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Jill Sidebottom, Ph.D.
Area Extension Forestry Specialist, Mountain Conifer IPM
Extension Forestry, College of Natural Resources
Department of Plant Pathology, College of Agriculture and Life Sciences
Original Christmas Tree Note written February 1, 1995. Updated November 1995, January 2004
Phytophthora root rot is the only serious disease of Fraser fir Christmas trees in western North Carolina. It is caused by a fungus 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 degrees F) and soils saturated with water, to infect plant roots.
Symptoms of Phytophthora
The above-ground symptoms of Phytophthora root rot on Fraser fir include yellow-green needles, wilting, slow growth, dead branches, and tree death. The needles remain on dead branches and turn cinnamon brown. Roots of affected trees are also cinnamon-colored to black and lack white growing tips. The outer surface of the root can be pulled away from the inner core. Feeder roots are absent. Cutting into the bark of the trunk of the tree may reveal butterscotch colored wood. Many of these symptoms may initially be present on only one side of the tree or on lower branches since the fungus first infects a root and grows toward the trunk on that side. Eventually the entire tree will die. Infected trees are usually found grouped together in a field or bed. Unfortunately, a tree may be infected with the fungus months to years before the above-ground symptoms are seen.
Phytophthora root rot is caused by several species of Phytophthora. In western North Carolina, the most important species is P. cinnamoni. Often called a water mold, P. cinnamoni produces spores in response to near-saturated soils. The fungus may remain dormant in the soil for many years as mycelium (fungal threads) in infected root pieces or as chlamydospores, a thick-walled resting spore. During the growing season when soils are warm and wet, mycelium or chlamydospores produce sporangia, which in turn liberate zoospores that can swim short distances by means of flagella as they are attracted to root tips where infection occurs.
Reducing the Risk
Controlling Phytophthora root rot requires an Integrated Pest Management 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 the disease and minimize financial loss. Each step outlined in this note will help reduce the risk of getting and spreading this disease.
FIRST STEP: CLEAN SEEDLINGS/ TRANSPLANTS
Phytophthora root rot spreads quickly in seed beds or transplant beds because the seedling roots grow closely together, allowing the fungus to grow from one tree to another. The fungus can be carried to the field on or in infected transplants. Unfortunately, a transplant can appear healthy for several months after it has been infected.
To ensure the cleanest transplants possible, follow these steps :
If you start to see yellowing or dying seedlings or transplants, contact your county Extension agent to determine if Phytophthora root rot is the cause. These symptoms can also be caused by white grubs, transplant shock, drought, over-watering, fertilizer burn, and other causes. 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. Contact your county Extension agent to help you determine if it is safe to use plants from unaffected parts of a bed. Remember that an apparently green and healthy plant may be infected.
Only purchase seedlings and transplants from a reputable dealer. Don't purchase plants from beds that have dead or dying plants.
SECOND STEP: FIELD SITE SELECTION AND FIELD CLEARING
Soils may 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 as well as drains quickly off the field. Examine a 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 placing 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. A 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. In the largest field study of Fraser fir of over 250 sites conducted by NC State University, clay content of the subsoil and shallow soil depth were the two most important site factors to determining Phytophthora incidence.
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 slight possibility that woodlands cleared for Fraser fir already have the fungus 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. Several species of Phytophthora are associated with apple roots and appear to infect Frasers planted nearby. Grass and clover are not hosts and Phytophthora should not be present in old pastures.
THIRD STEP: KEEPING ROOTS HEALTHY
The Phytophthora fungus is attracted to wounded roots. Keeping roots healthy may help reduce Phytophthora development. The following measures will improve root health:
FOURTH STEP: WHAT TO DO IF PHYTOPHTHORA DEVELOPS
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:
Quarantine areas of the field where trees are dying with Phytophthora. Soil from these infested areas can carry the fungal spores. When working in trees, visit infected areas last. Don't carry mud on boots or equipment to areas of the field where trees are not dying. Wash soil off of boots or equipment with water and chlorine bleach when moving from contaminated to clean farms. Keep a ground cover on quarantined areas to reduce the
The best way to manage Phytophthora root rot is to never get it. Disease-free seedlings and transplants, and site selection and sanitation continue to be the mainstays of Phytophthora root rot management.
Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by North Carolina Cooperative Extension nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension agent.
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Jill Sidebottom, PhD
Mountain Horticultural Crops Research & Extension Center
455 Research Drive
Mills River, NC 28759
Phone: 828.684.3562 ~ Fax: 828.684.8715