Oedema is a physiological disorder of orchids caused by overwatering. The excess water is absorbed by the roots quicker than it is lost by the leaves, which can cause swelling of plant cells and produce a lesion resembling a blister.edema is a physiological disorder of orchids caused by overwatering. The excess water is absorbed by the roots quicker than it is lost by the leaves, which can cause swelling of plant cells and produce a lesion resembling a blister.