Click on images to enlarge

Figure 1. Fraser fir root infected with Phytophthora, showing distinctive ‘root sloughing’.

Figure 2. Lesion caused by Phytophthora on trunk of infected Fraser fir.

Figure 3. Symptoms of Phytophthora root rot often appear first on lower branches, on one side of the tree.

Figure 4. Infected Fraser firs are often found within clusters in the field.

Figure 5. Mycelia of P. cinnamomi, showing distinctive ‘hyphal swellings’ (photo credit: Larry Grand, NCSU).

Figure 6. Thick-walled chlamydospores of P. cinnamomi.

Figure 7. Non-papillate, lemon-shaped sporangium of P. cinnamomi.

Figure 8. Zoospores produced within a sporangium of P. cinnamomi.

Authors

J.R. Sidebottom, Extension Forestry
K.L. Ivors, Extension Plant Pathology
D.M. Benson, Research Plant Pathology

Introduction

With the rapid increase in Fraser fir Christmas tree plantings, growers are facing a number of production issues. Most notably, Phytophthora species represent a serious limitation to the North Carolina Christmas tree market. Phytophthora root rot and stem canker, caused by several Phytophthora species, has been associated with significant damage to Fraser fir since the 1960s. Phytophthora is a fungus-like organism that inhabits the soil and infects many woody plants through the roots. It can lie dormant in the soil for several years, waiting for a susceptible host such as Fraser fir and the right environmental conditions, including warm soil temperatures (above 54°F) and saturated soils to infect plant roots.

Symptoms/Signs

The above-ground symptoms of Phytophthora root rot on Fraser fir include yellow-green needles, wilting, dead branches, and tree death. These dead needles typically turn cinnamon-brown and remain on the branches, eventually resulting in a bronze-colored tree. Roots of affected trees are cinnamon-red to black in color and lack white growing tips. The outer surface of the root can be pulled away from the inner core, also called 'root sloughing' (Figure 1), and feeder roots are often absent. Cutting into the bark of the trunk of the tree may reveal butterscotch colored wood (Figure 2). Often these symptoms are initially present on only one side of the tree or on lower branches, since Phytophthora first infects a root and then colonizes the trunk on that side (Figure 3). Unfortunately, above-ground symptoms of the disease are often not apparent until the roots are heavily infected, after which death of the tree follows. Infected trees are usually found grouped together in a field or bed (Figure 4).

Causal Organism

Phytophthora root rot is caused by several species of Phytophthora, although in North Carolina, the most important species is P. cinnamoni. In addition, a few undescribed Phytophthora species have been recently isolated from symptomatic fir Christmas trees growing in seedling beds and plantations in multiple states. During the growing season when soils are warm and wet, mycelia (Figure 5) or chlamydospores (Figure 6) germinate and produce sporangia (Figure 7). These lemon-shaped structures cause new infections, either by germinating and colonizing roots, or by releasing zoospores (Figure 8) in water that have formed inside each sporangium. Zoospores are able to swim using their two flagella, and are capable of directional movement to host plants based on chemical attraction.

Disease Cycle

The disease is correlated with abnormally high soil moisture caused by frequent precipitation, flooding and poor soil drainage, as Phytophthora produces spores in response to near-saturated soils. Phytophthora species can spread by contaminated equipment, with infected nursery stock, or with water runoff from nearby infested sites. Transplanting infected nursery seedlings represents a major contribution to disease incidence in the field, yet there are no current standard methods for certifying seedlings as pathogen free. Due to the increase of Phytophthora species in seedling beds in North Carolina, growers have been purchasing field-ready transplants grown in other states such as Michigan, Oregon, Washington and Pennsylvania, to avoid purchasing infected seedlings. However, fir seedlings grown in other states also have the potential of being infected with their own regional Phytophthora species. Therefore, purchasing and transplanting seedlings from other states increases the potential for new introductions of exotic Phytophthora species. After disease develops in the field, growers have few options beyond harvesting as soon as possible, as the pathogen can survive extended periods of time as spores in the soil, in pieces of organic matter, or in roots of fir seedlings and trees. Due to this persistence, once Phytophthora becomes established on a site, the area is often no longer used for Fraser fir production.

