When sufficient evidence, as determined by the Secretary, is presented to the Secretary by watermelon producers, handlers, and importers, or whenever the Secretary has reason to believe that a plan will tend to effectuate the declared policy of this chapter, the Secretary shall give due notice and opportunity for a hearing on a proposed plan. Such hearing may be requested by watermelon producers, handlers, or importers or by any other interested person, including the Secretary, when the request for such hearing is accompanied by a proposal for a plan.