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The disease is caused by the soil-inhabiting fungus Phytophthora fragariae. Healthy roots are infected by spores produced from other infected plants. These spores can move through the soil and penetrate the tips of roots, growing within the root system. A few days after infection, the roots begin to rot. The fungus produces spores within this rotted tissue; eventually, the spores and the rotted roots become incorporated into the soil. Frequently, the diseased area in the bed follows a definite pattern. Plants showing above ground symptoms occur in patches where the soil is wettest. Symptoms depend on the severity of the root rotting. Severely diseased plants are stunted, with the younger leaves turning a blue green and the older ones, red, yellow, or orange. Plants will eventually wilt and die. As the number of diseased roots increases, plant size, yield, and berry size decrease. When a young, infected root is cut open lengthwise, the stele or core above the rot is red. This diagnostic symptom occurs when the soil is cool. As the disease progresses, the lateral roots die, giving the main roots a "rattail" appearance.
Because the spores of this pathogen can travel long distances in surface water, maintaining good drainage in strawberry beds is important. This also minimizes runoff and avoids soil compaction. Plant resistant varieties and purchase stock that has been inspected and tested for P. fragariae. Also remember that there are several different strains of red stele fungi and not all varieties of strawberries marketed as "red stele resistant" are equally resistant to all strains of the pathogen
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