By Erin Marissa Russell
Have you just started a garden in your new home only to learn to your dismay that the yard has been plagued with Phytophthora in the past? If so, or if you’re just looking for the best ways to identify, treat, and prevent Phytophthora crown, collar, and root rot in your trees, keep reading to learn exactly what you should do to keep this potentially fatal disease out of your soil and off of your plants.
The list of host plants for Phytophthora root and crown rot includes the following: andromedas (Pieris), apple tree (Malus domestica), apricot tree (Prunus armeniaca), azaleas (Rhododendron spp.), beech (Fagus), cherry tree (Prunus), dogwood (Cornus), holly (Ilex), juniper (Juniperus), peach tree (Prunus persica), true fir (Abies) and yew (Taxus baccata).
Identifying the Symptoms of Phytophthora Rot in Trees
The rot diseases caused by Phytophthora affect different areas of the plant depending on which disease the plant has contracted. Trees can be affected by root rot and crown rot simultaneously.
- Phytophthora collar rot results in girdling of the scions, which are the new growth shoots and twigs. Infected trees can develop a sunken canker that is gray, purple, or dark brown on the bark near where it was joined in grafting.
- Phytophthora crown rot affects the roots that lie right underneath ground level as well as the part of the tree trunk that is close to the ground.
- Phytophthora root rot infects the smaller fine roots, resulting in necrotic tissue and eventually, if the disease goes untreated, causing the death of the plant.
The phytophthora rots are especially troublesome in gardens that have heavy soil and a climate with abundant rain. Soil that stays damp for too long after irrigation or rain, as well as any condition in the garden that contributes to an overly wet environment, increases the risk of phytophthora rot. Infections become more likely the longer the soil remains too damp.
The organisms that cause Phytophthora rots are called oomyctes, which are similar to fungi. They are common in the soil of existing orchards. Phytophthora may also enter an orchard or garden by hitching a ride in soil, water, or on a newly introduced plant.
Trees suffering from Phytophthora crown or root rot won’t necessarily exhibit symptoms specific to their illness, instead seeming to become vaguely unhealthy. Most trees will have a long period of illness before they succumb to a fatal infection, but susceptible plants tend to die more quickly if they become infected with Phytophthora rots. Trees that do show more precisely identifiable signs of the disease normally have symptoms that resemble fire blight, malnutrition, wet feet (which occurs when standing water remains around the plant’s root system too long), or cold damage.
If you believe a tree in your garden may be experiencing a Phytophthora rot disease, you’ll need to perform a thorough visual inspection of the tree to confirm your suspicions. Examine every part of the tree carefully, starting with the roots at its base. You can move away some of the soil around the tree’s roots to get a better look. One way to check for Phytophthora rot is to gently peel back the outer layer of bark so you can see the color of the cadmium underneath.
On a healthy tree, the cadmium will be green, but in trees with Phytophthora, it ranges from orange to brown. Infected trees may have brown or orange roots that may shed and eventually fall away from the tree’s primary root. Slicing away the outer layer of bark on an infected tree will reveal discolored, diseased tissue, which may also be orange or brown.
The diseased tissue will be distinctly different from the healthy tissue, with the line between the two clear and easy to see. On the other hand, if the tree is suffering from rot due to overwatering instead of the Phytophthora organism, its roots will be brown and you will notice a distinct smell of decay.
Preventing and Treating Phytophthora Crown, Collar, and Root Rot
Now that you know how to identify Phytophthora rots in your garden, you need to know how to address an outbreak of these diseases if they do occur. Even if your property doesn’t have a history of Phytophthora crown, collar, or root or (at least not to your knowledge), a prevention routine is a best practice that every gardener should implement. Listed here, you’ll find techniques to avoid dealing with Phytophthora in your garden or to treat the diseases if the need ever arises.
Inspect vulnerable plants regularly so you can catch Phytophthora early if it strikes.
