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How to Manage Pests
UC Pest Management Guidelines
Floriculture and Ornamental Nurseries
Pythium Root Rot
Pathogen: Pythium spp.
(Reviewed 1/02,
updated 1/02)
In this Guideline:
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SYMPTOMS
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.
COMMENTS ON THE DISEASE
The fungi that are responsible for Pythium root rot, also known as black rot or
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 is conducive to
infection by the Pythium fungi. Soil from a given field may contain
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 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 fungi.
MANAGEMENT
In the control of Pythium diseases, emphasis is placed on providing good
drainage and water management. Steam (at 140°F for 30 minutes) 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 potting mixed is reported to provide
some control of Pythium and Phytophthora root rots; also the mycoparasite,
Gliocladium virens, is used as a Pythium biocontrol agent.
| Common name |
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Amount to Use
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| A. |
MEFENOXAM |
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(Subdue Maxx) |
Label rates |
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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. |
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| B. |
FOSETYL-AL |
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(Aliette) |
2.5-5 lb/100 gal water for foliar spray
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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. |
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| C. |
GLIOCLADIUM VIRENS |
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(SoilGard) 12G |
Label rates |
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![[Precautions]](/IMAGES/btn-precautions_bul.gif)
![[UC Peer Reviewed]](/IMAGES/logo-ucpeerreviewed.jpg)
UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
R. D. Raabe (emeritus), Environmental Science, Policy, and Management
(ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside
C. A. Wilen, UC IPM Program, UC Cooperative Extension, San Diego Co.
A. H. McCain (emeritus), Environmental Science, Policy, and Management (ESPM),
UC Berkeley
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