Disease name: Phytophthora root rot
Pathogen name: Phytophthora species (water mold or oomycete)
Hosts: Most plant species
Phytophthora is a genus of water molds that cause root rots, as well as cankers and foliar blights. The name Phytophthora is derived from Greek and means plant (Phyto) and destroyer (-phthora), which is descriptive of what they do.
There are approximately 180 described species of Phytophthora. Some species are only found in certain regions of the world, some others have a wide distribution. Some species are host specific, while others have a large host list.
Phytophthora are capable of being moved and spread on infected plant material, in water, and in soil. Some species are capable of surviving in fallow soil for decades after their introduction.
Symptoms
Phytophthora can infect roots and cause root rot. The above ground symptoms can include wilting, stunted branches and leaves, and a general loss of vigor. These symptoms occur because the infected roots are not capable of absorbing water.
Below-ground symptoms of Phytophthora root rot are especially noticeable in the fine absorbing roots. The fine roots will be mushy and lack their normal firmness. They will turn brown to black, and as the disease progresses, the outer part of the root (the epidermis) can easily slip off and expose the inner root (the stele).
Infected plants can be more susceptible to secondary diseases due to their weakened state. Phytophthora root rot can lead to death of the host plant.
Disease Cycle
Phytophthora species can have up to four spore stages. The oospore is the sexual spore while the chlamydospore, sporangium, and zoospores are asexual spores. The two survival spores are called the oospore and the chlamydospore. They allow the pathogen to survive during times when conditions are unfavorable or a host plant is unavailable.
When conditions are favorable, usually when there’s free water, the survival spores will germinate and begin to grow. Both survival spores often grow to produce sporangia, which can either germinate or it can produce zoospores. The zoospores have thin, threadlike structures called flagella, which allows them to swim.
All spore stages are capable of causing infection and disease. Phytophthora are capable of infecting host plants directly, meaning they do not need a wound or opening to cause infection.
Management
Cultural Management
The best management of Phytophthora root rot is preventing an introduction. Be sure to purchase healthy plants. When buying plants, inspect the roots by pulling them out of the container or digging to find the roots.
Since Phytophthora are water molds, they benefit from wet conditions. Proper watering will slow the disease. Aerating the soil to add in more air pore spaces can also slow the disease.
Resistant Cultivars
There are some resistant cultivars available, but the Phytophthora organism needs to be identified to species. To have Phytophthora identified, molecular work (DNA sequencing) needs to be done by a lab.
Biological Controls
There are biological control options being developed for Phytophthora infestations.
Chemical Management
While they are often called fungi, Phytophthora are not true-fungi, which is an important distinction when determining a management plan. The same fungicides that work for true-fungi will not always work on Phytophthora. There are products available for Phytophthora management, but they must be applied early in the disease cycle to be effective. The treatments protect the newly emerging roots while also killing the Phytophthora in the soil. These products are restricted use pesticides available only to professionals with a pesticide license.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
For more information, contact The Morton Arboretum Plant Clinic at 630-719-2424 or plantclinic@mortonarb.org.