Why is it a problem? Tree lichens are common in all apple orchards. They are only a problem if allowed to cover branches and young shoots. In severe cases they cause drying of branches and reduce growth and fruiting. This normally only occurs in poorly managed orchards and can be rectified.
Where and when is it a problem? Lichens occur on apples trees throughout Bhutan, but normally only become a problem in poorly managed orchards. Serious cases have been reported from Thimphu and Paro.
In Bhutan foliose and fruticose lichens are found growing abundantly on apple trees. The rhizines (similar to roots) allow them to attach to the tree. They grow through the cork, cortex, and cambium, down to the living wood. The rhizines block lenticels, split cork layers horizontally, and by increasing air exchange in the bark indirectly cause the cork cells to thicken and become more permeable to water.
Small shrubs and trees densely covered with lichens can show clear signs of decline or death of branches.
Confusion with other pests: The lichens haven’t been identified to species. There may be a range of species affecting apple trees, but their management is the same.
Lifecycle: Lichens on trees are a unique organism because they are the result of a symbiotic relationship between two organisms — fungus and algae. The fungus grows on the tree and can collect moisture, which the algae needs. The algae, in return, can create food from the energy of the sun, which feeds the fungus.
When can damage be expected? Tree lichen is ubiquitous on apple trees. However, it normally only becomes a problem in poorly managed orchards where pruning and tree replacement is not properly practiced.
Serious lichen infestations are generally a symptom of poor orchard management. It is easily prevented through good orchard management, and can be rectified.
Monitor for lichen build up as part of general orchard monitoring. Management is needed if lichens cover the trees, which is likely to be sufficient to reduce fruiting, or if there are signs of branches drying up.
Effect of variety
Maintenance of good plant health
- Pruning is necessary to reduce the conditions favourable for the growth of lichens on apple trees.
Control using Lime + Water
- Mix 1 kg lime (fine commercial lime) in 50 litres of water. This is sufficient for about 40 trees. The mixture should be filtered before spraying to prevent clogging the sprayer.
- Spray trees uniformly to the point of run-off during the dormant season (December to February).
- After spraying, observe regularly till the lichens turn reddish-brown.
- Measure lime concentrations carefully: too weak and it won’t work and too strong risks cracking the bark.
- Use a gunny bag or a cloth to scrub the lichens right after rainfall. This can be done in spring.
- Be careful not to scrub the tree surface too hard, as this can damage the bark, making the tree vulnerable to insect pests and diseases
- This method is time consuming and lichen can regenerate.
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Image acknowledgements: NPPC.