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Diagnostic Methods

If you start to see yellowing or dying seedlings or trees, contact your County Extension Agent to collect samples to determine if Phytophthora root rot is the cause. Root and/or soil samples should be sent to the Plant Disease and Insect Clinic (PDIC) at North Carolina State University in Raleigh http://www.ces.ncsu.edu/depts/ent/clinic/ to determine if Phytophthora species are present. This is best done after soil temperatures have warmed above 54°F. Root rot symptoms can also be caused by white grubs, transplant shock, drought, over-watering, fertilizer burn, and other problems. In the PDIC, root samples are washed, and suspicious root segments are collected, surface sterilized with a bleach solution, and plated onto selective media for Phytophthora isolation, known as PARPH. Although root samples are preferred by the PDIC, soil samples can also be baited for Phytophthora, by floating rhododendron leaves in water containing the soil for a few days. These leaves are then plated onto selective media for isolation of Phytophthora. Cultures growing on the selective media are identified by morphology using a microscope.

Disease Management

Managing Phytophthora root rot requires an Integrated Pest Management (IPM) approach. No single control strategy will prevent or control this disease. As with most plant diseases, the best control is through healthy seedlings and transplants, and proper site selection. If trees become infected with Phytophthora, management should change to practices that reduce the spread of root rot and minimize financial loss. The steps outlined below will help reduce the risk of getting and spreading this disease.

• First step: Obtain clean seedlings/transplants
  
  Unfortunately, a transplant can appear healthy for several months after it has been infected. TO ENSURE THE CLEANEST TRANSPLANTS POSSIBLE, FOLLOW THESE STEPS:
  
  1) Select the site for seedling or transplant beds where there has never been an incidence of Phytophthora root rot. Beds should be located in an area where they will not be flooded, where water will not drain through the beds, and where soil is not tight or high in clay.
  
  2) Prepare raised beds that are 6-8 inches in height to increase water drainage. Soil should be carefully prepared to break up all clods and allow plant material to decay.
  
  3) Fumigate the soil before sowing or planting with methyl bromide or some other labeled soil fumigant. This will reduce the amount of Phytophthora in the soil. Methyl bromide should only be applied when the soil temperature is above 50°F. For any treatment, label directions must be followed.
  
  4) Irrigate seed beds and transplants with well water. Water from farm ponds, creeks and rivers may be contaminated with Phytophthora spores, which could infect plants.
  
  5) Use Subdue MAXX fungicide in the spring and fall to prevent disease development. Apply 2 1/2 pints Subdue MAXX in at least 50 gallons of water per acre in seed beds and 5 pints Subdue MAXX in at least 50 gallons of water per acre in transplant beds. Consider rotating with another fungicide such as Aliette. Aliette should be applied to the foliage until run-off using 5 lb of product per 100 gallons of water. (Note: waiting to use Subdue MAXX until after trees start dying from Phytophthora will only give you a false sense of security. Subdue MAXX will protect living tissue as long as the dosage remains in the plant. However, when it wears off, the plant is again at risk. Subdue MAXX will not eliminate Phytophthora from the soil or from dead roots. Once the trees are moved to the field and are no longer being treated with Subdue MAXX, Phytophthora will become active again and trees may start to die).
  
  6) Only purchase seedlings and transplants from a reputable dealer. Do not purchase plants grown in beds that have dead or dying plants.
  
  7) Do not set transplants if roots are discolored and exhibit 'root sloughing'.
  
  If Phytophthora is diagnosed and diseased seedlings are isolated to one corner or section, you may be able to use plants in the rest of the bed or adjacent beds. However, remember that an apparently green and healthy transplant may be infected.
  
  
• Second step: Site selection and field clearing
  
  When temperatures are warm enough, soils only need to be saturated for several hours for Phytophthora to infect roots. Fraser fir should only be planted in fields where water drains quickly down through the soil and quickly off the field. Examine the potential field site for any areas where water collects or drains. Are there wet weather springs? Does a culvert drain onto the field? Be sure when establishing field roads that problems with water drainage aren't created. It may be necessary to go to a site during a heavy rain to observe water drainage.
  
