How to Manage Pests

UC Pest Management Guidelines

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 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.

COMMENTS ON THE DISEASE

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.

MANAGEMENT

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.

PUBLICATION

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
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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