This tongue-twister of a plant disease, phytophthora (FY-toe-THOR-uh) root rot, is a menace every gardener dreads. The prevalence of phytophthora is on the rise in many places including Pennsylvania. If you haven’t heard of it, now’s the time to learn more. This disease can kill a lot of plants quickly if not halted.
I’m dealing with it right now, and it’s a bear! Ugh. So I called in the cavalry — the professionals. Here’s what I’m learning about it.
What is phytophthora root rot?
This disease encompasses several fungus-like microorganisms in the soil which produce spores. The spores, which exude toxins, are spread either by airborne or waterborne means, depending on the type. Most commonly, phytophthora is spread via storm runoff, flooding, poor drainage and frequent rains. These pathogens are present worldwide and constitute a global nuisance for home gardeners, commercial farmers and wholesale growers alike.
Heavy clay and soil compaction exacerbate the problem by reducing the amount of oxygen available to roots, and by retaining water. Their density can also impact plant health by making the roots work harder to penetrate the substrate and absorb nutrients. Urban and suburban plots are at high risk because their soils are often disturbed (compacted) by manmade development.
Once infection sets in, there is no cure, only management. A wide range of ornamentals, vegetables and shrubs are susceptible to phytophthora rot. Most will die without some kind of intervention. Fortunately there are many plants that are also resistant to it.
What are the symptoms?
Phytophthora is often present at low levels or dormant for years. Under normal weather conditions in well-drained soils, healthy, unstressed plants can fight off phytophthora. A garden can thrive for many seasons with no outward sign of its presence. However, if conditions change in ways that favor the pathogen, the chance of infection becomes much more likely.
The disease attacks the roots first, where it can take hold out of sight. Visible signs only become apparent once some above-ground damage has been done. Any or all of the below signs may be present:
- Blackening at the crown (also caused by other rots)
- Black streaks on stems arising from the crown
- Wilting of a few branches, seemingly at random
- Rapid dieback that seems abrupt
- Stunted growth, either generally or partially
- Discoloration of stem base in an inverted “V” shape
- Overall yellowing foliage with green veining (chlorosis)
- Loss of color vibrancy; conifers may turn grayish
- Neighboring plants may or may not show similar signs
If the affected plant is large enough to withstand further disturbance, you can expose a small section of the roots to look for more signs. If phytophthora is present you will likely notice:
- Lack of fine feeder roots
- Cinnamon-brown or black roots (healthy roots are pale)
- Sloughing-off of root skin
- Poorly anchored plant because roots have died off
Affected Plants in My Garden
Phytophthora is present in most of my soil, with the worst areas currently attacking a row of Schip laurels, a large mature Norway spruce, two peonies and a spirea hedge of some two dozen plants. Previously I lost a viburnum, holly, rose of Sharon, Chief Joseph pine, and a smoke tree to it. Several other perennials may also be affected but I’m not sure. (And this is after correcting for poor drainage and doing everything I can to halt the spread!)
Dealing with this menace is challenging, to be sure. So how do you deal with it? Well, nothing’s foolproof. But here are my suggestions.
What to Do
If you suspect phytophthora rot, call a licensed arborist (which I did) to take a sample and send it to a lab for testing; such evaluations are free of charge. Or take a sample yourself to your nearest ag-extension office for analysis. Many diseases present similarly, so you want to be sure of the diagnosis before deciding on a treatment regimen.
Unfortunately phytophthora is difficult if not impossible to eradicate. Once it’s in your soil it’s there to stay. If there aren’t many roots left on a plant or it’s more than half dead, it may be too far gone to save. Removal and thorough disposal of all infected plant matter then becomes the only option.
But if infection has not progressed too far, you have a few options.
For small plants, you can try an over-the-counter fungicide designed to treat phytophthora (among other fungal diseases) and follow instructions carefully. If the plant is large or the affected area represents a significant landscape investment, your best bet is to hire a professional. Unfortunately, such treatment is not cheap. But it may be your only choice if you want to save your plants.
Typically, a licensed specialist will apply a chemical “drench” around the infected roots in spring or fall (or both) which will halt uptake of the pathogen for a while. Repeat treatments may be necessary every season or annually, or only if symptoms reappear.
There are recipes for homemade drench concoctions on the internet, but I can’t recommend any of them without an expert’s endorsement. Some of them sound downright nasty. Only you can weigh the cost of treatment against the loss and replacement of a valued specimen.
Ongoing Preventive Measures
Since some of us are destined to live with this problem, the best way to deal with phytophthora long term is to mount a strong defense. Practicing good garden hygiene will go a long way toward staving off disease. Healthy plants with robust immune systems can often fight off phytophthora. But just like humans, when plants are under stress from too much heat, rain, storms or rapid temperature shifts, they become overwhelmed. To help them mitigate risk, do the following:
- Remove all diseased foliage promptly; cut out infected branches and remove dying plants to slow down the spread
- Loosen compacted soils
- Amend clay with compost, peat, loam or organic matter
- Keep beds mulched but not overly so
- Support earthworms and other beneficial microbes
- Rectify bad drainage areas; prevent ponding
- Add berms or raised beds to improve drainage
- Provide good air circulation; keep trees and shrubs pruned
- Repair leaky downspouts; redirect storm drains
- Plant phytophthora-resistant cultivars. Here is a list.
- Remove all dead and diseased plant matter seasonally
- During periods of high heat, irrigate or water either morning or evening. Allow time for the foliage to dry before nightfall.
Most Susceptible Species
The list of plants susceptible to phytophthora is depressingly long– much too long for this article. The top culprits (which may already be infected when you buy them) are rhododendron, camellia, kalmia, pieris and azalea. Even annuals like vinca, petunias and calibrachoa are hosts for phytophthora, possibly unwittingly introducing it into your garden regularly without you even being aware.
In short, this is one pathogen you can’t avoid. But knowing what to look for, how to treat it and proactively working to resist its grip will give you the confidence to know you’ve done all you can to minimize any future damage. If you’ve successfully treated for phytophthora, let us know in the comments. And good luck!
