Ministry of Agriculture

Diagnosing Diseases of Trees and Shrubs in the Landscape

Introduction:

Diagnosing and managing diseases and disorders of landscape trees can be challenging. Often, environmental factors cause damage or lead to poor tree vigour and any disease-causing organisms (or damaging insects) found are secondary. Be sure to look at the whole picture and consider the history of the site and pattern of damage when diagnosing problems.

Following is a description of some of the most common diseases encountered. If a disease-causing agent is suspected, you may wish to submit a sample for analysis to the Ministry of Agriculture Plant Diagnostic Laboratory in Abbotsford. The lab does not accept home garden samples directly, but will accept samples collected and submitted by professional landscapers, consultants, garden centres or Master Gardeners. Instructions for submitting samples and a fee schedule are attached.

Basic Information:

Landscapers should consult the "Nursery and Landscape Production and Pest Management Guide" published by the BCLNA/BC Ministry of Agriculture and the Ministry's "Gardener's Guide to Pest Prevention and Control in the Home and Garden" for information on specific diseases, resistant species and control recommendations.

Bacterial Diseases:

There are two common bacterial diseases on woody ornamentals - Erwinia fire blight and Pseudomonas bacterial blight (also called bacterial canker). Fire blight attacks only plants in the Rosaceae family, such as apple, pear, cotoneaster, and rose. Bacterial blight or canker attacks many species; lilac and all Prunus species such as flowering cherry are highly susceptible. Once twigs dry out, it is virtually impossible to isolate the bacteria in the diagnostic lab, even though the symptoms may be strongly suggestive of bacterial disease. Only fungicides containing copper will help control bacterial diseases, but these may cause spotting or burn of young shoots in spring if applied at higher rates or under slow drying conditions. Prune and dispose of dead wood, disinfecting pruners as you go.

Root Rots:

Some trees, such as Lawson's cypress are highly susceptible to Phytophthora root rot. Nevertheless, root rot is almost always the result of over-watering, root damage or prior drought stress, poor soil conditions, flooding, etc. Pythium and Phytophthora can almost always be found in dead, rotted roots, so may not be the primary cause. Except for highly susceptible species, re-planting with healthy trees should not be a problem if the soil and environment are improved and good horticultural practices are followed. Some species such as rhododendron and holly are quite susceptible to a black foliar and twig blight caused by Phytophthora.

Vascular Wilts (Verticillium):

The first symptom is usually dieback of young twigs and branches. A black or brown ring can usually be seen in the vascular tissue of dead twigs. Mature trees may "seal off" the infection and live for many years. Others may die quickly. The outcome is difficult to predict. Infection occurs via the roots, then spreads up through the vascular system in the tree. The fungus carries over in the soil for several years, so only resistant species should be used for re-planting.

Foliar Fungal Diseases:

Leaf spots, shoot blights, twig dieback and "anthracnose" can be caused by dozens of different fungi. These can usually be controlled in the landscape by removal of infected leaves plus a general broad-spectrum fungicide if necessary. For some diseases, fungicide sprays should be timed to the sporulation of the fungus - spring or fall applications are usually recommended. Many foliar diseases also cause twig and branch cankers.

Fungal Cankers:

If twig and branch cankers are present, these should be pruned back to healthy wood as much as possible. Pruning tools should be disinfected between cuts, using 10% bleach, 70% rubbing alcohol or shellac thinner. Fungicide applications should be timed to the sporulation of the fungus as for foliar diseases.

Viruses:

Many woody ornamentals are susceptible to virus diseases. These will not usually do any damage to the plant unless it is severely pruned or growing under stressful conditions. However, a stress episode can then lead to irreversible decline when combined with virus infection. The viruses do not usually spread to other species in the landscape. Some, such as rhododendron ringspot and rose mosaic are easily identified by the symptoms on the leaves. In other cases, the symptoms are more general and can resemble nutrient deficiencies. A nutrient test of both soil and leaf tissue is recommended first, when these symptoms are observed.

The Plant Diagnostic Lab is not equipped to identify most specific viruses. Once other causes are eliminated and a virus is suspected, it may be recommended that a sample be sent to Phytodiagnostics Co. in Saanichton, Vancouver Island for further identification. In most cases, the submitter will be asked to contact the company directly if they wish virus testing to be done.

