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Phytophthora: Plant Destroyer!

By: Lacey Russell, PhD | June 1st, 2018

The word phytophthora is Greek, meaning “plant-destroyer!” This pathogen is found in soils all over the world and often destroys plants from the bottom up, starting with the roots.

One of the most common tree diseases, there are more than 100 species of phytophthora and new species continue to be discovered. They are parasites on a wide range of agricultural, ornamental, and forest plants all over the world.

Learn how to identify Texas tree diseases

One of the most well known pathogens is Phytophthora infestans, the fungus responsible for the Irish potato famine. In 1845 this devastating fugus-like disease destroyed potato crops and caused 25% of Ireland’s population to decrease due to starvation and emigration. P. infestans is limited to crops in the Solanaceae, or nightshade plant family. This includes tomatoes, potatoes and eggplants.

Many species of Phytophthora are host-specific, but some have a wide range of hosts. For example, P. cinnamomi is a known parasite of more than 900 species of woody and herbaceous plants. P. nicotianae is reported to attack more than 250 plant species.

Many shade trees and ornamental shrubs planted in our yards are susceptible to Phytophthora root rot.

They can develop root and crown rot especially if the soil around the base of the plant stays wet for long periods of time. Typical symptoms of a root disease include leaves that appear drought stressed and may die quickly as the weather warms up in late spring.

How to identify Phytophthora in trees

First of all, it’s important to know that Phytophthora species thrive in wet soils. If your lawn is watered regularly to the point where the soil does not have a chance to dry out between watering, there is a good chance that Phytophthora may be living in your soil.

On your sick tree, the leaves will appear drought stressed, and sometimes turn dull green, yellow, red, or purple as they wilt. If left untreated, infected trees can survive for a few years before the disease kills the entire plant.

At the base of the plant, near the soil, the bark may appear darker than usual. If you cut away some of the bark, the wood should appear reddish brown. It is different from Armillaria root rot because fungal mats do not develop with Phytophthora root rot.

Phytophthora Life Cycle

Phytophthora species can survive in the soil for years, as long as the soil stays moist. Like other bacterial and fungal tree diseases, it can spread through splashing rain, irrigation water, and runoff water.

Contaminated soil can be moved from a garden center to your home, or on borrowed garden equipment. Be careful when loaning gardening equipment to a neighbor, and clean it well before using it again in your garden. Also be sure that the nursery where you purchase plants is effectively managing for Phytophthora. You would hate to introduce into your landscape by accident.

The rot is more likely to spread during cool, rainy weather. Excess water or poor drainage allows roots to become flooded for extended periods of time. This is when Phytophthora can produce swimming spores (zoospores). Zoospores are attracted to the roots and begin the infection process. After this, symptoms begin to show up in the summer, when the warm weather sets in.


How to make sure it's actually Phytophthora

Phytophthora-root-rot-2The first step in managing any of the several diseases caused by this fungus is to make sure it is actually Phytophthora. It is a very widespread problem but can often be misdiagnosed.

Landscape clues

You’ll need to gather evidence, like a detective, so you can confirm or deny Phytophthora as the problem-causing agent in your trees and shrubs. Look at your landscape and ask yourself, “Do I have areas in my yard that remain wet? Do my sprinklers come on everyday for a short period of time?”

Phytophthora is a water mold and thrives, grows, reproduces, and infects plant roots in water, saturated soil, and along riverbanks and ponds. Anywhere significant quantities of water collect, accumulate, or flow Phytophthora spp. can be found.

Diseased plants are usually found in the low areas of the yard, where excess rain or irrigation water accumulates. Hillcrests or steeply sloped areas are unlikely places to find Phytophthora diseases.

“High Tech” answers

Plant samples can also be taken to a laboratory for traditional or “high tech” tests to confirm the presence of this fungus-like organism. Your county extension agent or an arborist specializing in tree and plant health care can help you take a sample and properly send it to a lab.

What does the plant say?

It is necessary to thoroughly examine above and below ground parts of the plant. Aboveground symptoms are useful but do not tell the whole story. Many different problems can result in the same aboveground symptoms as Phytophthora root rots.


Phytophthora-root-rot-3Anything that girdles or cuts off water and nutrients to the top of a plant results in wilting, yellow leaves, brown dying leaves, and premature leaf fall and eventual plant death. Causes other than Phytophthora include feeding by wood-boring insects, frost injury, weed-eater or lawn-mower injury, wire or plastic used to keep trees straight after planting, nursery tags, lack of water, and other fungal root rots.

Many plants with Phytophthora root rot do not show aboveground symptoms until warm weather in late spring and summer. As the hot, dry weather sets in, the plant does not have enough functional roots left to keep up with the water they are loosing to the atmosphere through their leaves (transpiration). Plants usually wilt and collapse within a week. Because of the wilting, many people water plants even more that usual, flooding their roots, encouraging the pathogen, and potentially spreading the disease even more. If your plants look wilted but the soil feels moist, do not water them.