Now that you know exactly what Phytophthora crown, collar, and root rot would look like, create a routine so you’ll remember to inspect your trees for symptoms on a consistent basis. Especially during rainy seasons, keeping track of how your trees are doing will make it possible for you to diagnose a Phytophthora problem early. Catching these diseases before they have the chance to spread to other trees in your garden means less likelihood of severe or widespread infections.
Resolve any situation that causes excess moisture in the soil and lessen Phytophthora’s opportunity to spread.
When the soil in your garden is too waterlogged, Phytophthora rot diseases have the chance to pass from plant to plant. This is the only time you’ll notice new cases of Phytophthora. That‘s because one of Phytophthora’s forms is a zoospore, for which swimming is the only method of traveling from one place to another. The longer the soil remains in this slightly liquid state, the longer the zoospore has to find more plants in your garden to infect.
To lessen the risk of Phytophthora in your garden, keep soil at only the needed level of moisture, and reduce the time the soil remains wet after a rainfall however you can. One of the best ways you can do this is by choosing a place for plants susceptible to Phytophthora to grow that offers plenty of drainage.
Boost your soil’s drainage with an amendment that will improve its texture.
There are so many amendments on the market that you can mix into your soil to improve its drainage. Choose one of these treatments to give your soil a looser texture, which provides room for air to circulate and to allow water to flow through from the surface.
Organic materials like shredded dead leaves, well rotted compost, or clippings from mowing the lawn will do the trick just as well as a commercial product made to improve drainage in garden soil. Just spread your organic materials out in a layer two to three inches thick, then mix the amendment down into the top six to 12 inches of soil.
When you’re choosing new plants for your garden, go for Phytophthora-resistant varieties.
When you’re shopping for seeds or selecting some new plants for your garden this season, look for those marked as Phytophthora-resistant. You can also do a bit of research online to find which plants are recommended for gardeners in your USDA Hardiness Zone working with properties where Phytophthora has been a problem before. There are so many to choose from that you won’t feel too limited in your options by narrowing the field this way. Check the list we provide at the end of this article for resistant varieties to consider for your garden.
Do your gardening in the soil instead of in containers.
Container gardens are customizable or portable, so you can stow plants away in winter, and appropriate for pretty much anything you want to grow. But unfortunately, Phytophthora is most likely to strike plants when they grow in pots.
Whenever you can, choose direct planting instead of planting in a container. But if you must, make sure that the soil medium you use will facilitate drainage and that the container has holes in the bottom to allow excess water to escape. Nurseries and garden centers tend to use containers for their baby plants, so make sure to check any new plants over for signs of the disease before you bring them home.
Target the base of your plants when you water the garden.
It’s easy to water from above and let the liquid splash all over your plants and the ground around them. However, the moisture sitting on their leaves can lead to sunscald, which is caused when the sun burns the plant’s foliage. Another side effect of splashing water around instead of carefully controlling its stream from the garden hose is that you’re creating just the type of moist conditions where Phytophthora thrives. All you need to do is correct your aim. Point the water hose lower, where the plant meets the soil, and you’ll both get more hydration to its roots and help prevent Phytophthora in your garden.
Before winter sets in, make sure to clear all plant debris from the ground, or Phytophthora can hide there until next season.
It’s a good idea regardless of your garden’s situation to gather up all the plant debris you can and discard it or add it to the compost heap before cold weather sets in. (If you have any reason to suspect that a plant in your collection might have a disease or be infested with insects, don’t compost the debris from your garden, or you risk passing the problem on to your future self next spring. )
Take cuttings from parts of the tree that aren’t in contact with the soil.
Whenever you propagate a tree by taking cuttings from it, you have a chance to start the new cutting off right and reduce the chance it will carry Phytophthora diseases. Just take your cutting from a part of the tree that is growing high enough that it is free from contact with the soil. Because many Phytophthora organisms hide in the soil when conditions don’t allow them to travel, you’ll be avoiding a lot of the potential for disease in your new tree.
Minimize a bout of Phytophthora when you apply a nitrogen fertilizer.