Root/Stem Galls:

Crown gall disease caused by Agrobacterium is occasionally a problem on woody ornamentals. Crown gall bacteria have a very wide host range. Some plants particularly susceptible to crown gall include rose, walnut, stone and pome fruit trees, poplar and willow. The bacteria that cause crown gall carry over in the soil so it is important to re-plant with resistant species.

Unfortunately, there exists no good test to confirm crown gall disease. It is difficult to isolate the bacterium in culture since many older galls do not contain viable bacteria and non-pathogenic strains of the bacterium are common in soil.

Not all root galls on ornamentals are caused by crown gall. Black locust and other leguminous trees have nitrogen-fixing nodules on the roots that resemble small galls. Rhododendrons sometimes have "physiological" galls on stems, crowns and roots that look very similar to crown gall. Some rose varieties develop a graft-union gall which results in a weak scion with nutrient-deficiency symptoms. This is a physiological gall not caused by disease.

Many woody plants also exhibit branch and trunk galls and burls of unknown origin. Others have been associated with a Phomopsis sp. fungus. Some are caused by aphids, wasps or other insects.

Powdery mildew:

A white powdery fungal growth occurs on the upper surface of leaves and on buds. It is not usually damaging unless quite severe, but some varieties of azalea/rhododendron, rose and lilac may be quite susceptible. Rake up and dispose of fallen leaves (do not compost). Sulphur-containing fungicides may be used in the home garden, but be careful of phytotoxicity, especially during bloom. Do not use sulphur on Viburnum.

Rusts:

These fungi cause unsightly yellow, orange or brown spots on leaves, often accompanied by early leaf drop. Some need an alternate host to complete their life cycle each year (such as pear trellis rust on pear and juniper); others will continue to infect only one host species year after year. Control consists of removing one host in the first case, plus raking and disposing of infected leaves. Copper or sulphur fungicides can also be used if necessary.

Some Common Problems:

  • Prunus, Maple, Lilac, Magnolia - black twig die-back in spring, leaf spots, shothole and/or blossom burn -usually Pseudomonas syringae bacterial blight.

  • Apple, Pear, Cotoneaster - die-back in early summer - twigs and leaves brown and burnt - often fire blight (Erwinia amylovora).

  • Apple - branch and twig cankers - anthracnose or European canker fungi.

  • Oak, Dogwood, Maple, Sycamore - twig die-back in spring and/or summer leaf spots - often anthracnose fungi. May require a lab diagnosis to distinguish from winter damage.

  • Rhododendron - leaf browning in spring is often due to winter damage - blackening of crown tissue is usually evident also. Leaf and shoot blight may also be caused by summer drought stress, sunscald, root rot, stem cankers or Phytophthora foliar blight. Wilting is often caused by Phytophthora root rot.

  • Western Red Cedar (Thuja plicata) - scattered browning of foliage in summer/fall is usually "cedar flagging". A natural, physiological phenomenon - often worse in dry years.

  • Cedar Hedges - die after transplanting in summer. Usually transplant stress from drying out of root ball. Thuja are quite shallow rooted and will succumb to drought in hot dry weather. Dead trees are often replaced in mid-summer with the same effect. Wait to re-plant until fall when more moisture is in the soil. Thuja are quite resistant to root rot unless flooded, but have been known to succumb to Armillaria rot - large white fans of fungus occur under bark at the butt of trees and also black "rhizomorphs" that look like shoe-strings.

  • Birch and Willow - die-back - often borer insects - look for bore holes in trunk. Birches have a natural life span of 30-40 years. Willows are subject to fungal twig blight.

  • Prunus and Laurocerasus - leaf shothole - the most common cause in coastal B.C. is Pseudomonas bacterial blight. Shot-hole on leaves can be caused by fungi too, which are more common in the Interior.

  • Maple, Magnolia, Prunus - die-back from tips of branch first, often one-sided on tree - possibly Verticillium wilt - check for black /brown streaks in vascular tissue in branches and twigs.

  • Lawson's cypress (Chamaecyparis) - very susceptible to Phytophthora root rot which causes die-back, then death of the tree.

  • Maple, Horse Chestnut, Oak - leaf scorch in summer. Often drought-stress, but some anthracnose fungi and a bacterial leaf scorch can cause similar symptoms. The fungal spore bodies are difficult to detect so this usually requires a lab diagnosis.

