It shall be unlawful to use any poisonous material in the form of spray or dust upon fruit trees, shade trees, or shrubs that are in blossom; provided that such poisonous material may be used prior to the opening of the blossoms and after the petals have fallen from 90 percent of the blossoms; and provided further that the commissioner of agriculture, markets, and food may permit the use of specific materials for fruit blossom thinning sprays after consultation with the research staff of the New Hampshire agricultural experiment station. The commissioner of agriculture, markets, and food shall cause to be published annually in April in the Weekly Market Bulletin a list of approved blossom thinning materials.