It’s only natural to grab a garden hose when you see one of your trees looking parched. But if your tree is suffering from Phytophthora root rot, that’s actually one of the worst things you can do.
A common shrub and tree disease found across the DC Metro area, Phytophthora root rot is often associated with excessive soil moisture. So, while it can make trees appear like they need more water, the truth is, affected trees often need better drainage and slightly drier roots.
We’ll explain everything you need to know about this disease, the pathogen that causes it, and the best ways to address it so that you can protect your trees below.
What Is Phytophthora Root Rot?
Phytophthora root rot is a very common disease of plants and trees, caused by fungus-like organisms known as water molds (oomycetes) in the genus Phytophthora.
These organisms cause problems all over the globe. Some attack food crops, while others cause problems in natural habitats. And some attack landscape shrubs and trees. A few of the most noteworthy species in Northern Virginia and the surrounding metropolitan area include:
- Phytophthora cinnamomi: A major root-rot pathogen with a very broad host range that’s particularly problematic on wet sites.
- Phytophthora cactorum: A broad-host-range species associated with root, crown, and collar rots of numerous woody plants.
- Phytophthora citricola: Now treated as an entire species complex, these species can cause root rot, crown rot, and bleeding cankers on numerous woody hosts.
- Phytophthora cambivora: An important pathogen of landscape trees, this species often attacks beeches (Fagus spp.) and other hardwoods.
Then there is the species that strikes fear in the hearts of tree lovers everywhere: Phytophthora ramorum, the pathogen responsible for a disease called sudden oak death. This species hasn’t yet become established in our area, but it has been detected in nursery stock.
It’s rarely important to distinguish these organisms to the species level, which is fortunate, as doing so typically requires laboratory testing. Instead, many common Phytophthora root rot infections can be treated or managed in the same basic manner.
How Does Phytophthora Spread?
Unfortunately, Phytophthora organisms can persist in the soil for many years.
So, unlike some other pathogens that find their way to your property via insects, wind, or some other means, they may already be present in your landscape.
These organisms create motile spores called zoospores, which move through the environment via splashing rain or runoff water. These zoospores often infect the fine roots of plants and trees, though they may not cause obvious or serious illness.
However, when conditions become favorable to the organism’s growth, they can spread, causing the infection to move up through the major roots and even into the stem. At this point, the pathogen starts damaging vascular tissues and disrupting the ability of a tree or shrub to transport water and nutrients.
Phytophthora Root Rot: Symptoms of an Infection
Phytophthora root rot typically causes only subtle symptoms at first, though these can quickly become serious and result in the infected plant’s death (Phytophthora comes from the Greek words for “plant” and “destroyer”).
Symptoms can differ depending on the specific Phytophthora species in question as well as the plant or tree it’s attacking. However, some of the most common symptoms include:
- Wilting shoots or foliage
- Yellowing (chlorotic) foliage
- Leaf scorch or marginal browning
- Undersized leaves
- Reduced shoot growth
- Premature leaf drop
- Thinning canopy
- Twig and branch dieback
- Crown decline
- Red to brown discoloration beneath the bark
- Brittle or decayed roots
- Root-collar or basal cankers
The challenging part is that many of these symptoms mirror those produced by water stress. That’s part of the reason it is wise to secure professional assistance at the first sign of trouble — an ISA-certified arborist can assess your trees and determine what’s causing them to struggle.
However, the following table provides some clues for distinguishing Phytophthora root rot from drought stress. As you’ll see, some of the symptoms are similar in both cases, but there are a few helpful clues homeowners can consider.
Phytophthora Root Rot vs. Drought Stress
| Characteristic | Phytophthora Root Rot | Drought Stress |
|---|---|---|
| Soil Conditions | Often associated with wet, poorly drained, or frequently saturated soils | Typically associated with dry soil and prolonged droughts |
| Wilting | Common | Common |
| Leaf Symptoms | Yellowing, wilting, scorching, premature leaf drop, or undersized foliage | Wilting, curling, browning, scorching, and premature leaf drop |
| Canopy Symptoms | Sparse foliage, reduced growth, branch dieback, and canopy thinning | Sparse foliage, reduced growth, branch dieback, and canopy thinning |
| Root Condition | Fine roots may be decayed, discolored, soft, or absent | Roots generally remain structurally intact unless drought is severe |
| Root Collar/Trunk | Some infections cause darkened or discolored tissue and cankers | Typically no root-collar lesions or cankers directly attributable to drought |
| Response to Watering | Additional irrigation may provide little benefit and can make the disease worse | Symptoms may improve when adequate soil moisture is restored |
| Pattern of Decline | May persist or worsen despite apparently adequate water | Often associated with inadequate rainfall and high temperatures |
| Diagnosis | Usually requires evaluation of the roots/root collar and site conditions; laboratory testing may be needed for confirmation | Often diagnosed from soil moisture, weather history, site conditions, and the plant’s response to irrigation |
If you’re seeing cankers, yellowing or undersized foliage, or root damage, it’s probably wise to have an ISA-certified arborist evaluate your trees — particularly if the soil appears damp or wet.
Tree Species at Highest Risk of Phytophthora Root Rot
Given that there are more than 180 different described Phytophthora species, including several that can infect a variety of plant and tree species, it shouldn’t be surprising that a wide variety of trees are vulnerable to Phytophthora root rot.
With that said, some trees and shrubs appear more vulnerable — or at least, infected more commonly — than others. A few of the species at greatest risk include:
- Flowering dogwood (Cornus florida)
- Japanese maple (Acer palmatum)
- Red maple (Acer rubrum)
- Oaks (Quercus rubra, Q. palustris, Q. phellos)
- American and European beech (Fagus spp.)
