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How to Manage Pests
UC Pest Management Guidelines
Floriculture and Ornamental Nurseries
Phytophthora Root and Crown Rots
Pathogen: Phytophthora spp.
(Reviewed 1/02,
updated 1/02)
In this Guideline:
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SYMPTOMS
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 is 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.
COMMENTS ON THE DISEASE
The fungi 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 fungi 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 source. ELISA test kits are available for detecting
Phytophthora fungi.
MANAGEMENT
Emphasis in control of Phytophthora diseases is placed on providing good
drainage and water management. Whenever possible, plant resistant varieties. 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 are common in surface
waters. Both fungi are introduced via
contaminated 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.
Common name
(trade name) |
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 is 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: Foliar spray is more effective than the soil
drench. When applied as a foliar spray it is absorbed by foliage and
moves into roots. |
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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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