This blog is focused on one of the major diseases associated with poor water management, Phytophthora root and crown rot. Phytophthora, a mouthful, is pronounced [fahy-tof-ther-uh]. This blog is one of a series of articles on landscape trees and what causes trees to fail to thrive and then perhaps die. Topics include watering of landscape trees and managing oak root fungus (Armillaria).
Landscape trees are subject to diseases often associated with poor water management. When you notice a tree struggling, one of the first things to do is to examine how the tree is being watered, environmental conditions, and the soil moisture levels over time. Symptoms are often like ones associated with drought-stress and other diseases, hard to distinguish one from the other.
Diagnosis of tree problems can be difficult, and trees are a valuable portion of our landscape, not easy to replace; so hiring a consulting arborist may well be money well-spent. Consulting arborists are tree experts whose role is to bring a comprehensive, objective viewpoint to the diagnosis, appraisal, and evaluation of arboricultural issues. The correct diagnosis can help save or prolong the life of the tree.
A Common Root and Crown Disease—Phytophthora
Phytophthora is common in California and it can be found in many plant nurseries, natural ecosystems and in the home garden.
Phytophthora disease affects a wide range of trees and shrubs, including many California natives and crops such as tomatoes. Almost all fruit and nut trees, including citrus and avocados, can develop root or crown rot. Phytophthora root and crown rot is caused by soil-inhabiting water molds, also known as oomycetes, that thrive and spread in moist conditions.
Phytophthora root and crown rot is primarily a disease of heavy or waterlogged soils.
Trees and shrubs can develop root and crown rot if the soil around the base of the tree remains wet for prolonged periods, or when planted too deeply. There are a number of different species of Phytophthora. Phytophthora can persist in the soil for years.
Phytophthora species are an invisible threat to natural ecosystems, nurseries and the home garden.
These plant pathogens can remain undetected in container plants, field equipment and even footwear. They have been inadvertently introduced to wildland communities in the Western US and around the world. Infected plants in the nursery may show no symptoms until after transplanting.
Infected nursery stock has intensified the spread of Phytophthora.
Phytophthora ramorum causes sudden oak death (SOD), which has killed millions of tanoak and coast live oak since being first detected in 1995; it was introduced into California forests by infected nursery stock. P. lateralis, which was inadvertently introduced on infested ornamental nursery stock, has spread to wildlands in Southern Oregon, infecting and killing Port-Orford-cedar. According to Oregon State University, shipping and planting of infected nursery stock is considered the most common means for the introduction of new Phytophthora species into landscapes and habitats worldwide.
Phytophthora Symptoms
Aboveground symptoms of Phytophthora rot mimic many other conditions and diseases.
Anything that cuts off water to the top of a plant results in wilting, leaf yellowing, leaf browning, and premature leaf fall and plant death. Causes include too much water, lack of water, fungal root rots as well as Phytophthora root rot. Affected trees or plants often wilt and die rapidly during the first warm weather of the season. As hot, dry weather sets in, the plant may not have enough functional roots to keep up with transpiration.
Belowground symptoms of the roots, collar and stem base will reveal a poor root system.
The plant will have few, if any, feeder roots while remaining roots are brown or black internally and decaying. Phytophthora root rots generally start below ground and spread up the plant. Phytophthora can invade the collar or stem base of the plant causing a brown or black discoloration below the bark.
Additional information on exceptions to the rule and specific symptoms for particular plants can also be found here. An Oregon State University Extension article aimed at nurseries has informative photos of compare and contrast of healthy and infected plants.
Quick Tips on Managing Phytophthora Root and Crown Rot
Below is the list of the symptoms and tips on management taken from the UC IPM Quick Tips on Phytophthora. The list/tips have been lightly edited.
The symptoms described below do not apply to Phytophthora ramorum, a species that causes sudden oak death. P. ramorum has very different symptoms and management. See the Pest Note on Sudden Oak Death in California.
Overall symptoms of root and crown rot
- Leaves may wilt and turn dull green, yellow, red, or purplish.
- Generally, plants in the most poorly drained areas are affected.
- Unlike plants suffering from water stress, plants don't recover when irrigated.
Symptoms on roots and crowns
- Darkened areas in the bark around the crown and upper roots often develop, sometimes with gumming. How to distinguish Phytophthora root rot from bacterial canker
- Reddish-brown streaks or zones might be seen in the inner bark and outer layer of wood.
- If mushrooms are present or fungal filaments are visible within plants, damage is likely due to a fungal disease such as Armillaria root rot, not Phytophthora.
Water management is the key to preventing root and crown rot.
- Avoid prolonged soil saturation or standing water around tree bases.
- Adjust sprinkler heads and drip systems so water doesn't hit trunks and lower branches.
- Provide good soil drainage to rooting depth of plants: generally 3 to 6 feet for trees, 2 to 4 feet for shrubs, and 1 to 2 feet for bedding plants.
- Avoid allowing the roots and crown of the plant to remain wet for the 4 to 8 hours that are required for Phytophthora to infect the plant during favorable weather.
