Cause Phytophthora root rot, caused primarily by the oomycete Phytophthora cinnamomi, is a soilborne disease affecting blueberry in the Pacific Northwest. It is primarily a warm weather pathogen and does not survive well where soils freeze deeply or repeatedly in winter. This fungus-like organism has an extremely wide host range and infects a large number of tree and shrub species in the Pacific Northwest including other ericaceous plants (such as rhododendron and madrone) and many conifers.
The organism is spread in water, soil, and on infected nursery stock. It can survive for many years in the soil as chlamydospores or oospores. When soil moisture is abundant, it produces swimming spores called zoospores. These zoospores swim towards fine roots where they infect and kill the root system. As a consequence, root rot is much more severe in poorly drained, heavy soils. The disease is often considered mainly to be a problem of young plants during the establishment phase. Older plants with larger root systems are better able to compensate for root rot without apparent symptoms as long as the site is well-drained. Use of fresh sawdust is conducive to these fungi in other crops while use of composed bark is suppressive.
While all highbush blueberry cultivars are susceptible to the pathogen, some, such as 'Aurora,' 'Clockwork,' 'Legacy,' 'Liberty,' 'Overtime,' 'Patriot,' and 'Reka,' appear to be slightly more resistant to the disease. Cultivars Bluecrop, Bluetta, Blue Ribbon, Cargo, Draper, Duke, Last Call, Top Shelf, Toro, and Ventura are considered very susceptible.
Symptoms Symptoms are usually most severe on young plants. Initially, infected plants are stunted, appearing smaller than neighboring healthy plants. The leaves may be undersized and chlorotic, and may turn red prematurely. Leaf scorch symptoms (necrotic leaf margins) can also develop as the root system becomes compromised. Severe infection can lead to wilting and death, particularly in younger plants or under conducive conditions. However, older plants with larger root reserves can survive for years with a chronic infection and may never fully succumb to the disease. Infection in the roots is first observed causing a brownish-black rot on the fine feeder roots. Lesions may extend up into larger roots and eventually up into the canes, producing a firm, reddish-brown rot that is delineated from the surrounding, healthy white-green tissue at the lesion's leading edge.
Cultural control Integrate several cultural and/or chemical tactics to manage this disease.
- Plant only disease-free plants. Inspect and discard any incoming diseased nursery stock.
- Amend soils with composted sawdust or bark mulch to provide good aeration for roots.
- Plant in beds raised so that the top of the bed is at least 12 inches above the surrounding soil. Amend soil with gypsum (1 to 5 tons/A) before making raised beds and planting.
- Do not over-irrigate, especially in drip irrigation fields, so that the soil is saturated for 48 hr or more. Irrigate deeply and allow drying between irrigations. It is recommended to bury drip lines or place emitters 1 foot away from the base of the plant.
- Do not overfertilize with nitrogen. Higher levels of phosphorus and potassium appear to help by stimulating root production. High levels of nitrogen, which normally would cause only leaf burn on healthy plants, may kill Phytophthora-infected plants.
- Ensure drainage away from plants in wet or poorly drained soil. Subsoiling at 18 to 24 inches beside the row often helps. Planting into raised beds or hills will also help.
- Dig out plants with 50% or more mortality to roots and tops. They are unlikely to recover even with chemical treatment.
- Avoid reusing pots from a previous crop for propagation. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.
Chemical control Protect roots just before significant growth. Blueberry plants generally have two main root growth peaks. One in the spring just before shoot growth begins and another in late summer after harvest. Time chemical applications just before these peaks. Alternate chemicals from different groups with different modes of action to avoid problems with resistant strains. Chemical control not recommended for plants showing moderate to severe symptoms.
The Group 4 and Group P7 fungicides used to manage Phytophthora do not kill this organism. They can only prevent establishment of the organism before it gets into the plant. They can also prevent continued growth if the organism is already inside the plant thereby delaying symptoms that might have developed. Once chemical activity has subsided with time, the organism can resume growth within infected plants.
- Agri-Fos at 2.5 quarts/A. Group P7 fungicide. 4-hr reentry. WA only.
- Aliette WDG at 5 lb/A. Use as a foliar spray starting at budbreak. Do not use with or near copper applications. Can be applied on the same day as harvest with a 12-hr PHI. Do not use with adjuvants. Group P7 fungicide. 24-hr reentry.
- Fosphite at 1 to 2 quarts/A. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- MetaStar 2E at 2 gal/A broadcast to the soil at time of planting then at 1 pint/1000 linear ft of row in a three-foot band over the row once established. Group 4 fungicide. 48-hr reentry.
- Organocide Plant Doctor at 2 to 6 teaspoons/gal water as a foliar spray. Group P7 fungicide. H
- Orondis Gold at 28 to 55 fl oz/A as a drench, soil-directed spray or through irrigation water. Do not use within 1 day of harvest. Group 4 + 49 fungicide. 48-hr reentry.
- OxiPhos at 2.5 to 5 quarts/A as a foliar spray. Group P7 fungicide. 4-hr reentry.
- Phostrol at 2.5 to 5 pints/A. Group P7 fungicide. 4-hr reentry.
- Rampart at 1 to 3 quarts/100 gal water/A. Group P7 fungicide. 4-hr reentry.
- Ridomil Gold SL at 3.6 pints/A applied in 3-ft band centered on the plant row or through drip irrigation system before established plants start growth in spring or at planting for a new field. Group 4 fungicide. 48-hr reentry.
Reference Bryla, D.R. and Linderman, R.G. 2007. Implications of irrigation method and amount of water application on Phytophthora and Pythium infection and severity of root rot in highbush blueberry. HortScience 42:1463-1467.