Phytophthora Root Rot In Azaleas

Phytophthora Root Rot In Azaleas

Azaleas are often grown in the home landscape not only for their beauty, but for their hardiness. As hardy as they are though, there are still a few diseases that can affect azalea shrubs. One of these is phytophthora root rot. If you suspect that your azalea has been affected by phytophthora fungus, keep reading to learn more about the symptoms and ways to treat it.

Symptoms of Phytophthora Root Rot

Phytophthora root rot is a disease that affects azaleas. For an azalea owner, seeing signs of this disease can be devastating as the disease is difficult to control and cure.

Symptoms of a phytophthora fungus infection normally begin with reduced growth in the azalea plant. Overall growth will be less and what growth there is will be smaller. The new branches will not grow as thick as they once had and the leaves will be smaller.

Eventually, the phytophthora disease will affect the leaves. Leaves on the azalea will start to shrivel, curl, droop, or lose their shine. In some cultivars, the leaves will also change color to red, yellow, or purple in late summer through fall (this is only a problem if your azalea has not previously changed color at this time).

A sure sign that your azalea has phytophthora root rot is that the bark at the base of the azalea shrub will be dark and reddish or brownish. If the phytophthora disease is advanced, this discoloration may have already moved up the trunk to the branches. If you were to dig up the azalea plant, you would find that the roots also have this red or brown coloration.

Treating Phytophthora Root Rot

As with most fungus, the best way to treat phytophthora root rot is to make sure that your azalea plants don’t get it in the first place. This is best done through making sure that your azaleas grow in an environment that is not well suited for the phytophthora fungus to grow. Phytophthora root rot travels quickly through wet, poorly drained soil, so keeping your azaleas out of this kind of soil is key. If your azaleas grow in heavy soils, like clay, add organic material to help improve the drainage.

If your plant is already infected with phytophthora root rot, unfortunately, it is very difficult to treat. First, remove and destroy any damaged branches and stems. Next, treat the soil around the plant with fungicide. Repeat the fungicide treatment every few months. Continue to remove any infected branches or stems that you may find as time goes by.

If your azalea plant is badly infected with phytophthora root rot, it may be best to simply remove the plant before it infects other plants in your yard. Phytophthora root rot not only affects azaleas, but several other landscape plants as well. As mentioned, phytophthora root rot fungus moves quickly through wet soil. If you are experiencing heavy rains or if the soil in your whole yard drains poorly, you may want to consider removing the infected azaleas regardless of how advanced the phytophthora disease is in order to protect other plants.

If you need to remove your azalea shrubs, remove the entire plant as well as the soil it grew in. Destroy or discard both. Treat the area where the azalea shrub was with fungicide. Before planting anything else in that area, be sure to add organic material to improve drainage of the soil.


Azalea Diseases

Azaleas are lovely flowering shrubs that come in three distinct categories—our gorgeous US natives, Asian evergreens and hybrids. Out of the hundreds of species within each group, all have their own characteristics, but can commonly fall victim to devastating disease.

Typically fungal in nature, these diseases steal the vibrance of your Azaleas. Below are the predominant ailments that can occur, how you can identify and treat, along with which species may be more resistant or prone to becoming infected.


Water

Azalea leaves that turn brown and drop off during the summer months may be warning you that your plant is getting either too much water or not enough. Established azaleas need only 1 inch of rainfall or watering per week during the growing season. If the weather is hot and dry, you may need to increase watering, but if dryness is not the problem, carefully dig up the roots and inspect them. Azalea roots that are not over-watered are firm and crisp. Waterlogged roots are brown and mushy.


Evergreen azalea leaves turning red - Knowledgebase Question

I notice that some of my evergreen azaleas (Kurume)have leaves that are smaller and lighter green (some leaves are even turning red)while other azaleas of the same variety have leaves that are larger and very dark green. They all get the same amount of sun. Our soil Ph tests at 6.5. All the azaleas are in mixed beds with other shrubs and perennials. The beds were watered by a sprinkler system in the summer.

Do you have an idea of what may have caused the difference in leaf size and color?

High soil moisture and warm soil temperatures favor development of Phytophthora root rot. The disease is more frequent and severe in heavy clays or poorly drained soils than in well drained or sandy soils. The disease is common and severe in areas where run-off water or rainwater from roofs collects around plant roots. Setting woody plants deeper than the soil level in the nursery or container, over-watering plants, or long periods of heavy rain also favor disease development, especially in shallow soils with underlying rock or compacted hard pans.