Underground symptoms of Phytophthora root rot include the plant’s having few if any feeder roots while the remaining roots are dark and in some stage of decay. Symptoms will be most severe at root tips and least severe near the root crown. Decaying roots are generally due to other microorganisms’ feeding on the roots after being killed by Phytophthora.

If you think you may have a Phytophthora infected plant dig around the roots of the tree until you can cut into it. Use a pocket knife on larger root or fingernails on smaller roots, to expose the vascular cambium, or wood underneath. The area between the bark and inner wood will be highly discolored where Phytophthora organisms have been actively colonizing the root or root crown. Many times the cambium has a dull red to reddish brown color. Above this area, the cambium will be the normal color for the plant, generally some shade of white to light green. The transition between the discolored area and the healthy area may be sharp, with a distinctive margin.

More than just Rotten Roots


Symptoms of Phytophthora diseases are not restricted to root rots because some species attack only aboveground parts of the plant. Large necrotic (dead) leaf or stem blotches characterize late blight of tomato and potato. Under the leaf, along the leading edge of the dying tissue, a downy white growth may develop. Azaleas develop a leaf spot and blight in the winter caused by a species of Phytophthora called P. syringae. Crabapples or flowering pears can develop a black, sunken canker several inches above the soil due to the same organism. Flowering Dogwoods get cankers developing near the soil surface, the trunk may become flat or sunken the cankered area.


Phytophthora-bleeding-canker-1Phytophthora ramorum causes different symptoms on different hosts. ‘Sudden oak death’ on oak tree species is characterized by bleeding cankers that girdle the trunk of several oak species. On azaleas, viburnums and camellias, the disease is characterized by leaf blights and shoot diebacks.


Symptoms on azaleas include discoloration of the leaf petiole and midrib, the leaf tip or entire leaf blade may be dying. Leaf spots can occur where water accumulates on the leaf margins while shoot dieback occurs when disease is severe. On viburnum, infected leaves may die and fall off, leaving dark leafless stems. In more severe infections, viburnum can be killed. Other hosts such as camellia may be infected but have only subtle symptoms, such as small leaf lesions on the lower leaves. Infected leaves on these hosts often fall off.

Phytophthora Prevention

The best way to control a Phytophthora disease is before it starts. Managing your irrigation system to prevent the disease, being sure to purchase plants from a clean nursery, and using composted bark are all ways to that you can prevent Phytophthora from entering and spreading in your landscape.

Regulating water is an important way to control Phytophthora diseases. This includes both the amount, frequency, and duration of water coming to plants and the way water is moved away from the plants. Phytophthora species generally require standing water for a long period of time. These organisms are not active until the soil is saturated.

Waiting longer between watering is a good way to reduce the risk of Phytophthora. This gives the soil a chance to dry out and also encourages a deep root system for plants.

Adding composted hardwood bark mulch to your soil has shown to help reduce the spread of Phytophthora. The bark increases the air spaces in the soil and helps it to dry out between watering, discouraging the growth of the pathogen. Another interesting benefit to using composted bark in your yard is that organisms living in the mulch have been shown to kill Phytophthora living in the soil! This method can be used as a preventative step as well as a management tool if you do have Phytophthora is your soil.

I think I have Phytophthora!

Don’t worry, this disease can be managed so that your tree will continue to live a long life. There are many fungicides registered to kill Phytophthora. The best course of action is to contact an arborist or horticulturist specializing tree and plant health care. They can help you make a plan to save your tree.

A professor of plant pathology at Clemson University, Dr. Steve Jeffers recommends using foliar sprays along with an early spring and/or late fall soil drench. This will protect the plants early and include two different types of fungicide chemistry. He suggests that an arborist drench the soil as the plants begin to grow and the roots are active. Then after the leaves are fully open make an application of a product like Prudent 44 or Agri-Fos. A similar application regime should be used in the fall-but reverse the produces. Apply the foliar spray in the early fall when the leaves are still open and functioning then about a month later apply the soil drench. This should keep your plants free of Phytophthora infections.

Finally, apply foliar nutrients to make up for rotting fibrous roots’ loss of nutrient uptake. This a form of fertilizing is sprayed on the plant’s leaves where the nutrients are taken up. But avoid excessive nitrogen fertilization which encourage the spread of Phytophthora.

Phytophthora is a group of several species of water molds that cause various diseases on a wide range of hosts. They have the ability to survive environmental stress while in the soil or on the plant, making them difficult to get rid of. Managing your landscape to minimize soggy soils, using composted bark mulch, and being careful not to introduce the disease through infected plants will go a long way to preventing Phytophthora. If you feel you have a problem with Phytophthora, an arborist can help you determine if that's indeed the problem, and help you take steps to control it.

And always remember, tree diseases don't care about fences or property lines. The best way to prevent tree diseases in your yard is to know how to identify them and look out for them in your neighbors' yards. Neighbors coming together to fight tree diseases saves trees... and makes the world a nicer place :)