White smoke coming from the exhaust is caused from burning oil. I would check the oil level and if it is over full the level needs to be adjusted. The smoking should stop shortly after running. If the oil level is ok or low, that is a sing of an internal problem with the engine. Most likely, the problem is a blown head gasket. I recommend have a service technician repair the tractor.
Mowing on a steep angel can cause the oil to run the side of the engine and cause it to burn oil and smoke. If this is, the case just keep an eye on the oil level and avoid moving the ditches.
you can have it repaired at your home by a Sears technician. Here is a link for the website:
Sears Home Services
Here is a link to order any parts you need.
Sears Parts Direct