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.
We look at the whole picture and consider the history of the site and pattern of damage when diagnosing problems.
Here is a taster of the things to look out for:
Some trees 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.
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.
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.
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.
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.
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.
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.
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.
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