I had a conversation with Dr. Jay Pscheidt, OSU Extension Plant Pathology Specialist about root rot in blueberrries. I thought it's worth to share his thoughts.
A Phytophthora root rot problem is indicative of poor soil drainage and/or too much irrigation. Blueberries have the added problem of pH and simply do not grow well when out of the correct range. The top priority here is to be sure we are dealing with Phytophthora and not something else. If so, then the solution will be water management supplemented with other things such as chemical control. Correcting the pH and soil drainage would be the things to spend money on.
ALL of our surface waters can be considered contaminated with Phytophthora. Well water is not. Treatment of surface water is possible but not easy and would entail such things as large quantities of Chlorine containing materials such as bleach or copper metered into the water a 1ppm. There are other ways to do this from slow sand filtration to ozone but few operations outside of nurseries do this.
After that, we can talk about chemicals. High phosphorus materials that are used as fertilizer will not control this disease and materials that contain Phosphorus acid will not fertilize plants. Products such as Agri-Fos, Aliette, Fosphite and Phostrol are in general not as effective as Ridomil (given all things equal). These organisms can become resistant to Ridomil thus making the former materials work much better by comparison. We do recommend alternating from one to the other. Ridomil in the spring as roots begin to grow and phosphorus acid materials later when another flush of root growth occurs.