Answer:
The error is in the use of "was" before the subjects "pollution and disease".
The incorrect verb is "was".
The correct sentence will be <em>"pollution and disease were to blame for the decimation of the Chesapeake Bay oyster population in the 1990s"</em>.
Explanation:
When we say the subject-verb agreement, then it means that the helping verb used is dependent on the number of the subject(s). This means that if the subject is singular, then the verb will be singular, and if it is plural, then the verb also becomes plural.
Now, taking the given sentence, we can see that the subjects are <em>"pollution and disease"</em>, which is plural in nature. The use of "and" makes it plural, thereby making the use of the singular verb "was" wrong.
Thus, the correct sentence will be <u><em>"pollution and disease were to blame for the decimation of the Chesapeake Bay oyster population in the 1990s".</em></u>