  Examine the soil at potential field sites to determine how easily water will drain down through the soil profile. High clay content decreases water flow and holds water longer. Not only is clay in the topsoil a potential problem but also clay in the subsoil. Hardpans and shallow soils will slow water flow. Compacted soils also hold more water and slow water flow down through the soil.
  
  If fields are to be cleared of brush with heavy equipment, special care should be taken to reduce soil compaction and the loss of topsoil since this will increase the risk of Phytophthora root rot. Do not use heavy equipment when the soil is wet. Do not push topsoil off the site. Sow a cover crop in the field to help repair soil structure after clearing before trees are set.
  
  Phytophthora can infect several hundred species of plants including red bud, dogwood, rhododendrons, mountain laurel, white pines, and honeysuckle. There is a possibility that woodlands cleared for Fraser fir production already have Phytophthora in the soil. Also, growers setting Fraser fir in old apple orchards or where apple trees were growing in old pastures have had problems with Phytophthora. Grass and clover are not hosts and Phytophthora should not be present in old pastures where there were no apple trees.
  
  
• Third step: Keep roots healthy
  
  Phytophthora is attracted to wounded roots. Keeping roots healthy may help reduce root rot development. THE FOLLOWING MEASURES SHOULD IMPROVE ROOT HEALTH:
  
  1. Do not set Fraser fir transplants deeper than 1 inch above the root collar. Forcing a large root system into a small planting hole will cause the roots to grow in the shape of a 'J', weakening root growth. Avoid excessive root pruning when planting.
  
  2. Spread fertilizer evenly. Piles of fertilizer on the ground will damage roots growing directly underneath.
  
  3. Limit use of Simazine. High rates of Simazine will damage roots.
  
  4. Allow ground covers to grow between trees to keep the soil cool. Fraser fir roots will grow closer to the surface of the soil where there is more oxygen and less water.
  
  Contact your county extension agent to learn more about ground cover management with the use of suppressive rates of post-emergent herbicides.
  
  
• Fourth step: What to do if Phytophthora is found
  
  Even with care, Phytophthora root rot can develop, especially after heavy rainfall or flooded conditions. THE FOLLOWING STEPS MAY REDUCE DISEASE SPREAD AND TREE LOSS:
  
  1. Quarantine areas of the field where trees are dying from Phytophthora. Soil from these infested areas can carry spores. When working in the field, visit infected areas last. Don't carry mud on boots or equipment to areas where trees are not dying. Wash soil off of boots or equipment with water and chlorine bleach when moving from contaminated to clean farms.
  
  2. Keep a ground cover on quarantined areas to reduce the spread of infested soil and water runoff.
  
  3. Early harvesting near affected areas may reduce financial losses. Selling smaller trees is better than leaving those trees to die before they can be harvested the following year.
  
  4. Trees immediately surrounding diseased trees may be treated in the field with Subdue MAXX at the rate of 1 1/4 to 2 1/2 gallons per acre in a minimum of 50 gallons of water per acre, Subdue 2G applied at 125 to 250 pounds per acre spread evenly to infested areas, and/or Aliette applied to the foliage at 5 lb /100 gallons. Apply these products in the early spring and again in late summer. Application should be made 1 to 3 days before a predicted rain. Never apply Subdue MAXX to fir growing on bottom lands or poorly drained soils, or near surface water since it may contaminate streams. HOWEVER, these treatments are expensive and may not be cost effective if the trees are more than a year from harvest.
  
  5. Do not replant Fraser fir on sites where Frasers have died from Phytophthora root rot. Alternative species may be used including Colorado blue spruce and Norway spruce. Although white pine and hemlock may also be cultivated in infested sites, they are a host to Phytophthora; if pathogen populations are high or the site is poor, they may die. Phytophthora root rot is becoming more of a problem in second and third rotations of Fraser fir in both beds and fields, as Fraser fir is HIGHLY susceptible to this disease, and continuous cultivation on the same site may promote disease development. At current, there is no natural resistance in Fraser fir to Phytophthora. However, research is currently being conducted to determine if Fraser fir grafted onto the roots of more resistant fir species can survive in areas where Fraser fir has died.
  
  The best way to control Phytophthora root rot is to never get it. Disease-free seedlings and transplants, and site selection continue to be the most important aspects of Phytophthora root rot management.
  
  **
  April 2007