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Oedema Disease also called Corky Scab is characterized by hard corky growths on the underside of leaves.
This corcky growths is the houseplant's response to soggy soil coupled with low light intensity.
In such conditions, the plant will rapidly absorb water from the soil but since the light available is little, most of the water cannot be used up as quickly which results in the cells bursting. The hard corky spots are due to the burst plant cells.
1.Remove and discard the badly affected leaves as they will not recover.
2.Move the plant to a better lit spot. Ensure you get the correct light requirements for your plants. Check out this guide on understanding light for houseplants.
3.Reduce the frequency of watering. Ensure you get the right water requirements for your plant to avoid overwatering. Check out this houseplants a-z guide for your plant.