Foliar symptoms
Foliar symptoms are likely to be the first indication of crown or collar rot, but are not diagnostic, merely indicators of root or vascular problems in the tree.
- Affected trees are unthrifty, showing poor extension growth.
- Foliage is often sparse, chlorotic and may show premature colouring in the autumn.
- Fruit is often small and colours prematurely.
- Trees affected by collar rot usually decline progressively over several seasons.
- In comparison, trees with crown rot, particularly in young orchards, usually die quite quickly, especially after a wet autumn or spring.
Bark symptoms
Once foliar symptoms have been identified, examination of the scion or rootstock bark may provide more diagnostic opportunity.
- Typically diseased bark is orange – red/brown in colour or dark-brown if the rot is advanced.
- Active collar rot lesions may also be tiger-striped under the bark and may weep on the outer bark surface.
- In extending lesions, the margin between healthy and necrotic areas is indistinct. This is the tissue from which the pathogen is most easily isolated.
- In crown rot the necrosis often extends some way along the main roots and sometimes up to the graft union, but rarely above it unless the scion is a very susceptible variety like Cox.
- In collar rot, the necrosis may extend right up the trunk, into one of the main branches, but does not usually extend below the graft union.
- No fruiting bodies are visible on the lesions. This helps to distinguish it from other wood rotting fungi, but in advanced collar rot, secondary rot fungi may invade the dead tissues.
Other problems that may be confused with crown and collar rot
Crown and root rot are most easily confused with root death/tree death due to waterlogging or wet feet which occurs if roots are waterlogged for significant periods when trees are physiologically active.
- The problem results in similar foliar symptoms.
- If the roots and crown are examined, root death due to waterlogging usually shows as dark brown/blackened roots rather than the distinct orange red/brown colour of Phytophthora.
- In addition, there is a sour smell rather than the alcoholic smell associated with Phytophthora root rots.
- Waterlogging may also result in bark necrosis but this is probably caused by diffusion of toxins brought up in the water conducting tissues, rather than by fungal activity.
- Such bark necrosis is also associated with deep staining in the central woody part of the trunk which may extend for some way up the tree.
- This symptom is not typical of Phytophthora diseases.
Collar rot, particularly when lesions are well advanced, may be confused with other wood rotting fungi, such as some of the bracket fungi.
- However, examination of the tissue under the bark, especially the wet orange/red-brown rot, usually distinguishes collar rot.