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Daylily rust
Scientific classification
Kingdom:	Fungi
Phylum:	Basidiomycota
Class:	Pucciniomycotina
Subclass:	Pucciniomycetes
Order:	Pucciniales
Family:	Pucciniaceae
Genus:	Puccinia
Species:	P. hemerocallidis
Binomial name
Puccinia hemerocallidis von Thümen

Puccinia hemerocallidis (Daylily rust)

Puccinia hemerocallidis von Thümen is a macrocyclic heteroecious pathogenic fungus, causing rust disease in daylilies. Its main hosts are plants from genus Hemerocallis (H. flava, H. fulva, H. longituba, H. minor, H. thunbergii ) in the subfamily Hemerocallidaceae. Alternate hosts are Patrinia spp in the family Valerianaceae. The symptoms of disease in daylilies are leaf lesions appearing as chlorotic areas on both surfaces of the leaf 7-10 days after infection. They grow in size and amount and start to produce orange-yellow spores. It can ultimately lead to death of foliage.

Ecology

During the summer period, yellow-orange urediospore penetrate the leaf surface of daylily and form mycelium within the leaf, eventually erupting and spreading numerous urediospores. While germination, urediospore produce a germ tube, which elongates towards the stoma and then forms an appressorium. Appressoria penetrate through the open stoma, form a vesicle in substomal space and intercellular hyphae differentiate from this vesicle. At the tip of intercellular hyphae haustorial mother cells form branches called haustoria, invading the host cells. Each haustorium is encapsulated into a newly developed portion of host cell extrahaustorial membrane, which isolates it from host cell cytoplasm and nucleus. Via haustoria, pathogen intakes nutrients without killing the host cell. Hyphae aggregate and form uredia in the substomatal space. Uredia appear mostly on the lower leaf surface, but can also emerge on the upper. This cycle repeats several times, amplificating daylily infection, untill in autumn telia grow among uredia. Early on they are honeybrown, gradually darkening through reddish-brown to blackish. Teliospores overwinter in dead daylily leaves and in spring germinate to produce basidiospores. Basidiospores are dispersed by wind or by animals. They cannot infect daylilies, but need an alternate host Patrinia spp. to form spermogonia. Spermogonia then produce spermatia (also called pycniospores), which fertilize hyphae of the opposite mating type in spermogonia. This is the sexual stage of parasite life cycle. After that aecia are formed on the lower surface of Patrinia's leaf and begin to produce light yellow aeciospores. Those aeciospores, when reaching the daylily leaves with wind or by other means, germinate through the leaf and again form uredia, completing the life cycle of pathogen.In warm climates, where daylilies are perennial, rust does not depend on alternate host and sexual spermatial stage, and can reproduce asexually via urediospores. This makes daylily highly invasive beyond its native range. Overwintering of vegetative rust mycelium or urediniospores in the host daylily or on the leaf surface in its native range was not observed.

Certain environmental conditions are necessary to provide rust infection in daylilies. Temperature around 22°C and about 5 hours of leaf wetness are optimal for lesion formation. No lesion formation was observed at 36°C and above in greenhouse experiments, and at the temperature of 10°C and less the amount of developing lesions decreased significantly. However, once rust has established, the disease development is no more sensitive to humidity or temperature.

History

Puccinia hemerocallidis was initially described in 1880 by von Thümen, who observed its talial stage on the leaves of Hemerocallis flava in Siberia. He reported the disease that attacks leaves and scapes of this daylily. Dietel described its uredinia and urediniospores on Hemerocallis longituba in Japan in 1898. Tranzschel has proven heteroicism of fungus in 1914 in Russia. He observed spermogonial-aecial stage on Patrinia scabiosaefolia and P. rupestris, and uredinial-telial stage on Hemerocallis minor. Host-alternating of rust between H. longituba and P. scabiosaefolia was described by Hiratsuka in 1938 in Japan. The closely related rust P. funkae affects another ornamental plant hosta. Initially, it was thought to be the same species as daylily rust. In 1992 Hiratsuka in his book "The Rust Flora of Japan" treated P. funkae as a synonym for P. hemerocallidis, considering the similar morphology of uredinia and telia. However, there now it was proved to be a distinct rust fungus.

