Cause The fungus-like microorganisms Phytophthora lateralis and P. cinnamomi kill Port-Orford-cedar seedlings and trees. The disease was found in nursery stock in 1923 and has since spread throughout the Pacific Northwest, in the tree's native range, southwest Oregon, and in landscapes with extensive hedgerows. Spread into the southwest Oregon mountains has been slow but progressive. Dead trees can be found along streams and in drainage ditches on the down-slope sides of roads.
These fungus-like microorganisms survive as oospores or chlamydospores in plant debris such as rotted roots or foliage. Spores may be moved long distances in contaminated soil through human activities (for example, by logging equipment), by cattle, or through moving water. Spores germinate and produce sporangia and a swimming spore stage called zoospores, which attack fine roots and begin to rot the root system. Tree-to-tree spread can occur through root grafts. Spores splashed on leaves or stems can initiate a foliage blight. Infection also can occur through wounds.
Port-Orford-cedar is highly susceptible to P. lateralis while Alaskan cedar (C. nootkatensis) is seldom killed. C. taiwanensis has variable results. Other species of Chamaecyparis are considered resistant including C. obtusa, which can infrequently become infected. All are considered susceptible to P. cinnamomi, which has an extensive host range.
Symptoms The organism infects small roots first, then larger and larger roots until the tree is girdled near the collar. The fine roots are at first water soaked, then darken and quickly disintegrate. Infected inner bark turns cinnamon-brown in contrast to the normal cream color of healthy tissue. There generally is a sharp line between infected and healthy tissues. Infected trees' foliage is a slightly lighter color and may wilt on warm days. All foliage soon turns chlorotic, then bronze, and finally brown. Seedlings may die a few weeks after infection; large trees may take 2 to 4 years to die. In hedgerow landscape plantings, trees typically die one by one over several years as the organism moves between adjacent trees. Dead and dying trees may be attacked by bark beetles and are more likely to be blown down by strong winds.
Foliar symptoms are rare. Infections that begin on leaves or stems start locally but can spread down the stem or to other branches. Stem lesions are sunken and kill all foliage distal to the canker.
- Plant healthy seedlings in soil known to be free of the organism.
- Do not plant in soil with poor drainage or in areas that receive drainage from roads.
- Prevent wounding at the base of trees or to roots from nearby construction or harvesting operations.
- Avoid extensive gardening (mulching and planting susceptible flowering plants) under cedar hedges because it may introduce the disease and/or wound tree roots.
- Promptly remove and destroy dead and dying trees to help protect other trees in the area. In landscape hedges, also remove healthy-looking trees next to diseased ones. Cedar bark beetle activity in trees has been associated with trees with root rot and may be good indicators of which healthy appearing trees need to be removed. Avoid moving infested soil while removing trees and/or stumps.
- Plant resistant species in contaminated ground. If P. lateralis is the only problem (which is very likely) then Chamaecyparis formosensis, C. thyoides, C. pisifera, C. obtusa, and Cupressocyparis leylandii may be planted. If P. cinnamomi is the problem (which is possible), there are few options for planting besides a few crabapple and rhododendron species. Correct any drainage problems before replanting.
- Resistant Chamaecyparis lawsoniana are now available including susceptible scions grafted onto resistant rootstocks.
- In sensitive forested areas of southwest Oregon, unpaved roads are closed during the rainy season. Also, consider removal of cedars along the road side to prevent infections in those areas.
- 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 These chemicals may help prevent infection or allow slightly infected trees to survive the disease given annual spring and/or fall applications. Severely infected trees will not benefit and will die. 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 as a foliar spray to nursery plants. Do not reapply within 30 days. Group P7 fungicide. 12-hr reentry.
- Alude at 5 to 10 fl oz/100 gal water applied as a soil drench at a rate of 1 gal solution/sq yd. Follow application with irrigation. Can also be used as a foliar spray at 26 to 54 fl oz/100 gal water at 14- to 21-day intervals. Group P7 fungicide. 4-hr reentry.
- ArborFos (from Mauget) can be injected into trees at a rate of 5 ml/inch diameter at breast height. See label for details. Unknown efficacy in the Pacific Northwest.
- Banrot 40 WP at 6 to 12 oz/100 gal water for container- or bed-grown plants. Group 1 + 14 fungicide. 12-hr reentry.
- Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- Mefenoxam 2 AQ at 0.98 to 1.96 oz/100 gal water as a soil drench or at 1.23 to 2.45 oz/1000 sq ft followed by at least 0.5 inch rain or irrigation. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
- Organocide Plant Doctor at 16 fl oz in 16 fl oz water plus 1 oz Pentra-Bark as a basal trunk spray. Also labeled for injection, see label for details. Can be used in landscape sites. Group P7 fungicide. H
- Phospho-Jet is registered for tree injections. Rates are based on tree size. Group P7 fungicide. 4-hr reentry.
- Subdue MAXX as a soil surface spray at 1.25 to 2.5 fl oz/1000 sq ft; irrigate with 0.5 inch water within 24 hours of application. 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.
References Hansen, E.M., Goheen, D.J., Jules, E.S., and Ullian, B. 2000. Managing Port-Orford-cedar and the introduced pathogen Phytophthora lateralis. Plant Disease 84:4-14.
Robin, C., Brasier, C., Reeser, P.W., Sutton, W., Vannini, A., Vettraino, A.M., and Hansen, E. 2015. Pathogenicity of Phytophthora lateralis lineages on different selections of Chamaecyparis lawsoniana. Plant Disease 99:1133-1139.