The following was the correct answer to a GMAT sentence correction problem. I was wondering if someone could explain to me why the verb following "when supplies" should be "are" instead of "were". It seems to me that the usage of "could" here indicates a hypothetical situation which would indicate that the verb should take the second conditional form which would be past tense. Please let me know why I'm wrong.
Coffee prices rose sharply Monday, posting their biggest one-day gain in almost three years, after a weekend cold snap in Brazil raised concern that the world's largest crop could be damaged at a time when supplies are already tight.