HR – High resistance to rust. Cultivars in this class can exhibit a few pustules throughout the season or may show pustules only toward the end of the season, when the foliage is in decline. The presence of only one rust pustule on the foliage throws a plant from rust resistant into this high resistance category.
MR – Moderate resistance to rust. Numerous pustules, but not enough to damage plant health or performance. The rust generally occurs during or after the bloom cycle.
PR- Poor resistance to rust. Plant is covered with rust pustules, and the degree of rust infestation varies from year to year. Some of these plants are able to bloom reasonably well in spite of the rust. Some seasons, they may not show any rust at all, but the potential for severe rust exists within the plant.