Establishment of southern highbush blueberry cultivars and suppression of Phytophthora root rot using cover crops and soil amendment treatments

Authors: Smith, Barbara; STAFNE, ERIC - Mississippi State University

Submitted to: PhytoFrontiers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2025
Publication Date: 3/14/2025
Citation: Smith, B.J., Stafne, E.T. 2025. Establishment of southern highbush blueberry cultivars and suppression of Phytophthora root rot using cover crops and soil amendment treatments. PhytoFrontiers. https://doi.org/10.1094/PHYTOFR-09-24-0096-R.
DOI: https://doi.org/10.1094/PHYTOFR-09-24-0096-R

Interpretive Summary: Phytophthora root rot is a major disease of blueberry; however, disease control options are limited due to a lack of effective chemicals and resistant cultivars. Disease management relies primarily on cultural practices, including the addition of amendments to the soil before planting to increase organic matter, improve soil structure and drainage, and suppress pathogens. In a six-year field study, we evaluated the ability of cover crops to suppress Phytophthora root rot infection and reduced symptoms in three blueberry cultivars grown in a field known to be infested with the pathogen. Three living cover crops, an organic amendment, a fungicide drench, and an untreated control treatments were applied in the spring and again in the fall, plowed into the soil six months later; and after three years, all six treatments were plowed into the soil, and the field was planted with blueberries. Plant growth measurements were taken periodically over three years; and at the conclusion of the study, shoot and root measurements were collected. No differences were identified between the control treatment and the three living cover crop treatments (brassica, grass, and legume) in leaf or flower development scores or in plant vigor scores; however, plants in the organic amendment (ground pine bark) treatment scored highest in each of these parameters. Final plant measurements (stem count, total stem length, living stem length, stem weight, stem diameter, percentage of living stems, root length, root ball diameter, root ball weight) from the bark treatment were also higher than those of plants from all other treatments. Among the cultivars, ‘Gumbo’ outperformed ‘Pearl’ and ‘Legacy’ in most measured parameters. Neither of two bioassays detected differences among the six treatments in Phytophthora root rot levels. Even though this study failed to demonstrate differences among treatments in disease levels, it did dramatically show that the ground pine bark amendment improved blueberry plant growth compared to that of the other five treatments. This supports previous reports of the beneficial effect of organic soil amendments (e.g., peat moss, sawdust, bark) on blueberries. Blueberry growers, extension and research scientists, and homeowners will benefit from these findings.

Technical Abstract: Phytophthora root rot, caused by Phytophthora cinnamomi, is a major disease of blueberry in most regions where they are grown. Phytophthora root rot resistant blueberry cultivars are not available and chemical control options are limited; therefore, management of Phytophthora root rot relies primarily on cultural practices including the addition of soil amendments before planting to increase organic matter, improve soil structure and drainage, and suppress pathogens and nematodes. Our objective was to compare the ability of cover crops to suppress Phytophthora root rot infection and reduce symptoms in young southern highbush blueberry cultivars grown in a field known to be infested with P. cinnamomi. The six treatments [three cover crops (Legume, Grass, and Brassica), aged ground pine bark (Bark), fungicide drench (Ridomil), and untreated control (Control)] were applied in the spring and fall each year and tilled into the soil six months later. After three years, all treatment plots were tilled into the soil, and one-year-old plants of three southern highbush cultivars were transplanted into each treatment plot and grown for three years. Blueberry plant growth measurements were taken periodically over three years; and at the conclusion of the study, shoot and root measurements were collected. Two Phytophthora bioassays failed to detect any suppressive effect of the six treatments. There were no differences between the Control treatment and the three cover crop treatments (Brassica, Grass, and Legume) in leaf and flower development scores or plant vigor; however, plants in the Bark (organic amendment) treatment scored higher in each of these parameters. After three years growth, final plant measurements from the Bark treatment (stem count, total stem length, living stem length, stem weight, stem diameter, percentage of living stems, root length, root ball diameter, root ball weight) were all greater than those of plants from all other treatments. Among the cultivars, ‘Gumbo’ outperformed ‘Pearl’ and ‘Legacy’ in most measured parameters. These results support previous reports of the beneficial effect of organic soil amendments (e.g., peat moss, sawdust, bark) on blueberries.