Mortality of Pythium Zoospores in Chlorinated Irrigation Water

In October of last year, I published a blog, Disinfecting Irrigation Water for Disease Management, where I pointed out that plant disease can be introduced through irrigation water. In that blog, I described alternative disease control strategies that could be used when using recirculated irrigation water. Probably the easiest water disinfection, or more appropriately, disinfestation technology that a greenhouse grower can adapt to their existing irrigation system is a chlorine injection system.

Chlorine is a strong oxidizer and an excellent sanitizing agent. Chlorinating water by adding sodium hypochlorite, calcium hypochlorite, chlorine gas, or chlorine dioxide is an economical method for managing plant pathogens including Pythium. However, factors other than total chlorine concentration may affect the disinfestation potential of a chlorine solution. These factors include temperature, pH, and organic content in water.

During the process of oxidation, hypochlorous acid is reduced to hypochlorite, which has weaker disinfestation properties. Therefore, it is important to maintain and monitor the concentration of the oxidizer in the water to ensure sanitation.

The oxidation reduction potential (ORP), measured in mV, is an alternative method for expressing the sanitizing ability of a chlorine solution. ORP is the potential (voltage) at which oxidation occurs at the anode (positive) and reduction occurs at the cathode (negative) of an electrochemical cell. In other words, an oxidizing chemical, such as chlorine, oxidizes the cell membrane causing it to leak.

In collaboration with Dr. Ned Tisserat and his research associates Jillian Lang (who actually did the work) and Brittany Rebits, a study was conducted that used ORP to measure the disinfestant potential of various chlorine treatments against P. aphanidermatum and P. dissotocum zoospores. Since ORP is inversely related to pH at a given chlorine concentration, the water pH was lowered to observe its effect on ORP and subsequent Pythium zoospore mortality.

We exposed zoospores of Pythium aphanidermatum and P. dissotocum municipal water where the ORP was increased by adding 0.125, 0.5, and 2 mg/liter chlorine (pH 7.6 to 8.1) or to the same water source where the pH was lowered to 6.0 prior to the addition of chlorine, resulting in a final pH of 6 to 7.3. Some zoospores of P. aphanidermatum and P. dissotocum survived treatment to the highest chlorine concentration for 4 min in water where pH was not lowered.

When the water pH was lowered to 6.0 prior to chlorine addition, 100% of the P. aphanidermatum zoospores were killed after 0.5 min exposure to 0.5 mg/liter chlorine, where ORP ranged from 748 to 790 mV and pH 6.3. Lowering the initial water pH improved disinfestation of P. dissotocum zoospores at the highest chlorine concentration tested and a mean ORP of 790 mV.

Follow this link to read the entire article: Monitoring Mortality of Pythium Zoospores in Chlorinated Water Using Oxidation Reduction Potential