Cause:
Phytophthora cinnamomi, a fungus-like microorganism. Another unidentified species of Phytophthora, isolated from Tualatin River irrigation water, has been shown to cause minor root rot to blueberries under greenhouse conditions. The fungus is transported in water and soil and, once established in soil, remains indefinitely. P. cinnamomi is a warm-weather pathogen and is most active in the summer. Use of fresh sawdust is conducive to these fungi in other crops.
Symptoms:
Early symptoms include overall stunting, small leaves, premature reddening of leaves, and eventual death. Dead leaves stay on branches; the result is a twiggy type of growth. Frequently, several canes on one side of the plant die and can be traced to dead roots on that side of the plant. (Water and nutrients are transported poorly from healthy roots to canes supported by damaged roots.) Fibrous roots usually die first, then the entire root system. Rot continues up through the root crown to stems, sometimes in a spiral fashion. The edge of the advancing red-brown lesion is distinct from the adjacent unaffected tissue. The rot is firm, not soft. Infected young plants with small root systems may die the first year. Larger plants may slowly decline for 3 to 4 years before dying.
What looks like nutrient deficiency or high soil pH is due to lack of roots from root rot.
Cultural
control:
Plant only disease-free plants.
Amend soils with composted sawdust to provide good aeration for roots.
Do not overirrigate, especially in drip irrigation fields, so that the soil is saturated for 48 hours 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.
Chemical
control: Alternate chemicals to avoid problems with resistant strains.
Agri-Fos at 2.5 qt/A. 4-hour reentry.
Aliette WDG at 5 lb/A. Use as a foliar spray starting at budbreak. Do not use with or near copper applications. Do not apply more than 20 lb/A/season. Can be applied on the same day as harvest with a 12 hour PHI. 48-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. 4-hr reentry.
Phostrol at 2.5 to 5 pt/A. 4-hr reentry.
Ridomil Gold SL. To avoid developing resistance, treat only until good root systems have developed. Discontinue use then, but monitor roots for recurrence of disease and re-treat on a limited basis only as necessary. 48-hr reentry.
Established plantings: Ridomil Gold SL at 0.25 pint/1000 linear ft of row (3.6 pints/A broadcast basis) in 3-ft band centered on the plant row before plants start growth in spring. In severe situations, an additional application may be made in midseason when irrigating or in fall. A second application usually is not needed.
New plantings: Apply Ridomil Gold SL in an 18-inch band centered on the plant row at or after planting using the 3.6-pints/A broadcast rate. The amount actually used per acre will depend on row spacing. Make supplemental applications at 2- to 3-month intervals.
Content edited by:
Jay W. Pscheidt on
January 1, 2009
