Phytophthora Root and Crown Rots

Many, if not most, Phytophthora species can infect roots in the same manner as Pythium species. In addition, Phytophthora species infect crowns, stems, and larger roots, particularly in woody plants. Infection of the roots, crowns, and lower stems result in dark, discolored tissues. Tissues of diseased roots become soft and mushy and outer layers of the root may slough off. Plants with infected roots and crowns become stunted, are low in vigor, wilt, and appear as if they were water stressed. Foliage turns yellow, leaves fall off, and the plant may wilt and die.

Aerial plant parts, including branches and shoots, can also be infected by some species of Phytophthora under wet conditions if infested soil, water, or airborne spores contact these aboveground parts.

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. Root and crown rots are most common under wet or over-irrigated soil conditions. Ideal soil conditions for the growth of Phytophthora are wet soils 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 plant 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 with 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 as 20% of potting mix is reported to provide some control of Pythium and Phytophthora root rots. For flower production in open fields with warmer climates, solarization has successfully controlled most Phytophthora species in many crops. Care must be taken not to re-infest treated soil via contaminated plants, soil, media, or water. Solarization, steaming, and composting are acceptable for organic production.