See:
Cedar, Port Orford (Chamaecyparis lawsoniana) - Root Rot
Cause Several pathogens can cause root rots of cypress in the PNW including Phytophthora spp. and Armillaria sp. as on Cupressocyparis leylandii. A Fusarium sp. has been reported for Oregon. Most are diseases in nurseries and not in the landscape.
Phytophthora cinnamomi has been reported from California on nursery stock and likely occurs in the PNW. 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.
Notes: Phytophthora austrocedri was detected for the first time in the USA on juniper at two different Oregon nurseries and on Thuja sp. in 2024. Hosts of this new organism include arborvitae, cedar, cypress, and juniper including popular ornamental species such as Leyland cypress (Cupressus x leylandii) and Chinese juniper (Juniperus chinensis).
Symptoms Leyland cypress infected with P. cinnamomi have decayed and darkened roots. Aboveground trees exhibit stunted growth, general yellowing of the foliage, smaller branches and needles, and tip dieback.
Cultural control
- Plant healthy seedlings in soil known to be free of pathogens.
- 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.
- Promptly remove and destroy dead and dying trees to help protect other trees in the area.
- 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 by Phytophthora spp. 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 FRAC groups that have a different mode of action for resistance management.
- Aliette at 2.5 to 5 lb/100 gal water as a foliar spray to nursery plants. Do not reapply within 30 days. Do not use with adjuvants. Group P7 fungicide. 24-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.
- Monterey Garden Phos 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 at 1 to 2 quarts/100 gal water as a soil drench. Group P7 fungicide. 4-hr reentry.
- Phostrol at 51 fl oz/74 fl oz water plus an organosilicone surfactant used as a basal bark spray. Also labeled for injection, see label for details. 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 Martínez-Espinoza, A.D., Williams-Woodward, J. and Pearce, M. 2009. Diseases of Leyland cypress in the landscape. University of Georgia Cooperative Extension Bulletin 1229.
Zentmyer, G. A. and Munnecke, D. E. 1952. Phytophthora root rot of nursery stock. PDR 36:211-212.