By: Hannah Ayala, Sheila McBride, Kevin Ong
The literal Greek translation of the word Phytophthora is “plant” (phyto) and “destruction” (phthora). The Phytophthora pathogen damages a plant’s root system, interfering with its ability to move water and nutrients throughout the plant. It lives as a parasite in a variety of plant hosts and various Phytophthora sp. cause stem, crown, and root rots. Phytophthora rot typically affects the roots of a plant first; however, in some plant species, infection can occur aboveground.
Once thought to be a fungus, species in the genus Phytophthora are oomycetes, also known as water molds. Oomycetes produce zoospores (a spore capable of independent movement) in a structure called a sporangium (Figure 1, A and B). Phytophthora zoospores have two flagella (appendages) that make it possible for them to “swim” in water, such as in a hydroponic system, retention pond, or water-saturated soil, in search of a host to infect. When the mobile zoospores reproduce, they produce a thick-walled oospore that, in many Phytophthora species, allow the pathogen to survive in infected plant debris and in soil for many years.
Symptoms of a mild root rot caused by Phytophthora sp. can appear as abnormal foliage growth or as darkly discolored or dead feeder roots or stems (Figure 2). Severe root rot causes stunting or wilting, abnormal foliage growth, significant reduction and rot of root systems, reddish-brown discoloration, lack of new shoot development, or death. Aboveground foliage will exhibit yellowing, wilting, stunting, or interveinal chlorosis.
Damage to root tissue prevents the root system from absorbing and moving water and nutrients to the upper part of the plant, resulting in significant plant stress, tissue desiccation, and, ultimately, plant death. The damage from the lack of water and nutrient uptake can appear similar to drought symptoms. Symptoms in plants with low levels of Phytophthora infection may not be visible. Inspect the crown and roots since the infection may also resemble other plant pathogens or abiotic (non-living) factors. Since it is difficult to make an accurate diagnosis based solely on the aboveground symptoms, send a sample to a plant diagnostic clinic to confirm the presence of the pathogen.
Phytophthora infections do not always appear underground. Phytophthora aerial blight typically affects the aboveground plant parts rather than the roots. Affected plants display brown to black lesions on the foliage as well as brown, water-soaked lesions on the stems or at the base. The foliage above the lesions quickly begins to wilt and brown (Figure 3).
Like all plant pathogens, Phytophthora sp. are virulent only to a susceptible host in a conducive environment. Cool, wet weather typical of early spring or late fall promotes the spread of infection. Phytophthora aerial blight develops under warm, humid weather, extensive rainfall, overhead watering, or heavy fertilization. These conditions allow the zoospores to move greater distances in water.
Excess water is a crucial factor in the infection of plants. Zoospores and cysts can move in the soil through flowing irrigation water, rainfall runoff, and soil movement. Prolonged irrigation or rainfall, containers sitting in puddles of water, or poor drainage can all affect the severity and spread of Phytophthora and make plants more susceptible to infection.
No current chemical control can eradicate a Phytophthora infection. Fungicides can mitigate infection, but an application will not be beneficial unless irrigation, soil or media moisture, and drainage problems are corrected. Phytophthora can survive in the winter in infected roots or other infected plant parts, so be sure to dispose of infected plant material or media properly. The primary goal of controlling infection is to prevent disease through cultural practices, such as
- using host plants that have disease resistance, planting in well-drained locations, improving soil drainage (increase porosity by adding peat moss, perlite, and sand),
- not planting too deeply (the soil line should not be more than 1 inch over the upper roots),
- using new or sterilized containers (provide storage with no opportunity for contamination),
- increasing plant spacing and early irrigation to encourage rapid leaf drying, and
- mulching the area around the plants to prevent zoospores splashing from the soil onto the foliage
Erwin, D. C., and O. K. Ribeiro. 1996. Phytophthora Diseases Worldwide. St. Paul, MN: The American Phytopathological Society.
Gleason, M. L., M. L. Daughtrey, A. R. Chase, G. W. Moorman, and D. S. Mueller. 2009. Diseases of Herbaceous Perennials. St. Paul, MN: The American Phytopathological Society.
Phytophthora Root Rot. Royal Horticulture Society. Accessed October 24, 2018. https://www.rhs.org. uk/advice/profile?PID=542.
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