When is a tree unhealthy? The simple answer is when symptoms are observed in the crown, on the foliage or on stems (shoots, branches, trunks). The impracticality of examining tree roots or failure to do so may mask important evidence, but for serious problems symptoms are usually manifest in other parts of the tree.
The detection of ill health depends on the initial recognition of symptoms. Some symptoms are easy to identify, for example wilted leaves and stems with cankers, but others are not and may be difficult to distinguish from events that occur in the normal cycle of growth. Many trees drop their leaves and stop growing during winter or dry seasons. It is therefore important to understand the tree's normal pattern of growth throughout the year and from one year to the next, according to prevailing conditions at particular sites.
A scheme for classifying symptoms is presented in Table 3. This is based on a comprehensive review of symptom descriptions and categories presented mostly in books on tree pests. The basis for the scheme is the CABI crop protection compendium (see Annex 2), which includes a comprehensive listing of symptoms resulting from biotic and abiotic factors. As shown in the colour plates, similar features can have very different causes. A classification of tree health problems by symptoms will inevitably include more than one type of cause in some categories.
Table 3. Categories of symptoms of ill health
|
CATEGORY /FEATURE |
NOTES |
CORRESPONDING |
|
Altered growth or development |
||
Colour changes in crown |
Change of colour; loss of colour (discolouration) |
1A, 1B |
Change in form or shape |
Galls, swellings an d knots; cracked or split surface, distorted leaves and stems, malformation |
2, 3 |
Growth disturbance |
Growth stimulation; stunted or reduced growth |
4, 5 |
Premature loss or development |
Early leaf drop, senescence or ripening |
6 |
General death |
||
Blight |
Characterized by widespread and rapid killing of plant parts (i.e. leaves, flowers, stems) |
7 |
Dieback |
Progressive death of shoots, leaves or roots, beginning at the tips |
8 |
Wilt and collapse |
Drooping of plants as a result of insufficient water supply |
9 |
Localize d death or necrosis |
||
Spots and lesions |
Many words are used to describe these small localized areas in addition to the common “leafspot”, e.g. blotches, scabs, pits |
10 |
Cankers |
Cankers vary from those with sunken centres to others with raised edges and some with more general swellings |
11A, 11B |
Rots and decays |
Rots and decays, commonly occurring inside major stems or trunks |
12 |
Physical evidence |
||
Damage by animal and insect feeding |
Exit holes, frass, webbing, internal or external shredding, spittle mass |
13A, 13B |
Pest infestation |
Visible insects, visible fungal sporing structures (e.g. bracket fungi), mycelium, growth of moulds or sooty appearance |
14A, 14B |
General damage |
Bleeding, ooze (not bacterial); mechanical damage; adverse climatic conditions |
15A, 15B |
Other growths on trees |
Parasitic plants, epiphytes, lichens, mosses, algae |
16 |
