Incidence of Phytophthora and Pythium Infection and the Relation to Cultural Conditions in Commercial Blueberry Fields

Fifty-five commercial blueberry (Vaccinium spp.) fields were sampled in north- west Oregon in 2001 to determine the incidence of Phytophthora and Pythium root rot pathogens and identify cultural factors that increase the probability of developing infection. Phytophthora was detected in 24% and Pythium was detected in 85% of the fields sampled. The only species of Phytophthora identified in the study was P. cinnamomi. Root infection by P. cinnamomi was significantly related to cultivar with incidence observed more frequently than expected in 'Duke' and 'Bluecrop'. Both blueberry cultivars are two of the most popular grown in the region, representing 42% of the fields in this survey and '46% of the total area planted in Oregon. Two other cultivars found infected by P. cinnamomi were 'Rubel' and 'Briggitta Blue', together accounting for an additional 24% of the fields surveyed. Phytophthora was not detected in fields planted with 'Berkeley', 'Bluejay', 'Bluetta', 'Darrow', 'Earliblue', 'Elliott', and 'Powderblue', each of which represented only 2% to 7% of the fields surveyed. Pythium spp. were identified to genus only, but one or more species ofPythium was found in all 11 cultivars included in the survey. Occurrence of either Phytophthora or Pythium was unrelated to soil type, planting age, or cultural practices such as bed type, cover crop, mulch, irrigation system, fertilizer application, fungicide use, or the source of plant material used in the fields. Overall, most fields with Phytophthora or Pythium remained largely symptomless under good soil drainage conditions and had similar levels of vigor as those without the pathogens.