Landscape Evaluation of Daylily Cultivars for Daylily Rust: Association of Ploidy with Disease Severity

Daylilies (Hemerocallis species and hybrids) are popular perennials for landscapes, performing well in full sun, heat, humidity, and periods of dry weather. As of early 2015, over 78,000 cultivars have been registered with the American Hemerocallis Society. Daylilies are generally considered to be pest-free; however, a rust disease (Puccinia hemerocallidis), which was introduced into the U.S. on imported plants in 2000, has become a prevalent problem on daylilies in the lower South. Through the cooperation of a daylily grower in south Mississippi, we surveyed a landscape planting of 575 newer cultivars (including 163 diploids and 412 tetraploids) which had not been sprayed with fungicides to prevent infection by daylily rust during the summer of 2013. The warm, damp summer of 2013 was ideal for spread of daylily rust. Plants were rated at the end of the summer on a 1 to 3 scale: 1 (no or little visual sign of infection), 2 (moderate infection), or 3 (severe infection). Tetraploid cultivars were associated with higher daylily rust severity ratings than diploid cultivars based upon our analysis using Fisher's Exact Test (Freeman-Halton Test) (p < 0.0001). This association was also shown in our analyses of ratings data on older daylily cultivars published by other researchers in 2002 and 2003. This association may be of interest to daylily hybridizers and as a basis for future research.