Rainy Season Equals Phytophthora Disease

Spring rains can create growing conditions that are devastating to most landscape plants.  Wet soils are favored by a group of pathogens called water molds, or oomycetes, which cause a range of root and stem diseases.

Water molds are found in most soils, but plant stress and high pathogen numbers can lead to severe disease. One common water mold is Phytophthora. This pathogen is common in Kentucky and has recently been diagnosed causing root rot on numerous plants, such as blueberry, arborvitae, and Colorado blue spruce.

Phytophthora Facts

  • Symptoms vary greatly due to disease severity and host characteristics.
  • Roots are concealed, so disease often goes undetected until plants begin to decline or upper plant parts wilt (Figure 1) as a result root reduction (Figure 2).
  • Disease often begins during rainy spring weather, but it is typically not noticed until hot dry weather initiates wilting.
  • Aboveground infections may result in symptoms ranging from yellow mottling of leaves to water-soaked lesions on leaves or succulent stems. Woody tissues may develop cankers, often near the soil line.
  • Free water is required to allow for “swimming” spores to move to new sites of infection.
  • Spores are spread by splashing water and movement of contaminated soil particles.
  • The pathogen can produce survival structures that allow it to lie dormant during hot dry seasons or during winter.
Figure 1: Upper portions of the plant may decline or die-back as a result of Phytophthora root rot. Note excess water puddling. (Photo: Nicole Ward Gauthier)

Figure 1: Lower portions of the plant may decline or die-back as a result of Phytophthora root rot. Note excess water puddling. (Photo: Nicole Ward Gauthier)

Figure 2: Phytophthora root rot results in root reduction. (Photo: William M. Brown Jr., Bugwood.org

Figure 2: Phytophthora root rot results in root reduction. (Photo: William M. Brown Jr., Bugwood.org

 

Management

Most Phytophthora diseases can be prevented or managed using cultural practices.  Consider the management tips below to prevent infections or to help manage infected nursery or landscape plants.

  • Improve drainage through management of surface water, limited irrigation, diverting downspouts, or planting in raised beds.
  • Disinfest tools and containers.
  • Dispose of infested potting media.
  • Inspect plants prior to purchase or during production to insure that plants are healthy prior to installation.
  • Do not compost infected plant material.
  • Remove plant debris and other sources of inoculum.
  • Mulch plants to reduce spore splash.
  • Use resistant cultivars whenever possible.

Phytophthora spp. are not true fungi, so not all fungicides will be effective against these pathogens.  Fungicides must be specifically labeled for oomycetes. Homeowners can utilize fungicides containing phosphorus acid to protect plants from infection or suppress disease development. Commercial production fungicides include products containing cyazofamid, etridiazole, mefenoxam, or phosphorus acids. For additional information on fungicide use, please contact a local UK Cooperative Extension Service agent. Always follow label directions when utilizing fungicides.

Resources

  • Fungicides for Management of Diseases in Commercial Greenhouse Ornamentals (PPFS-GH-03)
  • Fungicides for Management of Landscape Woody Ornamental Diseases (PPFS-OR-W-14)
  • Homeowner’s Guide to Fungicides (PPFS-GEN-07)
  • Landscape Sanitation (PPFS-GEN-04)
  • Relative Effectiveness of Various chemicals for Disease Control of Ornamental Plants (PPFS-GEN-13)

 

By Nicole Ward Gauthier, Extension Specialist and Kim Leonberger, Extension Associate

 

 

Posted in Landscape Trees & Shrubs, Ornamentals