Phytophthora: The Gardener’s Worst Nightmare

Phytophthora can be a great problem in the garden. This soil-borne pathogen will infect woody plants, trees, even vegetables.

It can cause a tree to collapse after several years of planting. This kind of organism is not visible and it does not come from insects or anything.


What is Phytophthora?

In some Phytophthora species, sporangia can detach and be blown or splashed with water to nearby plants. Sporangia release short-lived, one-celled, flagellated zoospores that can swim through thin films of water on leaf surfaces or in water-filled soil pores, and can accumulate in puddles and ponds.
Sporangium Phytophthora

This organism is an oomycete which is very similar to a fungus. It is a genus of microorganisms that is in the same family as water molds, brown algae, and diatoms.

The kingdom is Stramenopile. These species grow hyphae (fine filaments). It also produces spores.

Even though this organism looks like Phytophthora fungus, it is actually a different thing while coming to the life cycle.

Unlike the true fungi, this organism produces zoospores which are swimming spores in one stage of the cycle.

There are over 80 species of Phytophthora you can find in the world. Most of them are plant pathogens.

The name itself comes from Greek which means a “plant destroyer”. It really lives up to its name, right?

Some species of Phytophthora would favor warmer weather, the rest of them like the cooler ones. Mostly, they will appear in the season of spring and fall.

Several signs may appear when a plant gets infected by this organism. Generally, it can cause slow collapses and weaknesses.

You can also learn about the signs that you need to notice in your plants. When they appear to be drought-stressed even though you think you do enough watering, it would be a sign of Phytophthora.

You may also find that the leaves start to appear dull and a bit yellowish, purple, or red even though the fall season is still a long way.

Problem Identification

Phytophthora disease is a plant pathogen that spreads to trees and plants by contaminating their roots. This disease is easily transmitted from plant to plant. Once inside the root system, Phytophthora grows into and follows the water conducting vessels of the roots; it quickly grows into the trunk or stem and eventually reaches the plants' extremities.
Phytophthora Infectious Tree

There are some specific signs or evidence that quickly lead you toward the diagnosis of this disease. You can see it in the field of history, characteristics, and symptoms.

It is essential to understand the biology of this oocyte. Also, understand the conditions that would help it to develop.

The infected plants’ pattern and the drainage become the two main factors to look for. You will need both of them to do the identification or diagnosis.

You can observe these factors just about anywhere, including the greenhouse.

Commonly, these organisms act as water molds. If you grow the plants on the soil with significant quantities of water, you may find this species there.

In other cases, the infected plants are found in low bogs nearby the coast. Furthermore, this organism can also be in the mountains to infect the trees that grow a thousand feet above sea level.

The conditions that attract the Phytophthora is when the soil has excess water and poor drainage. As said before, this organism can produce swimming spores that infect the roots of plants.

Generally, gardeners would face this issue in the plants that grow in the lower field area where water or rain accumulates.

So, you will not find it in steeply sloped areas. The poor drainage is also the problem that attracts this organism.

There are still other patterns to identify or diagnose the existence. Well, the information above is already enough for gardeners.

The Symptoms of Phytophthora

The phytophthora species are often referred to as water mold fungi, because they need free water to spread in soil and to infect orchards and nursery trees.
Phytophthora Symptom on Apple

Looking for the best evidence of Phytophthora’s existence? Check for the plant’s symptoms. Start searching for them by thoroughly examine below and aboveground parts of the plant.

However, you cannot rely fully on the examination of aboveground parts because it is not completely diagnostic.

Some other problems can cause the evidence you find. For example, the browning and yellow leaf. It can also come from the root weevil larvae, winter injury, many others.

How about the belowground symptoms? Well, If you see the plants have a few feeder rots while the remaining ones are dark and look decay.

At the root tips, the symptoms will look most severe than the area near the crown. Generally, the cause of decaying roots is the feeding of other organisms that are killed by this species.

The name of the disease is Phytophthora root rot. It generally starts from the underground. Then, it would work up the plant.

However, it does not occur in some plants such as Apple, Dogwood, Madrone, True fir, and Strawberry.

That is due to the plants’ features. Well, some species may attack only the aboveground parts. The disease is called root rot because it is mostly found below the ground.

