Kiran Shetty
University of Idaho, Extension Potato Specialist

Potato storage diseases are often difficult to control once they occur inside storages for two reasons: a) there are very few post harvest chemicals for control of diseases registered for use on potatoes and b) the storage environment and conditions can favor the spread of certain diseases. Disinfection is a preventive procedure designed to kill disease-causing agents. Successful disinfection of potato storages and handling equipment will greatly reduce disease problems in potato storages.

One of the popular group of products available for use in potato storage is the chlorine based disinfectants. Some of the chlorine products have special considerations to be effective as disinfectants. This article outlines those considerations.
Special considerations for chlorine based disinfectants:
Chlorine based disinfectants are one of the popular group of disinfectants used in the industry. Chlorine is a surface sterilant. Chlorine does not disinfect the internal tissue of the potato but does prevent the build-up of disease-causing fungi and bacteria on the surface. The most common chlorine-based disinfectants are sodium hypochlorite, chlorine dioxide and calcium hypochlorite. The hypochlorite containing chlorine compounds form the spore-killing active ingredient hypochlorous acid when added to water. In this case the disinfectant activity of a chlorine solution is influenced by its pH. pH is the measure of whether a solution is acidic or alkaline. The lower the pH (more acidic), the greater the amount of hypochlorous acid formed from the chlorine compound. This is why it is important to apply the right mix in water. At very low pH levels, odor problems and corrosion of equipment will occur. At levels above pH 7.5 (slightly alkaline) hypochlorous acid will dissociate to form the hypochlorite ion, which is less toxic to fungi and bacteria. Therefore for the most effective chlorine disinfectant the pH should be maintained between 6.0 and 7.5. At this pH the damage to the equipment will also be minimum. The pH of the tank mix can be checked by using pH test paper (which changes color at different pH's) or by using pH meters. The chlorine dioxide-based disinfectants are active at pH range from 2.3 to 2.5.

Other factors to consider when using chlorine disinfectants:
Organic Matter: Organic matter reacts with chlorine to lower its effectiveness. The more organic matter in the solution, the more chlorine is tied-up. This organic matter in the water can be a critical problem. Consequently, flumes and dump tanks should be cleaned at regular intervals.
Temperature: Chlorine exhibits a complex reaction to temperature. Generally, as temperature increases chlorine becomes more active as a disinfectant. However, raising the temperature increases the ability of organic matter to tie-up chlorine. In addition, raising the temperature also speeds up the loss of chlorine into the atmosphere.
Exposure time: As exposure time to chlorine is reduced, less disinfection occurs. A minimum of 15 minutes of exposure to chlorine is required for adequate disinfection. However, efficient disinfection depends upon accurately monitoring the concentration of chlorine in the tank. Treatment is most effective when chlorine is metered continuously. It is ineffective to simply dump chlorine into the tank. For fungi an effective concentration of 500 ppm of free chlorine is required. This is not a case where "if a little is good, more is better". If the concentration of chlorine is too high, or if the pH of the chlorine solution is too low, chlorine gas is wasted into the air. This creates an odor problem, wastes chlorine, and corrodes equipment. It is better to meter in the chlorine and control the pH of the solution than to double the amount of chlorine in the solution.