The following suggestions may aid in the prevention of root rot:

1. Purchase disease-free plants. Choose plants with normal green color and white or light colored roots. Avoid plants that appear wilted in the morning, have reddish-brown discolored roots or evergreen plants that have excessive winter defoliation.

2. When purchasing new azaleas for the landscape, select disease resistant cultivars.

3. Plant in well-drained areas or establish raised beds. If excessive water from any source collects in the planting site, avoid planting root-rot susceptible plants. If soil is clay, set plants in raised beds and thoroughly mix pine bark mulch (not sawdust or peat) into the bed. The material should be incorporated to a depth of about 6 inches this also may help reduce excessive soil moisture. For new plantings in poorly-drained soils, set the plant on top of the soil surface, then build up the raised bed by filling in between plants with pine bark mulch.

4. Do not set the new plant any deeper than the soil level in the nursery container. Firm the soil beneath the soil ball so that the plant will not settle deeper into the soil after watering.

5. Avoid use of azalea in areas where root-rot susceptible plants have died. Instead, replant with plants that are not susceptible to root rot such as Chinese holly, hybrid hollies (eg. Nellie R. Stevens), ligustrum or others.

6. For azaleas in irrigated landscapes, do not overwater. Established landscape plants need an inch of rainfall or irrigation per week in the growing season. Size irrigation equipment for your soil type so that rate of water application does not waterlog the soil.


This soil pathogen exists in low levels throughout Maryland but becomes a problem in wet sites.

  • As roots are killed the leaves begin to turn a lighter green and eventually yellow.
  • Infected plants initially appear wilted. As symptoms progress leaves roll inward towards the midrib and turn brown.
  • Highly susceptible cultivars can die within two weeks, where as more resistant plants may not die until many weeks after the plants have developed the initial wilt symptoms.
  • The entire root system may become diseased or portions may escape infection and support the plant until other stress factors cause death.
  • On older plants, symptoms of root rot may be present a season or more before death. In such cases, plants often exhibit symptoms on part of the plant. The plants also decline in vigor and suffer additional damage from other pathogens or insect pests.
  • Phytophthora dieback, although uncommon in the landscape, is a distinct phase of the Phytophthora disease syndrome on rhododendrons, azaleas, leucothoe, and Japanese andromeda. It can be brought into the landscape on infected plants and can be severe on plants grown under overhead sprinkler irrigation. The disease occurs when the pathogen is splashed onto the foliage. Thus, infected plants may show symptoms on leaves and shoots, but may have healthy root systems. Plants with dieback develop symptoms on the current season growth. Mature leaves are often resistant, however, if they become infected, they usually fall prematurely. Infected leaves show chocolate brown lesions that often expand and cause dieback of the shoot tips. Infected leaves droop and curl towards the stem. Diseased leaves remain attached to the stem. Growth of the pathogen through the midrib tissue often produces a V shaped lesion that extends along the leaf midrib into the stem.


    Rhododendron infected with Phytophthora. Notice the rolled leaves


    Phytophthora infected roots


    Azaleas Showing Symptoms of Root Rot

    Picture of Azaleas Showing Symptoms of Root Rot

    Picture of Azaleas Showing Symptoms of Root Rot

    Picture of Azaleas Showing Symptoms of Root Rot

    There is a wilting, pale section in my huge bank of reliably blooming azaleas. Also, please see the photo of some other developing issues. I have not pruned or fertilized them yet.

    Is this more rain damage? Is there an all-purpose solution?

    Thanks, Ireys

    This appears to be root rot. The roots of an azalea absorb the water the shrub needs and the minerals the shrub needs.

    When a section of the roots gets sick and does not properly absorb mineral nutrients from the soil, the section of the shrub nourished by those roots turns yellow and chlorotic.

    When the roots are killed and stop functioning, the section of the upper part of the shrub connected to those roots is deprived of water and suddenly wilts and dies.

    It is virtually impossible to properly treat root systems growing in the ground, so there are no practical treatments for this problem. The frequent and large amount of rain this spring/early summer is the main issue. The water molds that cause most root rot problems, like Phytophthora, are far more likely to attack roots when extended periods of wet soil occur.

    Prune out the dead areas back to living tissue. The azaleas may survive this and recover if the weather will turn drier.

    Dan Gill
    Consumer Horticulture Specialist


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    Watch the video: Phytophthora root rot on Soybean