Agriculture: Floriculture and Ornamental Nurseries Pest Management Guidelines

Pythium Root Rot

  • Pythium spp.
  • Symptoms and Signs

    Pythium attacks juvenile tissues such as the root tip and newly germinated seedlings. After gaining entrance to the root the fungus may cause a rapid, brown to black rot of the entire primary root and may even move up into the stem tissue. As the soil dries, new roots may be produced and the plant may recover or never show symptoms of disease. Under wet conditions brought about by poor soil drainage or excess irrigation, more and more roots are killed and the plant may wilt, stop growing, or even collapse and die. Bulbs of susceptible plants turn brown to black, gradually desiccate, and form a hard mummy.

    Comments on the Disease

    The pathogens that are responsible for Pythium root rot, also known as water molds, are present in practically all cultivated soils and attack plant roots under wet conditions. These fungi can be spread by fungus gnats and shore flies and end up contaminating potting mixes in containers. There are many species of Pythium; a few of these species are beneficial in that they compete with or parasitize the pathogenic species. Of the many pathogenic species, some have limited host ranges while others, such as Pythium ultimum, have very wide host ranges.

    Some Pythium species, such as P. aphanidermatum, are pathogens only at high temperatures (above 77°F), and some are active only at low soil temperatures. Soil moisture conditions of 70% or higher of available water capacity are conducive to infection by Pythium. It is likely that soil from a field contains several pathogenic Pythium species.

    Pythium species form several types of spores, but not all species form all types. Zoospores, which are produced in sporangia, are motile in water. Oospores, which result from a sexual process, usually undergo a period of dormancy and can withstand long periods of drying. Some species also form chlamydospores, which are asexual and have thick cell walls. These structures can serve as survival or overwintering structures. Sporangia and zoospores in general do not survive in air or dry soil for long periods of time.


    In the control of Pythium diseases, emphasis is placed on providing good drainage and water management.

    • Steam (at 140°F for 30 minutes), solarize (double-tent at 160°F for 30 minutes or 140°F at 1 hour), or chemically treat the growing medium.
    • Sanitize well because Pythium spp. can survive in dust, planting medium, or soil particles on greenhouse floors and in flats and pots.
    • Remove and discard diseased plants.
    • Use of properly composted pine bark as 20% of a potting mixture is reported to provide some control of Pythium and Phytophthora root rots. Additionally, the mycoparasite Gliocladium virens is used as a Pythium biocontrol agent.

    For flower production in outside fields, solarization has been successful for control of damping–off in many crops that are grown in warmer climates. There are reports of inadequate control of some high temperature species (e.g. P. aphanidermatum). Solarization and steaming are acceptable for organic production.

    Common name Amount to use REI‡
    (Example trade name) (hours)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
    (Adorn) 1–4 fl oz/100 gal water 12
    MODE OF ACTION GROUP NAME (NUMBER1): Benzamides (43)
    COMMENTS: Toxic to aquatic organisms.
    (Segway) 1.5–3.0 fl oz/100 gal water 12
    MODE OF ACTION GROUP NAME (NUMBER1): Ubiquinone reductase, Qi site (21)
    COMMENTS: Toxic to aquatic organisms.
    (Fenstop) 7–14 fl oz/50–100 gal water 12
    MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
    COMMENTS: Toxic to aquatic organisms.
    (Pageant) 12–18 oz/100 gal 12
    MODE OF ACTION GROUP NAME (NUMBER1): Carboximide (7) and quinone outside inhibitor (11)
    (Regalia CG) Label rates 4
    MODE OF ACTION GROUP NAME (NUMBER1): Anthraquinone elicitor (P 05)
    (Subdue Maxx) 0.5–1.0 fl oz/100 gal water 48
    MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)
    COMMENTS: Applied at planting as a drench and periodically thereafter as needed. Available also in a granular formulation to use before planting. It is water-soluble and readily leached from soil. It is absorbed primarily through roots and may be translocated in the plant through the xylem. Use of this material over a period of time may lead to resistance.
    (Aliette WDG) 2.5–5 lb/100 gal water for foliar application 12
    MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (P 07)
    COMMENTS: When applied as a foliar spray, it is absorbed by foliage and moves into roots. Soil drench is less effective than a foliar application.
    (Terrazole CA) 4–6 oz/100 gal 12
    MODE OF ACTION GROUP NAME (NUMBER1): Heteroaromatics (14)
    (SoilGard) Label rates 4
    1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action group number.
    # Acceptable for use on organically grown ornamentals.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.
    Text Updated: 11/20
    Treatment Table Updated: 11/20