An agricultural scientist is evaluating the efficacy of a newly formulated copper-based bactericidal spray designed to control Erwinia amylovora infections on apple orchard foliage. The scientist swabs the upper surface of several leaves before and 1 hour after applying the spray, transfers these samples onto nutrient agar plates, and incubates them at 25∘C25^\circ\text{C}25∘C for 72 hours.
Following incubation, several bacterial colonies are observed on the agar plate inoculated with the 'after' swab.
Suggest two reasons why this experimental procedure does not prove that the copper-based spray successfully killed all Erwinia amylovora bacteria on the treated leaves.
