As a former plant pathologist, pluralised root rots sounds absolutely fine, if you're talking about rots caused by different pathogens (e.g. rots caused by phytophthora vs rots caused by Pythium).
If the study referred to the suppression of one particular type of root rot, then you would specify what kind of root rot in the title, such as: "xxx suppression of Fusarium root rot in tomatoes" (or even more specific).
It's a good example of a term that is plural for some, singular for others. For specialists who understand the multiple varieties of root rot, root rots is fine. For those ignorant of such things, rotting roots is an observation rather than a specific plant pathology and therefore seems better singular.