Phytophthora Root Rot

Trees that do not thrive after planting or quickly develop reddish-brown needles
and exhibit dieback should be checked for Phytophthora.
Phytophthora Root Rot - Articles
Phytophthora Root Rot

Severe tree browning, a symptom of Phytophthora root rot. Courtesy of Tracey Olson, PDA

Phytophthora spp.

Hosts

  • Most Christmas tree species
  • True firs, Douglas-fir, spruce, and eastern white pine highly susceptible

Damage Potential

  • High

Symptoms and Signs

  • Reduced or stunted growth
  • Chlorotic or red-brown needles
  • Needle loss
  • Root decay
  • Bleeding basal cankers
  • Death of the tree

Causes of Similar Symptoms

  • Armillaria root rot
  • Procera root rot
  • Bark beetles

Identification

Phytophthora identification requires laboratory analysis, but some symptoms in the field should make the grower suspect this disease. Trees that do not thrive after planting or quickly develop reddish-brown needles and exhibit dieback should be checked for Phytophthora. Examine roots for symptoms of decay and absence of an extensive feeder root system (Figure 1). Low-lying areas with poor water drainage are especially prone to root rot diseases such as Phytophthora (Figure 2).

Figure 1. Roots decay and trunk base may discolor from Phytophthora infection. Courtesy of Tracey Olson, PDA

Figure 2. Phytophthora infection often follows the slope of a hill. Courtesy of Tracey Olson, PDA

Biology and Life Cycle

Phytophthora is a soilborne water mold that can spread from an infested field to a new field through the movement of water in the soil or on the surface. New infections can occur when the temperatures exceed 59°F (15°C). Resting spores (chlamydospores and oospores) that are capable of surviving for many years in the soil or plant are formed during cold and/or dry periods. When required temperature and moisture conditions are present, these resting spores will germinate and form another type of spore-producing structure called a sporangia. When mature, numerous motile, infectious spores, or zoospores, are released. Zoospores swim for up to an hour through the soil water and are attracted to the plant roots by chemicals that are produced during growth. When they come in contact with susceptible tissue, they germinate and penetrate into the roots, form mycelium, and cause infection. Zoospores are spread farther distances from an infested field to a new field through the movement of flowing surface water. It should be assumed that any plant, soil, or water that is transported from an infested field is contaminated with some type of Phytophthora spore.

Figure 3. Browning and loss of infected tree feeder roots. Courtesy of Edward L. Barnard, Florida Department of Agriculture and Consumer Services, Bugwood.org (#4822096)

As the mycelium continues to develop inside of the roots, the roots will die and turn brown (Figure 3). The fungus will spread from the outer roots toward the larger roots, the root crown, and eventually the stem. The conductive tissue of the plants will decay and prevent flow of water and nutrients to the upper portion of the tree. Needles will first turn chlorotic and then a reddish brown, while branches wilt (Figure 4). This infection will lead to death of the tree.

Figure 4. Sudden wilting and browning of infected tree. Courtesy of Tracey Olson, PDA

Monitoring and Management Strategies

Plantation Establishment

  • Buy and plant healthy seedlings from a registered grower.
  • Do not plant in a field known to be infected with Phytophthora.
  • Monitor seedlings before planting. Look for plants that show reddish-brown roots or other symptoms of root rot and do not plant these in the field.
  • Have suspicious plants analyzed for Phytophthora by a diagnostic lab.
  • Avoid planting Fraser fir in areas that retain considerable moisture.
  • Some growers in Pennsylvania have tried mounding the soil in the rows before planting as a preventative strategy, though no research has been done on the effectiveness of this technique.

Preseason

  • Do not overfertilize or overwater trees.

Growing Season

  • Monitor fields for symptomatic trees. If a tree is suspected to be infected, remove the tree and the root ball from the field and burn them, unless the tree is to be tested for the pathogen.
  • To receive confirmation that the pathogen is Phytophthora spp., send sample to an extension laboratory or state regulatory laboratory.
  • Trees that are in close proximity to known Phytophthora-infected trees should be marketed only as cut Christmas trees as soon as possible.
  • At the end of the season, evaluate results and update records.

Control Options

Biological

  • No recommendations are available at this time.

Mechanical

  • No recommendations are available at this time.

Biorational

  • No recommendations are available at this time.

Chemical

  • No recommendations are available at this time.

Next Crop/Prevention

  • Replanting with susceptible hosts in known Phytophthora-infected fields is not recommended.
  • Most conifers grown in Pennsylvania are susceptible to Phytophthora spp. to some degree. Tolerance to Phytophthora has been shown with some species of Abies, most notably Turkish (A. bournmuelleriana) and Momi (A. firma).
  • Some success has occurred with using grafted Fraser firs to Turkish and Momi fir rootstock in Phytophthora-infected sites, though sources for these plants are limited.