- Flowering cherries (Prunus spp.)
- Apple and crabapple (Malus spp.)
- Eastern white pine (Pinus strobus)
- Leyland cypress (×Cuprocyparis leylandii)
- Arborvitae (Thuja occidentalis)
- Eastern redcedar (Juniperus virginiana)
- Rhododendrons and azaleas (Rhododendron spp.)
- Boxwood (Buxus spp.)
- Yew (Taxus spp.)
- Japanese holly (Ilex crenata)
- Mountain laurel (Kalmia latifolia)
- Camellia (Camellia japonica)
Phytophthora Root Rot Treatments & Management: Can a Tree Recover?
There’s no doubt about it; Phytophthora root rot poses a serious threat to shrubs and trees across the DC Metro area. But that doesn’t mean it’s always a death sentence for the plants it infects. With prompt care, you can often limit the damage caused and help some trees recover.
A few of the most common treatment and management strategies used to combat Phytophthora root rot include:
- Improving Soil Drainage: Correcting poor drainage and reducing excessive irrigation can reduce the saturated soil conditions that favor Phytophthora.
- Correcting Root-Collar Problems: Removing excess soil or mulch from around the root flare can improve aeration and reduce moisture around crown and root tissues.
- Adjusting Irrigation Frequency: Watering deeply but less frequently and allowing soils to drain between waterings can help limit conditions favorable to the pathogen.
- Applying Fungicides or Phosphites: Products containing phosphites, phosphonates, mefenoxam, or other oomycete-targeting active ingredients may suppress Phytophthora.
- Supporting Overall Tree Health: Appropriate mulching, irrigation, and other cultural practices can help affected trees tolerate root loss and support root growth.
- Removing Severely Affected Plants: Trees or shrubs with extensive root or crown damage may not recover and may eventually require removal.
Just remember that you’ll have the best chance of saving your trees if you obtain professional help quickly.
Phytophthora Root Rot: Frequently Asked Questions
Given that it is potentially deadly, relatively common, and presents a lot of confusing symptoms, Phytophthora root rot is a subject that sparks a lot of questions. Here are answers to some of the most common ones homeowners have about this disease.
Can a tree recover from Phytophthora root rot?
Trees with mild or early infections may recover or remain stable with proper management. However, extensive root or root-collar damage can be irreversible, and severely affected trees may eventually require removal.
Can Phytophthora root rot be treated?
Yes, but treatment typically focuses on management rather than eliminating the pathogen. Improving drainage, correcting irrigation practices, exposing buried root collars, and applying appropriate fungicides or phosphites can help suppress the disease.
Can Phytophthora spread from one tree to another?
Yes. Phytophthora produces spores that can move through water and contaminated soil. The pathogen can also be introduced to a property through infected nursery plants, soil, or growing media.
Does Phytophthora stay in the soil?
Yes. Some Phytophthora species produce long-lived survival structures that can persist in soil or infected plant material, making site management and careful plant selection important even after an affected plant is removed.
Can I replant after losing a tree to Phytophthora root rot?
Often, yes, but simply replacing the tree without addressing the conditions that contributed to the disease can lead to another failure. Drainage problems should be corrected first, and a resistant or less-susceptible species may be a better choice.
How can I tell Phytophthora root rot from drought stress?
The symptoms can look remarkably similar. Both can cause wilting, leaf scorch, canopy thinning, and dieback. One important clue is soil moisture: A tree showing drought-like symptoms in persistently wet or poorly drained soil may be suffering from root disease rather than a lack of water.
RTEC Treecare: Diagnosing and Treating Root Disease
Wilting leaves, thinning canopies, and branch dieback don’t always mean your tree needs more water. When Phytophthora or another root problem is involved, adding water can sometimes make matters worse.
The ISA-certified arborists at RTEC Treecare can evaluate your tree, its root system, and the surrounding site conditions to help identify what’s causing the decline and recommend an appropriate treatment or management plan.
Contact us today to schedule your onsite evaluation.
Sources and References
- UC IPM – “Phytophthora Root Rot”
Peer-reviewed Pest Notes on Phytophthora root and crown rot, covering the water mold’s biology, symptoms in landscape trees and shrubs, and prevention through drainage, irrigation, and planting practices.
https://ipm.ucanr.edu/home-and-landscape/phytophthora-root-rot/ - University of Maryland Extension – “Root Rots of Trees and Shrubs”
Extension guide to identifying root rot diseases of woody plants, including those caused by Phytophthora, with symptoms, site conditions that favor them, and management options.
https://extension.umd.edu/resource/root-rots-trees-and-shrubs/ - Virginia Cooperative Extension (Virginia Tech) – “Phytophthora Root Rot of Rhododendron and Azalea” (Publication 450-615)
Extension publication by Mary Ann Hansen on Phytophthora root rot of rhododendron and azalea, covering symptoms, the causal species (P. cinnamomi and P. parasitica), and the other woody hosts commonly affected.
https://vtechworks.lib.vt.edu/server/api/core/bitstreams/c0e7ea63-7e7a-48eb-8ee0-909be02f0246/content - Missouri Botanical Garden – “Phytophthora Root Rot of Trees and Shrubs”
Home-gardener factsheet on Phytophthora root rot, describing symptoms such as wilting, canopy dieback, and dark decayed roots, along with cultural controls.
https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/diseases/rot/phytophthora-root-rot-of-trees-and-shrubs - Morton Arboretum – “Phytophthora Root Rot”
Arboretum plant-care resource on Phytophthora root rot, covering its symptoms, favorable soil conditions, and practices that reduce infection in landscape trees.
https://mortonarb.org/plant-and-protect/tree-plant-care/plant-care-resources/phytophthora-root-rot/
