Daylily (Hemerocallis sp.) is a popular and widely planted herbaceous perennial in the landscape, with over 78,000 cultivars registered with the American Hemerocallis Society. Daylily performs well in full sun, heat, humidity, and periods of dry weather, and has generally been considered to be pest free. However, a rust disease (Puccinia hemerocallidis) was introduced in the United States on imported plants in 2000, quickly spreading to become a widespread problem on daylily in and beyond the southern United States. In Aug. 2013, 575 daylily cultivars (mostly newer hybrids) were surveyed for daylily rust in a large landscape planting that had not received any fungicide treatment during the 2013 growing season. Weather conditions during the growing season were favorable for daylily rust. Individual clumps were rated as 1 (no or little visual sign of infection), 2 (moderate infection), or 3 (severe infection). In this survey, 119 cultivars (21%) received a median rating of 1 or 1.5, 230 cultivars (40%) received a rating of 2, and 226 (39%) received a rating of 2.5 or 3. Most cultivars were represented by a single clump, and may thus be more susceptible to daylily rust than a single rating might indicate. Diploid cultivars were associated with lower daylily rust severity ratings than tetraploid cultivars.
Daylily is highly valued throughout the world as an easily grown herbaceous perennial for the landscape, with a proper selection of species and clonal hybrids providing an ongoing display of flowers from spring through fall. Daylily varies in growth habit; bloom season; and size, shape, and color of flowers. The robust and often fleshy roots of daylily withstand drought and their compact and sturdy, clumping growth habits allow daylily plants to tolerate competition from garden weeds (Hill and Hill, 1991; Stout, 1986). Daylily flowers may be used to flavor and garnish culinary dishes (Pollard et al., 2004).
Daylily generally has few insect and disease pests in the landscape; however, daylily rust, caused by the fungus Puccinia hemerocallidis, was observed for the first time in the United States on plants of the cultivar Pardon Me at a nursery in Georgia in Aug. 2000, apparently introduced via plants imported from South America (Williams-Woodward et al., 2001). Daylily rust spread quickly throughout much of the United States through movement of infected stock (Buck and Ono, 2012). Before 2000, the pathogen was known only from eastern Asia (Hernández et al., 2002). Multiple pathotypes of the pathogen have been identified in the southeastern United States (Buck, 2013). Disease management strategies include quarantine of new nursery stock, use of disease-resistant cultivars, scouting for signs and symptoms of rust, and use of preventative fungicides (Buck and Ono, 2012).
The daylily rust pathogen produces yellow-orange to rust-brown pustules containing urediniospores on upper and lower leaf surfaces within 1 to 2 weeks after infection (Williams-Woodward et al., 2001). Additional symptoms reported on infected plants include bright yellow-orange spots and streaks, water-soaked tan spots with darker borders, large yellow lesions, and small discrete spots (Williams-Woodward and Buck, 2001). Mueller and Buck (2003) determined that temperatures around 72 °F with 5 h of leaf wetness are required during infection; however, disease development is not as sensitive to environmental conditions once infection has taken place. Cultivar-specific resistance and partial resistance to the pathogen involve hypersensitive cell death and a delayed latent period and reduced sporulation resulting from restriction of intercellular hyphal growth (Li et al., 2007).
Identification of rust-resistant and rust-susceptible daylily cultivars is of benefit to breeders, commercial growers, and home gardeners, particularly in helping to reduce the time and expense involved in repeated fungicide applications in nurseries and landscapes. In Georgia, Williams-Woodward and Buck (2001) conducted a preliminary greenhouse study with pathogen-inoculated plants of 22 cultivars and Mueller et al. (2003) followed with a greenhouse study with pathogen-inoculated plants of 84 cultivars to assess relative resistance to daylily rust. Robbins and Vann (2002) surveyed 500 field-grown cultivars at a small commercial nursery in central Arkansas in 2001, with a follow-up survey of 500 cultivars in 2002 (Robbins, 2003), with 75% of these cultivars also surveyed in the prior year.
As of early 2015, over 78,000 cultivars have been registered with the American Hemerocallis Society (2015). Little information is available on resistance and susceptibility of daylily cultivars that have been developed and registered since 2000. This report presents results from a survey of 575 mostly modern (registered from 2000 onward) hybrid cultivars of daylily growing in a landscape in southern Mississippi under conditions favorable for daylily rust.
Materials and methods
In Aug. 2013, a large, established, residential landscape planting of 575 daylily cultivars (Table 1) in Hattiesburg, MS, was made available on a short-term basis for a survey of daylily rust severity. Because of planned removal of the planting, the daylily plants had not been treated with any fungicides during 2013. The 2013 growing season was characterized by warm temperatures, humid conditions, and regular rainfall, with conditions being favorable for daylily rust (Table 2). The planting was irrigated daily with overhead sprinklers.
Daylily rust severity ratings on 575 daylily cultivars growing in a large landscape planting in southern Mississippi in Aug. 2013. Plants had not received any fungicide treatment during the 2013 growing season. Weather conditions during the growing season were favorable for daylily rust.
Mean maximum, average, and minimum daily air temperature; mean daily dew point, and total precipitation in June, July, and Aug. 2013 in Hattiesburg, MS.z
The 575 cultivars in the surveyed planting consisted of mostly modern hybrids, with 93% of the cultivars registered in 2000 or later and 76% of the cultivars registered in 2005 or later. Most cultivars (85%) were represented by a single clump of plants, 12% by two clumps, and 3% by three or more clumps. Clumps were well developed and contained a minimum of five fans (ramets), with fan count per clump varying by cultivar. Clumps of the same cultivar were in different locations in the planting. Clumps were spaced on ≈3-ft centers.
