Phytophthora Root Rot

Phytophthora root rot is caused by Phytophthora species, water molds that are classified with fungi but belong to a separate lineage (oomycetes) and behave differently from true fungal pathogens. They produce motile zoospores that swim through saturated soil toward root tissue, which means standing water is not just a stressor for the plant but an active delivery mechanism for the pathogen. Once established in root tissue, the organism spreads rapidly, destroying the fine feeder roots that blueberries, raspberries, and strawberries depend on for nutrient and water uptake.

The conditions that favor infection are predictable: heavy, poorly drained soils; prolonged wet periods; and planting depths that set crowns too low. The disease is most destructive in spring and fall when soils stay saturated for days at a time. Symptoms (yellowing, wilting, and sudden decline after wet weather) often appear weeks or months after the initial infection event, which makes tracing the cause difficult.

Site selection and drainage are, by a wide margin, the most cost-effective management tools. Raised beds and berms physically remove root zones from the saturated zone where zoospores concentrate. Improving soil organic matter helps with drainage and microbial competition against the pathogen. Where infections are already established, phosphonate fungicide drenches (such as potassium phosphite) can slow progression but do not eliminate the pathogen from infested soil. No commercially available blueberry varieties carry strong resistance to Phytophthora root rot in current production systems, so structural site corrections remain the primary defense.