If it appears from an investigation that the revenue from the assessment levied hereunder is inadequate to accomplish the purposes of this chapter, the commission shall file with the director of the department of agriculture a report showing the necessity of the industry, extent and probable cost of the required research, market promotion and advertising, extent of public convenience, interest and necessity, and probable revenue from the assessment desired to be levied. It shall thereupon increase the assessment to a sum not to exceed ten cents (10¢) per forty (40) pounds shipped in bulk, container or any style of package; but no increase shall be made prior to filing of said report and finding. Provided, however, that no increase in such assessment shall become effective unless the same shall first be referred by the commission on a referendum mail ballot of the apple growers of this state, and be approved by two-thirds (2/3) vote of the growers of fifty per cent (50%) or more of the acreage represented in the voting.