EPLP-043 8/19

Phytophthora

Hannah Ayala, Extension Assistant Sheila McBride, Extension Program Specialist Kevin Ong, Professor and Extension Plant Pathologist Texas A&M AgriLife Extension Service, The Texas A&M University System

The literal Greek translation of the wordPhytoph - search of a host to infect. When the mobile zoospores thora is “plant” (phyto) and “destruction” (phthora). reproduce, they produce a thick-walled oospore that, in The Phytophthora pathogen damages a plant’s root many Phytophthora species, allow the pathogen to sur- system, interfering with its ability to move water and vive in infected plant debris and in soil for many years. nutrients throughout the plant. It lives as a parasite in a variety of plant hosts and various Phytophthora sp. Symptoms cause stem, crown, and root rots. Phytophthora rot Symptoms of a mild root rot caused by Phytoph- typically affects the roots of a plant first; however, in thora sp. can appear as abnormal foliage growth or some plant species, infection can occur aboveground. as darkly discolored or dead feeder roots or stems (Figure 2). Severe root rot causes stunting or wilting, Cause abnormal foliage growth, significant reduction and Once thought to be a fungus, species in the genus rot of root systems, reddish-brown discoloration, lack Phytophthora are oomycetes, also known as water of new shoot molds. Oomycetes produce zoospores (a spore capable development, or of independent movement) in a structure called a spo- death. Above- rangium (Figure 1, A and B). Phytophthora zoospores ground foliage have two flagella (appendages) that make it possible will exhibit for them to “swim” in water, such as in a hydroponic yellowing, wilt- system, retention pond, or water-saturated soil, in ing, stunting, or interveinal chlorosis. Damage to root tissue pre- vents the root system from absorbing and moving water and nutrients Figure 1. Characteristic structures of Phytophthora. A) to the upper Figure 2. Boxwood crown displaying Sporangia dyed with Lacto-Fuschia B) Zoospores inside a part of the discoloration. Source: Kendall Grier, Texas A&M sporangium. Source: Sheila McBride, Texas A&M AgriLife Extension Service plant, resulting University

1 in significant plant stress, tissue desiccation, and, ulti- Excess water is a crucial factor in the infection mately, plant death. The damage from the lack of wa- of plants. Zoospores and cysts can move in the soil ter and nutrient uptake can appear similar to drought through flowing irrigation water, rainfall runoff, and symptoms. Symptoms in plants with low levels of soil movement. Prolonged irrigation or rainfall, con- Phytophthora infection may not be visible. Inspect the tainers sitting in puddles of water, or poor drainage crown and roots since the infection may also resemble can all affect the severity and spread of Phytophthora other plant pathogens or abiotic (non-living) factors. and make plants more susceptible to infection. Since it is difficult to make an accurate diagnosis based solely on the aboveground symptoms, send Management Strategies a sample to a plant diagnostic clinic to confirm the No current chemical control can eradicate a presence of the pathogen. Phytophthora infection. Fungicides can mitigate Phytophthora infections do not always appear infection, but an application will not be beneficial underground. Phytophthora aerial blight typically unless irrigation, soil or media moisture, and drain- affects the aboveground plant parts rather than the age problems are corrected. Phytophthora can survive roots. Affected plants display brown to black lesions in the winter in infected roots or other infected plant on the foliage as well as brown, water-soaked lesions parts, so be sure to dispose of infected plant material on the stems or at the base. The foliage above the or media properly. The primary goal of controlling lesions quickly begins to wilt and brown (Figure 3). infection is to prevent disease through cultural prac- tices, 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 stor- age 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. Figure 3. Vincas exhibiting wilting and dieback. Source: Sheila McBride, Texas A&M AgriLife Extension Service Sources Erwin, D. C., and O. K. Ribeiro. 1996. Phytophthora Environmental Factors Diseases Worldwide. St. Paul, MN: The American Like all plant pathogens, Phytophthora sp. are vir- Phytopathological Society. ulent only to a susceptible host in a conducive envi- Gleason, M. L., M. L. Daughtrey, A. R. Chase, G. W. ronment. Cool, wet weather typical of early spring or Moorman, and D. S. Mueller. 2009. Diseases of late fall promotes the spread of infection. Phytophtho- Herbaceous Perennials. St. Paul, MN: The Ameri- ra aerial blight develops under warm, humid weather, can Phytopathological Society. extensive rainfall, overhead watering, or heavy fertil- Phytophthora Root Rot. Royal Horticulture Society. ization. These conditions allow the zoospores to move Accessed October 24, 2018. https://www.rhs.org. greater distances in water. uk/advice/profile?PID=542.

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