(As noted in the Pest Note, some Phytophthora species are favored by warm weather, some by cool weather. Root rots of avocado, citrus, and tomato are favored by warm conditions, developing most extensively in late spring and early summer. Decay of crown, trunk, and branches of other tree species are favored by cool, wet conditions. These decays develop most rapidly in late fall and early spring.)
Take steps to avoid crown and root problems when you plant.
- Don't plant trees or other plants deeper than they were planted in the nursery, and never cover the graft union with soil or mulch.
- Select certified nursery stock and resistant rootstocks or varieties when available.
- Try not to have irrigated turfgrass around the base of trees.
- If Phytophthora has been a problem on tomatoes, plant a nonsusceptible crop such as sweet corn in its place for a year or two.
- Avoid planting susceptible species on poorly drained or shallow soils.
- If you do plant in poorly drained soil, or in an area where you know Phytophthora is present, consider planting trees and shrubs on mounds that are 8-10 inches high.
Trees can sometimes be saved if crown and root rot is caught early.
- At the first sign of leaf wilting, examine the tree at the soil line for rot.
- Cut away any bark that looks affected.
- Remove soil from around the base of the tree down to the top of the main roots and allow the crown tissue to dry out.
- Keep water away from the crown and trunk by only watering at the dripline, the area underneath the canopy.
- Young infected trees may die rapidly; older trees may survive for several years.
Additional management steps for crown and root rot
Sanitation
Avoid movement of infested soil, water and plant parts from where Phytophthora rot has developed. Drainage water above and below ground, and anything (like tools and footwear) that carries moist soil can move the pathogen to a new area. During wet weather especially, channel water from areas infested so that the water is directed away from uninfected garden areas and unaffected plants.
Rotation
If a plant is affected by Phytophthora root rot, avoid planting susceptible plants in the same soil for at least one to two seasons. Alternatively, leave the soil unplanted and do not irrigate; work the soil to allow it to dry as deeply as possible.
Selection of healthy, resistant plants
Plant only certified nursery stock from a reputable source.
Carefully select individual plants, free of symptoms, inspecting the roots where possible. Avoid plants that are severely pot-bound, or with brown roots on the outside of the root ball.
Choose the most resistant rootstock or varieties.
Below is some information on susceptibility to Phytophthora and information on native plant nurseries working with the Nursery Phytophthora Best Management Practices (NPBMPs).
Trees and shrubs not reported to be susceptible to Phytophthora cinnamomi
Information on diseases that plants are susceptible are available on UC IPM home and landscape Trees, Shrubs and Woody Ornamentals website.
Nurseries that are participating in the cleanliness of restoration and native plant nursery stock
Chemical control
As stated in the Pest Note on Phytophthora Root and Crown Rot,
“the most effective way of preventing Phytophthora rot diseases is to provide good drainage and to practice good water management. Along with the appropriate cultural controls, the fungicide fosetyl-al (Aliette) may be used on a number of ornamental plant species to help prevent Phytophthora infections. When applied as a foliar spray it is absorbed by foliage and moves into roots. However, do not rely on fungicide applications alone to control root and crown rot diseases.”
Information on Fosetyl-Al (Aliette) from the Environmental Protection Agency (PDF)
Summary of Management of Phytophthora Root and Crown Rot
California's Mediterranean climate (mild, wet winters and dry summers) provides favorable conditions for the spread of Phytophthora. The best way to manage Phytophthora root and crown rot is to provide good drainage and practice good water management. If possible, use nursery stock that has been certified to be free of Phytophthora, or, at a minimum, inspect roots and reject plants that show root or crown rot.
Resources:
- UC IPM Pest Note Phytophthora Root and Crown Rot
- Quick Tips on Phytophthora
- Accreditation to Improve Restoration (AIR) Protecting Native Habitats by Excluding Phytophthora in the Nursery
- Missouri Botanical Garden Phytophthora Root Rot of Trees and Shrubs
- Phytophthora overview from Master Gardeners in Fairfax County, Virginia
- Pacific NorthWest Handbook -- Plants Susceptible to Phytophthora Diseases
- Pacific NorthWest Handbook -- Diagnosis and Management of Phytophthora Diseases
- Preventing Phytophthora Infestations in Restoration Nurseries
- Phytophthora Diseases of CA Citrus by Greg Douhan, UC Cooperative Extension Ventura County, citrus farm advisor
- Phytophthora Species Are Common on Nursery Stock Grown for Restoration and Revegetation Purposes in California
- Sudden Oak Death
Want to learn more?
This blog post is brought to you by the Help Desk of the UC Master Gardeners of Alameda County. Subscribe to our blog!
Have a gardening question? We'll help. You can reach us by:
- Emailing acmg@ucanr.edu. Please include a photo of the problem, if you can, plus your name, phone number, city and a description of the problem.
- Using our online form.
- By phone, during our office hours, 10 am to noon Wednesday and 11 am to 1 pm Thursday: 510-670-5645. At other times, please leave a message and we'll return your call during our office hours.
- In person at our Hayward office, during our office hours, only by appointment