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Authors
Introduction
Symptoms/Signs
Causal Organism
Disease Cycle
Diagnostic Methods
Disease Management

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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
Department of Forestry and Environmental Resources, North Carolina State University


K.L. Ivors, Extension Plant Pathology
D.M. Benson, Research Plant Pathology
Department of Plant Pathology, North Carolina State University

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.

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April 2007