Repairing a Split in Wood Before Staining

Is it too small for a narrow filler piece? May not look great, but it might hold together better than sawdust and glue.

I just saw in a woodworking newsletter that if you can clamp it well enough, cyanocrylate (super) glue will work on small cracks. Of course, small is a relative term, but it might work for you.

You could try it by mixing up some saw dust and glue and staining it before you apply it to the crack to see if it will take the stain. At home Depot, and I am sure at Lowes as well, there is a product sold in the paint dept that is a two part wood filler. One part is similar to a body filler, the other part is a hardner that is mixed together and used as a wood filler. After it cures one can sand and paint it. I have used it on a wood riser outside but I covered it with another board. The other idea is to replace the tread if possible. What ever product you use to repair the tread stain a sample piece before applying to the tread to make sure you are satisfied with the results.