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How to Manage Pests

UC Pest Management Guidelines

Foliage damaged by Pythium sp.

Floriculture and Ornamental Nurseries

Pythium Root Rot

Pathogen: Pythium spp.

(Reviewed 3/09, updated 3/09)

In this Guideline:


Pythium attacks juvenile tissues such as the root tip. After gaining entrance to the root the fungus may cause a rapid, 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 black, gradually desiccate, and form a hard mummy.


The pathogens that are responsible for Pythium root rot, also known as water mold, 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. 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 are conducive to infection by Pythium. Soil from a given field may contain several pathogenic Pythium species.

Pythium speciesform 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 resting structures. Sporangia and zoospores in general do not survive in air or dry soil for long periods of time. ELISA test kits are available for detecting Pythium.


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 growing medium. Sanitation is important 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 at 20% in a potting mixture is reported to provide some control of Pythium and Phytophthora root rots; also the mycoparasite, Gliocladium virens, is used as a Pythium biocontrol agent. For flower production in open fields, solarization in warmer climates has been successful for control of damping off in many crops. Reports of inadequate control of some high temperature species (e.g. P. aphanidermatum) have been made. Solarization and steaming are acceptable for organic production.

Common name Amount to Use R.E.I.+
(trade name)   (hours)

Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
When choosing a fungicide, consider the general properties of the fungicide as well as information relating to environmental impact.
  (Subdue Maxx) Label rates 48
  COMMENTS: The granular formulation may be applied preplant or the liquid formulation can be applied as a drench at planting. Mefenoxam is water-soluble and readily leached from soil. It is absorbed primarily through roots and may be translocated in the plant through the xylem.
  (Aliette) 2.5–5 lb/100 gal water for foliar spray 12
  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.
  (SoilGard) 12G Label rates NA
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). 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 (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.
NA Not applicable.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
S. T. Koike, UC Cooperative Extension Monterey County
C. A. Wilen, UC IPM Program, UC Cooperative Extension San Diego County
Acknowledgment for contributions to Diseases:
R. D. Raabe, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
A. H. McCain, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside

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