killed some but not all of the bacterial pathogens on the blade.

Unfortunately although product names remain the same, the ingredients in the bottle may change. In fact the formulations of Pine-sol and Lysol have both changed significantly since 1991. New research is needed to determine if these new formulations are effective in sterilizing pruning tools. In the meantime, a 1:10 dilution of household bleach (5.2% sodium hypochlorite) in water remains the most effective solution to sterilize pruning tools. It can be applied as a spray or by dipping or soaking the blade in the solution.  Bleach can be corrosive to metal.

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