This is a common problem in understanding and speaking English and probably other langauges.. People who know English well have the structure of the whole sentence in their heads. They "feel" which word is the subject of the sentence. They use the verb form that agrees with it. Here the skeleton of the sentence is "resistance was reported." "Resistance" is the subject. It's singular, so we use the singular verb "was." The phrase "to substances X and Z" modifies the word "resistance" and gives additional information. The problem occurs whenever the sentence has a form like: "[Singular subject] [Lots of other words] [Something plural] [Verb]" If we aren't paying attention, it feels natural to make the verb agree with the words we have just recently spoken. But it is incorrect. It will sound "wrong" to good speakers of English. This kind of error doesn't interfere with understanding. It is a common kind of error even with native speakers. If you are writing, and have time to think about it, the key is to correctly identify the subject of the sentence.

Because resistance t is singular you should use was (the number of substances doesn't matter). Also, it's more correct to say "substances x and z" or "substance x and substance z". Hope this helps.^^

Resistance is singular, so you should say “was reported”. Also, it’s better to say “substance X and substance Y”, or “substances X and Y”.

You can say: Resistance to *substances* X and Y was reported. In short: Resistance was reported. Therefore it is singular. Hope this helps, Angela! :)

I recommend you teach yourself how to "diagram sentences". If you learn how to do this, you will never again be confused by situations like this.