Writing plural noun as a singular noun

There is nothing wrong with it. Two nouns are never grammatically required to agree with each other in number; instead, the grammatical number of a noun is supposed to make logical sense (or in some set phrases, follow conventional usage). It makes logical sense to say that “mycoviruses” (plural, because you are speaking of multiple viruses) constitute a “powerful tool” (singular, because you are describing them as a single tool).