  • Holly - blackening of twigs and leaves in winter - often due to a foliar Phytophthora disease.

  • Juniper, Cypress, Pyramid cedars - browning foliage in "patches" - spring/summer - fungal foliar blights. Many are relatively weak pathogens that attack plants under stress or following winter damage or root rot. (Check also for girdled twigs from insect or rodent feeding).

  • Pine - Shoot and branch die-back - many conifers suffer winter damage or drought stress and some naturally shed their needles at certain times of the year. Needle cast fungi can be primary or secondary pathogens. Branch die-back can also be caused by insects - look for tiny bore-holes at the base of dead branches.

Submitting Samples to the B.C. Plant Diagnostic Laboratory

  1. Packaging:
    A dried out or rotten, mushy sample leaves little to work with. When you take a leaf or shoot sample, put it in a paper bag first, then into the plastic bag while fresh. Place in the fridge or cooler and send as soon as possible. Woody twigs or root balls should be wrapped in slightly damp newspaper or waxed paper to keep them from drying out. Leave some soil on roots.

  2. Sampling:

    1. Wilt symptoms? Dark streaks in the vascular system of dying branches or twigs? Select some that are not yet completely dried out.
    2. Is it a canker or die-back? Check where is healthy tissue vs. dead tissue and submit a sample that includes both. The edge of the dead/green tissue is where active fungal and bacterial pathogens can most often be found.
    3. Browning on leaves? Often a symptom of stem cankers, crown necrosis or root rot/damage. Check for these things before sending a leaf sample only. For leaf diseases, send whole shoots if possible.

  3. Analysis Requested:
    Be as specific in your request as possible. For example, if you just want the lab to check for one specific disease, such as fire blight, please state this. If you already suspect an "abiotic" cause, such as herbicide, environmental or nutritional factors, but you just want the lab to check for possible diseases to be certain, please state this with the submission.

  4. Provide a Description:
    The "pattern" of symptoms is very important: If more than one tree, how many trees are affected? All, in patches only, or scattered? Are symptoms on the lower, upper branches or scattered all over the tree? Is only old growth or new growth affected? Are there symptoms on other plants or vegetation? When were the symptoms first observed? Is this a new problem this year or an ongoing one? Please include:
    • Age or size of the trees.
    • When planted?
    • Where located? (street, backyard, park, small orchard, etc.)
    • Species and variety, if known.

  5. Replies to Submitter:
    A Fax number for return of diagnosis or full address plus Postal Code for mailing is required. If you do not specify, the diagnosis will go to the submitter only. If you wish the diagnosis to go to grower/owner also, include their address and/or Fax.

  6. Other Problems:
    Before sending a sample of "disease" or "insect damage" to the lab, have you checked the tree completely for other problems that may be caused by animals, nutritional or environmental factors? Are you sending the right sample and symptoms?

  7. Timing:
    Submit suspected problems promptly. If the tree had die-back in April-May, the lab will not likely be able to diagnose the cause in August or September. (However, during the lab's busiest season, May to August, please send only necessary samples).

  8. Separate Samples:
    Number and identify samples clearly with waterproof marker or pencil. Different problems at the same site will be considered as separate samples for diagnosis and charged separate fees, even if submitted together. For example, an oak with dieback and a maple with leaf spots from the same park or landscape would be considered 2 different samples (different symptoms and host) and charged 2 separate fees, even if they are submitted at the same time. In this case the causal agents are probably different and each plant will require a separate diagnosis. However, if the same cause, such as a herbicide or an environmental factor is suspected to have damaged several different species at the same time in the same general area, these may be submitted together as one "sample" and charged one fee, since it will require only one diagnosis to determine the cause. Similarly, a consultant may submit 3 or 4 branches from different apple orchards all with the same symptoms as one sample for one diagnostic fee.

  9. Submission Forms:
    Please fill out and submit a form with each separate sample. For samples sent by mail or courier, the form is available on the ministry website, in the Nursery and Landscape Production and Pest Management Guide and from most Ministry of Agriculture offices.

  10. Fees:
    Refer to the laboratory website for current fee structure. Payment in advance must accompany each sample submitted.


April, 2000