There are various possible translations for this term as used in this context. "Treatment" is one, but somewhat limited to trials where the different "modalites" actually are fungicide, insecticide, etc. treatments. A client for whom I translate this kind of stuff on a fairly regular basis prefers the more inclusive term "objects", which would also cover different cultural practices, different varieties, or whatever and not just different pesticides.

It refers to the tree formation methods (depends on the distance of plantation etc.)