Cause Several. Many samples sent to the OSU Plant Clinic are from poorly drained and wet sites where oxygen depletion to the roots is likely. Occasionally, pathogens such as Phytophthora lateralis or Armillaria solidipes (formerly A. ostoyae) are associated with rotted roots. The latter may be found on ground recently cleared of native forest vegetation.
Cultivars and species differ in susceptibility to Phytophthora root rot. Western red cedar (Thuja plicata) is most resistant, T. occidentalis 'Pyramidalis' is intermediate, and T. occidentalis 'Smaragd' is most susceptible.
Symptoms Aboveground symptoms are general discoloration of foliage and eventual death. Roots are discolored below the bark in the region of the cambium. Roots in advanced stages are highly decomposed and break off easily.
- 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.
- Plant only in deep, well-drained soil.
- Correct drainage if water puddles near bushes.
- Use resistant species or cultivars.
- Permanently removing soil and exposing the crown and main root areas has been effective for Armillaria control in tree fruits grown in California and Australia and may be of benefit for managing infected bushes in the Pacific Northwest.
Chemical control Not recommended unless Phytophthora sp. has been identified. Even then, plants are most likely too far gone for chemical therapy to work. The Group 4 and Group 33 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. Rotate fungicides from different groups that have a different mode of action for resistance management.
- Aliette WDG at 2.5 to 5 lb/100 gal water for a foliar application. Group 33 fungicide. 12-hr reentry.
- Alude at 6.3 to 12.7 fl oz/100 gal water applied as a soil drench at a rate of 25 gal solution/100 sq ft. Follow application with irrigation. Use only once per month. Can also be used as a foliar spray at 1 to 2 quarts/100 gal water at 14- to 21-day intervals. Group 33 fungicide. 4-hr reentry.
- Empress at 1 to 3 fl oz/100 gal water can be used for seedlings. Group 11 fungicide. 12-hr reentry.
- Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 7 days of treatment and do not use spray adjuvants. Group 33 fungicide. 4-hr reentry.
- Mefenoxam 2 AQ at 0.98 to 1.96 fl oz/100 gal water as a soil drench or at 1.23 to 2.45 fl oz/1000 sq ft followed by at least 0.5 inch rain or irrigation. Group 4 fungicide. 48-hr reentry.
- PhosphoJet is registered for tree injections. Rates are based on tree size. Group 33 fungicide. 4-hr reentry.
- Subdue MAXX at 1.25 to 2.5 fl oz/1000 sq ft, irrigated in with 0.5 inch water. Group 4 fungicide. 48-hr reentry.
- Terrazole 35 WP at 3.5 to 10 oz/100 gal water as a soil drench. Use only in commercial nurseries and greenhouses. Group 14 fungicide. 12-hr reentry.
Reference Hansen, E., Reeser, P., Sutton, W., and Sims, L. 2012. Host and habitat index for Phytophthora species in Oregon. Forest Phytophthoras 2(1). doi: 10.5399/osu/fp.2.1.3026