Individual clumps of each cultivar were visually assessed for daylily rust on 16 Aug. 2013 and rated by agreement of three raters using an ordinal scale of 1 (no or little sign of infection), 2 (moderate infection), and 3 (severe infection) (Table 1). When there were two or more clumps of the same cultivar, the median is reported as the rating. Additional information on each cultivar (bloom season, winter foliage, ploidy, originator, and year of registration) was obtained from the database of the American Hemerocallis Society (2015).
For statistical analysis, median rust ratings were grouped into three ordinal categories: 1 and 1.5 (apparently resistant), 2 (apparently moderately susceptible), and 2.5 and 3 (highly susceptible). Ploidy level (diploid or tetraploid) of the cultivars was evaluated for association with the three ratings groups using Fisher’s exact test (Freeman–Halton test). Bloom season, as ordinal categories (extra early, early, early midseason, midseason, late midseason, late, and very late), and winter foliage, as ordinal categories (dormant, semievergreen, and evergreen), were evaluated for correlation with the three ordinal ratings groups using the Cochran–Mantel–Haenszel statistic for nonzero correlation and as nominal categories using the Cochran–Mantel–Haenszel statistic for differing row mean scores (analysis of variance-type analysis). For comparison, the same statistics were calculated for cultivars in listings of rust-resistant and rust-susceptible cultivars previously published by Mueller et al. (2003), Robbins and Vann (2002), and Robbins (2003), with cultivar characteristics in the listings supplemented by information obtained from the American Hemerocallis Society (2015) when not provided by the authors. Statistical analyses were conducted using the FREQ procedure of SAS (version 9.4; SAS Institute, Cary, NC).
Results and discussion
A total of 119 (21%) of the 575 cultivars in the survey received a rating of 1 or 1.5 and 230 (40%) cultivars received a rating of 2 (Table 1), indicating apparent resistance and moderate susceptibility, respectively; however, some of these cultivars may be more susceptible to the disease than a one-time, single-clump rating might reveal. A total of 226 (39%) cultivars received a rating of 2.5 or 3, indicating them to be highly susceptible to daylily rust, and would likely require frequent applications of fungicides to control the disease. Although most of the daylily cultivars in the survey were represented by only single clumps of plants, the ability to evaluate 575 cultivars in one location in a season with weather conditions favorable for daylily rust, along with the plants having not been treated with fungicides, provided a unique opportunity to collect data on potential disease resistance and susceptibility of these mostly modern hybrid cultivars.
In a preliminary study at the University of Georgia by Williams-Woodward and Buck (2001) with pathogen-inoculated plants in a greenhouse, 4 cultivars were reported as showing little infection, 9 cultivars being moderately susceptible, and 13 cultivars as being highly susceptible. In further work at the University of Georgia reported by Mueller et al. (2003) with 84 cultivars obtained from three commercial nurseries and inoculated in a greenhouse, 17% were classified as resistant, 15% as moderately resistant, 26% as moderately susceptible, and 42% as fully susceptible. In a survey of rust severity conducted with nursery field-grown plants in Arkansas in 2001 and 2002 (Robbins, 2003; Robbins and Vann, 2002), 75% and 70%, respectively, of the cultivars examined in each year received ratings of ≤2 (out of 5) and were recommended for breeders and consumers. Ratings of ≥4 were received by 4% and 16% of the cultivars in 2011 and 2012, respectively; thus, Robbins and Vann (2002) recommended that these cultivars should best be avoided. Intermediate ratings (>2 and <4) were received by 21% and 14% of the cultivars in 2001 and 2002, respectively. A total of 500 cultivars were examined in each year, and the majority (>75%) were rated in both years. None of the cultivars reported in the current survey in Mississippi were included in the aforementioned studies in Georgia and Arkansas. Results of the current survey of daylily cultivars in a Mississippi landscape under conditions favorable to daylily rust are more in line with results reported by Williams-Woodward and Buck (2001) and Mueller et al. (2003) using inoculated plants in a greenhouse in Georgia, whereas percentages of nursery field-grown cultivars in Arkansas reported by Robbins and Vann (2002) and Robbins (2003) as being very resistant to moderately resistant appear to be higher. Environmental conditions in Arkansas may have been less favorable for daylily rust and the authors did not state if fungicides were used in the growing seasons before the two surveys or discuss the likelihood that all plants had been exposed to inoculum.
In exploring relationships between daylily cultivar characteristics and severity of daylily rust in this study, the tendency for tetraploid cultivars to be associated with greater severity of daylily rust than diploid cultivars was of particular interest (Table 3). This association was also noted with cultivars surveyed in 2001 and 2002 in Arkansas (Robbins, 2003; Robbins and Vann, 2002) and cultivars examined in a greenhouse study in Georgia (Mueller et al., 2003) based on our analysis (Table 3). Also, there was a tendency for earlier-blooming cultivars in this survey to have higher ratings for disease severity than later-blooming cultivars (Table 4) and a tendency for a greater percentage of cultivars with dormant winter foliage to have lower disease ratings than cultivars with semievergreen or evergreen foliage (Table 5); however, such associations were not seen with cultivars in the aforementioned surveys in Georgia or Arkansas based on our analyses (data not presented). Such associations may be of interest to other investigators in future studies.
Distributions of diploid and tetraploid daylily cultivars among ratings for daylily rust severity in the current survey in a Mississippi landscape in 2013, previous nursery field surveys in Arkansas in 2001 and 2002, and a greenhouse study in Georgia reported in 2003.
Distribution of 575 daylily cultivars by bloom season among ratings for daylily rust severity in a survey in a southern Mississippi landscape in Aug. 2013.
Distribution of 575 daylily cultivars by winter foliage among ratings for daylily rust severity in a survey in a southern Mississippi landscape in Aug. 2013.
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