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How to Manage Pests

UC Pest Management Guidelines

Stunting and discoloration from Phytophthora root rot on pepper roots.

Floriculture and Ornamental Nurseries

Phytophthora Root and Crown Rots

Pathogen: Phytophthora spp.

(Reviewed 3/09, updated 3/09)

In this Guideline:


Many, if not most, Phytophthora species can infect roots in the same manner as Pythium species. In addition, Phytophthora species infect larger roots and stems, particularly in woody plants. Infection of the bases of stems or root crowns is very common under wet conditions. Aerial parts, including branches and shoots, also can be infected under wet conditions if soil water or airborne spores are splashed on these aboveground parts.

Plants become stunted, low in vigor, and appear as if they were water stressed. Foliage yellows and the plant may wilt and die. Roots rot and stem is girdled by the fungi at or below the soil line, resulting in a dark brown rot.


The pathogens that cause Phytophthora root and crown rots are related to Pythium species. Pythium and Phytophthora are sometimes collectively referred to as the water molds and are grouped in the family Pythiaceae. Ideal soil conditions for the growth of Phytophthora are wet soils (70% moisture or higher) with temperatures in the range of 59° to 74°F. Like Pythium spp., these fungi can be spread by fungus gnats and shore flies.

Phytophthora species have the same type of reproductive structures as Pythium species (i.e., oospores, sporangia, chlamydospores, and zoospores). Sporangia of some Phytophthora species (e.g., P. infestans and P. nicotiana) are airborne and aerial parts are the principal infection sites. ELISA test kits are available for detecting Phytophthora.


Emphasis in control of Phytophthora diseases is placed on providing good drainage and water management. In addition, because aerial parts often are infected, propagative material can be a source of infection. Deep planting where soil covers the base of the stem encourages infection by Phytophthora. The same fungicides active against Pythium species also have activity against Phytophthora species. Copper-containing fungicides are also useful in protecting aerial parts of plants from infection by Phytophthora spp.

Both Pythium and Phytophthora species can be introduced to planting areas via contaminated surface water and soil. Aerial infections by Phytophthora species of a number of plants have been observed where overhead irrigation of water from streams is practiced or where untreated, recirculated water is used. For more information, see MANAGEMENT OF SOILBORNE PATHOGENS.

Steam (at 140°F for 30 minutes), solarize (double-tent at 160°F for 30 minutes or 140°F for 1 hour), or chemically treat growing medium. Sanitation is important because Phytophthora 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. For flower production in open fields, solarization in warmer climates has been successful for control of most Phytophthora species in many crops. Care must be taken not to reinfest treated soil via contaminated plants, soil, media, or water. Solarization, steaming, and composting 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 is translocated in the plant through the xylem.
  (Aliette) 2.5–5 lb/100 gal water for foliar spray 12
  COMMENTS: Foliar spray is more effective than the soil drench. When applied as a foliar spray it is absorbed by foliage and moves into roots.
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.
+ 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.



[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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