Phytophthora root rot is a serious soil-borne disease that kills the roots and root crown of infected plants, causing wilting, discoloration, stunted growth, premature leaf drop, decline, and eventual plant death. Poor water drainage at the base of the plant encourages the development of this disease.
Susceptible Hosts
Plants predisposed by stress and species that are intolerant of poor soil drainage, including: azalea, rhododendron, boxwood, hemlock, mountain laurel, dogwood, andromeda, fir, camellia, white pine, oak, beech, fruit trees, sugar maple, and zelkova.
Signs of Damage
- Wilting and rapid death of plant with the first warm weather of the season.
- Leaves may become discolored.
- Symptoms may develop on one branch, spreading to the rest of the plant.
- Trees may decline over several years before completely dying.
- Dark areas on the bark around the root flares.
- A dark ooze coming from the diseased trunk area.
- Red to brown streaks may be seen in the inner bark when the bark tissue is cut away.
Biology / Lifecycle
- Phytophthora is a soil-inhabiting fungus-like organism (oomycete) that requires high moisture to complete its life cycle and is most common under conditions of saturated soils.
- The pathogen might initially invade small roots as well as crown tissue. The pathogen is spread by movement of contaminated soil, diseased plants, or by splashing water.
- Even after host trees have died, Phytophthora spp. can survive for months until conditions are appropriate for growth.
- On its own, phytophthora spp. spores can only move several centimeters while swimming with flagella. With flowing water as a vehicle, though, the spores can move much larger distances.
Black leaf spots on Ash (Fraxinus)
Typical bottom up defoliation
Black lesions on Maple (Acer)
Treatment Strategy/Expectations
Soil and water management are crucial to phytophthora control. The disease can only occur where water is present, and usually where water is excessive. Monitor irrigation carefully and only water as much as the plant needs. Inspect soil type and structure to always provide adequate drainage. This may mean breaking up compaction and eliminating hardpans. Make sure the crown or graft union of the plant is always above the soil line. Decompacting/Root Enhancement mulch, and proper watering can prevent infections from beginning. This can also help trees defend against the pathogen once they are infected. Cambistat can help trees recover from stress. Along with the above cultural practices and Reliant, Cambistat can help trees regrow fine roots, increase leaf thickness, and increase trichome leaf hairs.
| Product | App. Method | Dosage | Timing | Re-Treatment |
| Soil application | 6 ¼ - 12 ¾ fl.oz./100 gal water. Apply 25gal per 100 sq ft | Anytime during growing season prior to symptom development | Monthly as needed | |
| Foliar spray | 1 3/8 pints/100 gallon water | Apply at bud break | 2-3 times on 14-day intervals | |
| Foliar spray | 20 fl oz/100 gallon water | Apply at bud break | 2-3 times on 14-day intervals |
*Product is Rainbow Ecoscience’s industry standard recommendation for most industry situations.
**Cambistat is a plant growth regulator that induces a disease resistance response in trees.
Reach out to Rainbow Ecoscience Technical Support for additional protocol support.
Foliar spray with Maruyama
Soil application with HTI 2000
Always refer to product label for rates and approved uses. Some images courtesy of forestryimages.org or Wikimedia Commons. Use of the images does not imply endorsement of treatments.
