The reasoning is not very sound, because the citizen uses ad hominem to attack the parks department.
The citizen's argument primarily critiques the credibility of the parks department instead of directly addressing the issue of whether the trees truly have black-spot disease. This line of reasoning relies on an attack on the department's past performance to undermine its current claims, which is not a valid way to assess the health of the trees. Without addressing the evidence or expertise about the disease itself, the argument lacks a solid foundation.