Question
Can i spray zinc sulphate on apple orchard at the dormancy stage.what is benefit.

Answer
If you are interested in treating your apple during dormancy, zinc sulfate is probably not the best choice.  If you have had a history of insect problems such as scale, you may want to treat for this pest during dormancy with dormant oil.  However, if you have not noticed a problem, it is not necessary.

Some growers will make a fungicide application in late winter or early spring for fire blight disease control.  This disease commonly affects apples, but the severity varies by region and apple cultivar.  To treat during dormancy, the spray should be made as late in the winter or spring as possible, but before the apple has leaves.  If applied late, the fungicides recommended can cause russeting on fruit.  The products generally recommended for this application contain copper, for instance copper hydroxide, copper sulfate or Bordeaux mixture (lime copper sulfate).  

I am not sure of your location, but if you are in the United States, most of the states where apple is commonly grown have a recommended spray schedule available.  If you search on the internet for your state and apple sprays, it is likely that you will find a guide.  This information will help to learn what insects and diseases are a problem in you location and what products are recommended.

If you do choose to make an application, be sure to follow the label directions.  In addition to telling you how to mix up the product and what it can be sprayed on, the label often gives information regarding temperature.  Many products applied during dormancy need to be applied at certain temperatures and growth stages so they do not damage the plant.

Good luck!