Geography

P. hemerocallidis originates from Asia. Its native range includes Siberia, Korea, Japan, Taiwan and China. With international trading of daylilies, P. hemerocallidis expanded across the world and became invasive in many countries. According to EPPO distribution map, to date it has been reported in Brazil, USA, Australia, Portugal, Columbia, Vietnam, Thailand, Taiwan and South Africa.

Methods of control

Resistant cultivars

To date, the number of daylily cultivars registered by the American Hemerocallis Society is nearly 80,000. Some of them are not susceptible to daylily rust. Mueller et al. examined 84 commercially important daylily cultivars for resistance to P. hemerocallidis inoculation. They classified 14 (17%) of them as resistant, 13 (15%) as moderately resistant, 22 (26%) as moderately susceptible, and 35 (42%) as fully susceptible. They recommended several cultivars with very high levels of resistance to be utilized by breeders to create resistant to daylily rust cultivars. Prairie Blue Eyes, Carolyn Criswell, Mardi Gras Parade, Hush Little Baby, Green Flutte, Plum Perfect cultivars showed the most remarkable resistance in their greenhouse assay, with no signs of disease 21 days after inoculation. Identification of genetic traits of those cultivars responsible for resistance thus is important task for future research. Breeding of resistant cultivars is the most effective way to fight rust epidemics.

Noteworthily, not only daylily cultivars vary in resistance to parasitic fungus, but rust itself exhibits pathotypes of varying virulence. Experiments conducted on 19 daylily cultivars with 16 pathotypes of P. hemerocallidis revealed a broad range of disease phenotypes from susceptible to highly resistant. Although susceptible cultivars were susceptible to all pathotypes of rust and resistant cultivars generally resisted most of them, the type of reaction varied from susceptibility to resistance among intermediate cultivars, depending on pathotype given.

Fungicides

Fungicides are one of the most effective methods for managing daylily rust. Among them azoxystrobin or pyraclostrobin (Fungicide Resistance Action Committee [FRAC] group 11), chlorothalonil (FRAC group M5), myclobutanil, propiconazole, and triadimefon significantly reduced lesion development when applied up to 15 days before inoculation. Azoxystrobin could also significantly reduce lesion formation when applied 7 days postinoculation in the same study. In another research azoxystrobin remarkably dicreased disease severity for up to 9 weeks after inoculation when applied as a soil drench and as a root dip. Emitt et al. have recently evaluated sensitivity of 35 isolates of P. hemerocallidis to 3 different fungicides. They revealed, that those isolates were most sensitive to pyraclostrobin and least sensitive to thiophanate-methyl.They also observed great variation in sensitivity to flutolanil (36-40-fold difference between the highest and lowest EC50 values from two experiments. Available fungicides can to be used in rotation on 7-, 14-, 21-, or 28-day intervals.

EPPO phytosanitary measures

In 2007 EPPO added P. hemerocallidis to the A2 List, and recommended endangered EPPO countries to manage it as a quarantine pest. EPPO therefore considers excluding the pathogen from the greater region, where it has not been yet found, as the best strategy. It recommend to require all imported plants for planting of Hemerocallis (and possibly also Patrinia as an alternate host) to originate from a pest-free area.

Potential economic impact

Daylilies are highly valued worldwide as ornamental plants. Daylily rust can lead to ultimate foliage death and spreads rapidly. Eradication of infected stocks, fungicide implications and quarantins significantly affect daylily market. Little is known to date about commercial importance and negative impact of daylily rust for nurseries or for gardeners in its native range. Intriguingly, in Japan P. hemerocallidis affect mostly wild Hemerocallis populations, but there were no reported outbreaks for daylily cultivars. The reason is uncertain. In North America it became widespread and causes serious problems. After it has been reported in 2000, mandatory quarantine was enforced by federal and state regulatory agencies. This and other eradication measures were costly for growers. However, the disease widespread quickly and by 2003 quarantine was recognised uneffective and abolished.

Useful links

http://web.ncf.ca/ah748/rust.html - Daylily rust information pages

http://www.apsnet.org/edcenter/intropp/lessons/fungi/Basidiomycetes/Pages/DaylilyRust.aspx - APS site