Phytophthora Root Rot

Phytophthora Root Rot

Phytophthora root rot is a common issue that can affect many landscape trees and plants.  Learn more about the condition and ways to reduce its likelihood in today’s TrueTreeTalk post.

What is Phytophthora Root Rot?

Phytophthora root rot is a condition that affects plants.  It is caused by species in the genus Phytophthora.  It can affect a large variety of plants, including trees that are common in landscapes and it can infect them throughout the year.

What are Some of the Symptoms of Phytophthora Root Rot?

The symptoms of Phytophthora root rot resemble those of other conditions.  They include yellowing of foliage, wilting leaves, and branches dying back.  The symptoms might not appear until after there is significant root decay, which can take months in some cases, and the severity of these symptoms progresses as the condition worsens.  The symptoms are indicative of a plant that is not absorbing nutrients and water through its roots as it should.  This is how Phytophthora harms plants.  The plant roots rot and don’t function as they should and the plants suffer.  Although the plant may look wet, the roots aren’t functioning, and therefore they can’t take up water.  The plant looks like it is stressed from drought even though the conditions may be wet.  Plants can eventually die, though it can take some time.

The symptoms can be somewhat general in nature and can be caused by other things such as Armillaria, drought conditions, and waterlogged soil.  This can make it difficult to diagnose the condition from the symptoms alone.

What are Some Signs of Phytophthora Root Rot?

Signs of Phytophthora root rot can be seen by examining a plant’s roots as well as the collar and the base of the stem or trunk.  When the condition has progressed, the thinner roots (the first to be infected) of the plant will be rotted and the more substantial roots of the plant (which are later infected) will have a black or brown color inside and will be softer compared to healthy roots.  Roots that are healthy will be a cream or a light shade of brown in color.  Roots that are infected will appear black or a red shade of brown.  In addition to the roots, the stem or trunk coming up from the ground (the last to be infected) can also show signs of infection.

One difference between root rot caused by Armillaria and root rot caused by Phytophthora, is that with Armillaria, there will be mats of mycelia and with Phytophthora there won’t be.

Can You See Phytophthora in The Soil?

Unlike a fungus that causes root rot such as Armillaria, the species in the genus Phytophthora are microscopic and can’t be seen with the naked eye.  Samples can be taken and examined in a laboratory to confirm whether there is Phytophthora or not.

Is Phytophthora a Fungus?

Although it shares some similarities with fungi, the species in the genus Phytophthora that cause root rot are water molds called oomycetes. There are hundreds of oomycetes, and some cause plant diseases.

Oomycetes are similar to fungi in appearance and behavior.  Like fungi, oomycetes get nutrients by absorbing them and there are many which produce mycelium, the same way fungi do.  Despite these similarities, oomycetes are more like algae or plants.

One of the main differences between fungi and oomycetes is the spores produced by oomycetes.  Oomycetes produce three spore types.  There aren’t any fungi that can produce all three types.  These spore types are:

Oospores – These live a long time and can survive during the winter in leaves on the ground or in soil.  They can persist for years even if an area is kept clean and they don’t need a host.  These spores can be the ones that initially cause an infection.

Sporangia – These spores don’t live a long time and are spread in the air.  They can be produced a number of times during the year.  In a number of species, these spores carry spores called zoospores from one host plant to another.

Zoospores – These spores live a very short time and are dispersed by water.  They can move a little like a tadpole in water.  When sporangia are in enough water they release zoospores.  Zoospores have to stay wet to survive and generally will only live a few hours.  Although they can move in water, they go farther distances when water moves, such as in runoff or in flooded soil, and wet conditions favor their transfer.  Zoospores aren’t produced by every oomycete, but many produce them.

How Can The Likelihood of Having Phytophthora Root Rot in Your Landscape Be Reduced?

There are some ways you can reduce the likelihood that the trees and other plants in your landscape will develop Phytophthora Root Rot.

One step you can take is to only buy plants that don’t have the infection.  This can be difficult because not all plants that are infected will show indications of the disease, but you can check for the signs and symptoms noted above and avoid any plants that exhibit them.  Check out the source of your plants and only buy from a reputable seller.  Ask questions about their plants and if they have had any issues with Phytophthora.

In addition to plants that aren’t already infected, you can also look for plants that are less susceptible to the disease.  You can check with your nursery to find out what they have available.

Another step you can take is to group plants in your landscape by their needs.  For example, you can group plants that need more water together and those that need less water together.  By doing this, you can reduce the likelihood that you will inadvertently overwater a plant susceptible to the disease.

Touching on this idea, avoiding overwatering is a general step you can take to reduce the chances of the disease.  Overwatering can stress plants and it can promote conditions where Phytophthora can spread.

Similarly, if possible, planting in soil that drains well can reduce the chances of the disease.  When soil drains well, plants are healthier (all else being equal) and the conditions are less conducive to the spread of Phytophthora.  Soils that are heavy in clay, that are compacted, or that don’t drain well can promote the spread of Phytophthora.  Soil that is waterlogged is a significant contributor to the infection taking hold.

In addition to having soil that drains well, you can also take steps to correct drainage issues in your landscape.  A qualified landscaper can help you to develop solutions to meet your needs.

Another step is to make sure trees and other plants are planted properly.  One aspect of this is ensuring that they aren’t planted lower in the ground than they were in their container or in the soil at the nursery.  When plants are planted too low, it can lead to a number of issues, including conditions that leave them susceptible to Phytophthora root rot.

Proper fertilization can also help your plants by generally keeping them healthy.  When plants are healthy they are better able to ward off diseases.

If you do have infected plants, having them removed, if it is feasible, can help to reduce the spread of the disease.

Can Phytophthora Root Rot be Treated With Fungicides?

Under certain conditions, plants that are infected can be treated with a fungicide.  One example are fungicides that contain phosphorous acid salts.  How fungicides are applied (for example by a foliar spray or by a soil drench), how much is applied, and what types are applied vary based on conditions.  Fungicides can be used after an infection, to prevent infection, and to kill spores.  Fungicides should only be applied by a professional.  Fungicides can work well in certain situations, but aren’t effective in all, and their use should be preceded by the preventative steps noted above.

If you found learning about Phytophthora root rot interesting, check out our other posts on our TrueTreeTalk blog.  Follow us on Facebook to keep up with these and other posts.  If you have concerns about Phytophthora root rot affecting your trees, contact us to set up an arborist consultation.

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ArborTrue is a science-based tree-service company in the greater Houston area.  We also serve Austin and other parts of Central Texas.  We provide a range of services including tree trimming, tree pruning, tree removal, tree planting, arborist consultations, and more.  Call us today at 832-980-8733 (Houston) or at 512-546-3833 (Austin) or reach out to us online to schedule an appointment.