Question
Hi.I have a collection of about 80 orchids and have noticed white marks that look like dried water on three of my dendrobiums. Have looked and treated for most common problems with no luck it seems to affect the older growth mainly.The plants seem fine other than this.Thank you
John.

Answer
John, this sounds like mineral deposite.  Thia is a common problem for many orchid growers and is especially prevalent in hard watwr areaas where the main culprit is calcium which can be deposited on a plant part when watering with water containing dissolved calcium salts.  Calcium salts are alkaline in nature and can be dissolved by repeated applications of a weak acid such as vinegar to the affected plant part. Another option is using RO water or rain water which can be added to hard water, reducing the concentration of calcium salts in your plant water