General Types of Symptoms

Phytophthora root rot causes a slow decline of the tree, especially in new plantings. This pathogen causes seed rots, pre- and post emergence damping off of seedlings and stem rot of plants at various growth stages. The leaves turn light green or yellow and may drop, depending on the amount of infection.
Phytophthora Root Rot

To sum up the previous explanation, these are 7 problems you can find on the infected plants:

  • Blight

The leaves show the water-soaked appearance that has randomly shaped brown spots that becomes larger and larger.

There are also wedge-shaped lesions that do not feature a yellowish halo around them. The lesions probably spread from or to the leaf petiole and stem

  • Dieback

You may find the death of the shoot tip of the twig or branch. Then, it goes on to infect the main stem. This symptom may occur with the loss of leaf.

  • Decline

The infected plants would become unthrifty and fail to grow. When the high tree gets the disease, its canopy would appear to be thin because of foliage loss.

It would need several years for Phytophthora to kill it entirely.

  • Canker

In the inner bark of a big tree, notice the darkly discolored necrotic lesions. You should remove the outer bark to see what the inside looks like.

These symptoms generally come with the liquid. You may witness the reddish-brown liquid that comes out of the tree that looks like it’s bled.

  • Rot

This problem is often found at the roots. Sometimes, you can find the symptoms goes right above the ground.

That is corral rot which can occur at the trunk’s base. You should eliminate the bark to see the actual rot.

  • Wilting

The lack of water uptake caused by this organism would make the leaves become flaccid. Mostly, you will see this as the first symptom of root rot that can be found above the ground.

  • Death

Have you ever seen a dead tree? Well, that might be the symptom. You may see the whole canopy or sections of it die.

The Disease Control

High temperatures have been used to control Phytophthora in many ways. Steam heat is effective to kill Phytophthora in contaminated soil, media or on planting containers such as pots.
Controlling Phytophthora

When you find a tree or plant that shows the symptoms of Phytophthora, you can still save it. The initial task to do is eliminating the soil that is around the base all the way down.

Make sure it reaches the main roots’ top. This method will leave the root exposed and away from the soil.

After completely dry, it would eventually slow down the spread of this organism. In most cases, control is only about prevention. You cannot beat the species when it already infects.

For example, prevent shrubs and trees like Azalea from the infection of this organism. So, it should be planted in areas with good drainage.

Keep the roots dry all day long. For your information, Phytophthora only needs about 4 hours to germinate in standing water.

That is why an area with poor drainage would be the best place for this organism. It seems difficult to deal with the disease.

Compared to the well-established trees, the younger ones have a higher chance to get an infection from Phytophthora.

During this stage of the plants, the roots are widely spread roots which give avenues for nutrition and water.

For smaller plants like flowers, shrubs, or vegetables, the Phytophthora root rot would cause them to die before you even realize it.

The Best Phytophthora Treatment

Generally, salicylic acid was the most effective chemical inducers followed by hydrogen peroxide and potassium phosphate.
Phytophthora Chemical Treatment

Having fungicide treatment can be a good option to deal with this harmful species. The use of this solution will make sure that the organism will not cause further harm to plants.

Something like Subdue MAXX mefenoxam becomes the industry standard to deal with this disease.

The substance continues to give great control. According to research, using that product would keep all the plants healthy. This registered product is proven and it performs well.

There are other products that also deliver. They all look promising. Compare the treated plants with the untreated ones.

You will notice a significant difference between them. The products include Stature DM, Insignia 20WG, and Terrazole 35WP.

In addition, there is a new fungicide product in the market called Fenstop. It also performs very well. All of those treatments is perfect for your plants to deal with their Phytophthora root rot diseases.

Phytophthora Root Rot Management

There are relatively few fungicides available for controlling Pythium. Propamocarb (Previcur N), etridiazole (Terrazole and Truban), fosetyl aluminum (Aliette), and metalaxyl (Subdue) have been tested on one or more ornamental crops.
Phytophthora Treatment Management

To manage this specific disease, there are a few things you can do. The first one is to use seed treatment fungicide. The use of a lot of mefenoxam and metalaxyl is very needed.

Besides, you can also do some cultural management. Avoid planting early in the season because you have to wait until getting the best temperatures for the soil.

During that time in the season, it would be below 60 degrees Fahrenheit.

So, it is better to increase the drainage of the water in the field if possible. The last management step you can do is to choose the varieties of plants that are resistant to root rot.


Phytophthora would be the worst nightmare for gardeners if the plants they have for several years collapse just like that. From this article, you can recognize the symptoms from immediately.

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