The rots associated with the Phytophthora organism mainly impact a plant’s most recently grown, most tender shoots as the garden wakes up and starts putting out new foliage each spring. A low- to medium-sized dose of a fertilizer containing nitrogen will stave off Phytophthora. Nitrogen fertilizers have this effect because they stop plants from producing new, delicate shoots, so the Phytophthora has nothing to latch on to.
Throw out your used soil after each growing season.
Phytophthora doesn’t only hide in plant debris on the surface on the soil. The organisms also spend their winters in any planting medium they can find, often hiding out in potting soil. Although some gardeners may tell you it’s OK to recycle soil from one year and use it again the next, if you want to keep your garden free of Phytophthora, the savings just aren’t worth the risk. Start each new spring in your garden with a fresh new batch of soil.
Keep gardening tools clean and sanitized.
Before and after each gardening session that requires use of your tools, you should clean and sanitize them to avoid spreading diseases like Phytophthora. In fact, the best way to prevent diseases traveling from plant to plant as you work is to wipe your shears, pruners, and other tools down with rubbing alcohol between each plant you work on. It may seem like a pain, but this preventive measure really doesn’t take much time at all, and it goes a long way toward keeping your garden clear of Phytophthora and other plant diseases.
Plants Resistant to Phytophthora Crown, Collar, and Root Rots
As we’ve already mentioned, one of the easiest and most effective ways to avoid doing battle with Phytophthora rots in your garden is to choose resistant plants. That way, you’ll bypass the need for preventive treatments entirely. And although it can be nearly impossible to remove the organisms that cause Phytophthora rots from your garden, you’ll be able to relax knowing that your plants are safe from this potentially fatal disease.
However, keep in mind that even resistant or tolerant plants can be infected with Phytophthora when conditions are too wet or soil doesn’t drain well enough, so it’s still important to manage your garden’s moisture level.
Here’s a list of many of the resistant plants available to help guide you as you plan your garden for next season.
- Brazilian Vervain (Verbena bonariensis): Also called purpletop vervain and tall verbena
- Burford Holly (Ilex cornuta ‘Burfordii’)
- Chinese Holly (Ilex chinensis): Also called Kashi holly, Oriental holly, and purple holly
- Dawn Redwood (Metasequoia glyptostroboides)
- Geranium (Pelargonium): Also called storkbill
- Glossy Abelia (Abelia x grandiflora): Also categorized as Linnaea x grandiflora
- Mexican Sunflower (Tithonia rotundifolia)
- Moss Rose (Portulaca): Also called little hogweed and purslane
- Ornamental Cabbage (Brassica oleracea var. acephala)
- Ornamental Kale (Brassica oleracea)
- River Birch (Betula nigra): Also called black birch, red birch, and water birch
- Sweet Potato Vine (Ipomoea batatas)
- Sweetshrub (Calycanthus): Also called spicebush
- Tree marigold (Tithonia diversifolia): Also called Japanese sunflower, Mexican sunflower, Mexican tournesol, Nitobe chrysanthemum
- Wintersweet: Also called Japanese allspice (Abelia)
- Yaupon Holly (Ilex vomitoria)
When you make it a priority to keep diseases like Phytophthora out of your garden, you’ll need to start a preventive control regimen. Luckily, most of what you need to do to keep plant diseases and infestations at bay is a best practice in general, even if your plants are the healthiest on the block.
You’ll find that management techniques like these are actually easy to put into place and keep up with. So pick the ones you think will fit best into your gardening style and start building a healthier environment where your plants can really thrive now.
Learn More About Phytophtora Root Rot
https://ag.umass.edu/landscape/fact-sheets/phytophthora-root-crown-rot
https://extension.psu.edu/phytophthora-root-rot-on-woody-ornamentals
https://www2.ipm.ucanr.edu/agriculture/citrus/Phytophthora-Root-Rot/
https://www.rhs.org.uk/advice/profile?pid=542
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