The Cobb scale and the modified Cobb (U.S.D.A.) scale were devised for estimating, by means of diagrams, the proportion of the area of a leaf or stem occupied by rust pustules. Existing diagrams illustrating these scales are inadequate in being arranged in intervals that are too large and irregular, and in not depicting a sufficient range of pustule sizes. New diagrams are presented using smaller regular intervals of rust intensity and a greater range of pustule sizes, thus providing a basis for more objective and accurate determinations.

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