Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/19/2013
Publication Date: 1/29/2013
Citation: Wang, M., Chen, X. 2013. First report of Oregon grape (Mahonia aquifolium) as an alternate host for the wheat stripe rust pathogen (Puccinia striiformis f. sp. tritici) under artificial inoculation. Plant Disease. 97:839. Interpretive Summary: The wheat stripe rust pathogen was recently found to be able to infect barberry (Berberis spp.) plants to complete its sexual life cycle, which has been known for the stem rust pathogen for hundreds years. Oregon grapes (Mahonia spp.), evergreen shrubs closely related to genus Berberis, are also known to be alternate hosts of the stem rust fungus. However, it was not clear whether Oregon grapes can be alternate hosts for the stripe rust fungus. Under controlled greenhouse conditions, seedlings of Mahonia aquifolium, a common native and ornamental plant species in the Pacific Northwest, were inoculated with teliospores of the stripe rust pathogen produced on wheat plants. Pycnia, one of the five spore stages in the complete life cycle of cereal rust fungi, appeared on the upper side of leaves 12 days after inoculation and aecia, another spore stage, were produced on the lower side of leaves 16 days after inoculation. Isolates of urediniospores, the most common spore stage on wheat and some other grass hosts, produced on wheat plants inoculated with aeciospores from mahonia plants were confirmed to be resulted from the sexual reproduction of the wheat stripe rust fungus on mahonia plants. The complete life cycle of the five spore stages for the wheat stripe rust was demonstrated in the greenhouse experiments with Mahonia aquifolium and wheat plants. The susceptibility of the mahonia species makes it useful for studying the genetics of the stripe rust fungus and epidemiology and evolution of the pathogen.
Technical Abstract: Cereal rusts are heteroecious fungi requiring cereal crops and other unrelated host species to complete their life cycle. Puccinia striiformis Westend. f. sp. tritici Erikss. (Pst), the wheat stripe rust pathogen, has five distinct spore types. Although wheat has been recognized as the primary host for production of urediniospores, teliospores, and basidiospores of Pst for centuries, only recently, barberry (Berberis spp.) was discovered to be an alternate host for production of pycniospores and aeciospores to complete its macrocyclic life cycle under artificial inoculation (1). Mahonia aquifolium (Pursh) Nutt. (syn. Berberis aquifolium Pursh) is an evergreen shrub closely related to genus Berberis. It is a native plant in the Pacific Northwest (PNW) distributing across British Columbia to northern California and commonly used as ornamental foliage. Aecia that were sometimes found on the leaves and fruit of mahonia plants in the PNW during our rust survey were identified mostly as the stem rust fungus (P. graminis). Stripe rust is an important wheat disease in the U. S. PNW and many parts of the world. To determine if mahonia can serve as an alternative host for Pst, like some Berberis spp. (1), we conducted artificial inoculation experiments under controlled greenhouse conditions. Mahonia seeds were sown in pots filled with soil mixture and the plants were grown in a greenhouse. Two sets of Pst telial samples were used as inoculums, race PST-127 collected from wheat straws in an experimental field near Pullman, WA and isolate 09-134 (also PST-127) generated on wheat plants grown in a greenhouse. Matured teliospores were transferred into distilled water under a dissecting microscope. The teliospore suspension was brushed onto leaves of 10-15 days old mahonia plants. The inoculated plants were kept in a dew chamber at 10oC for 72 h without light and then moved to a growth chamber with a diurnal temperature cycle gradually changed between 10oC at 2:00 am and 24oC at 2:00 pm with an 8-h dark/16-h light cycle. Reddish pycnia appeared on the upper side of leaves 12 d after inoculation and reddish aecia were produced on the lower side of leaves 16 d after inoculation. All of the 15 mahonia leaves inoculated with Pst teliospores produced pycnia and aecia. Five cups of aecia were transferred to a drop of water and the released aeciospores were brushed onto seedlings of wheat cultivars Nugaines and Avocet S, which are susceptible to all and most Pst races identified so far, respectively. The inoculated wheat plants were kept in a dew chamber at 10oC for 24 h and grown in the growth chamber at the same temperature and light settings as above. Uredinal sporulation started 15 days after inoculation and telia were formed about one month after inoculation. Virulence tests of randomly selected ten urediniospore isolates that were produced by single aeciospores showed that the progeny isolates and the parental isolate 09-134 had similar but different virulence spectra. Four Pst SSR markers (PstP001, PstP003, PstP005, and PstP029) (2), which identify homozygous loci in PST-127, showed that the ten progeny isolates had the same SSR marker pattern of the parental isolate. The results showed that M. aquifolium can serve as an alternative host for Pst. The susceptibility of M. aquifolium makes the species useful for genetic studies of Pst. Although we have not identified Pst aecia from mohonia samples with natural rust infection in the PNW in the last three years, mahonia plants could be infected by Pst when the production of teliospores meets the similar weather conditions as specified in this study. Due to the wide distribution of mahonia plants and the importance of wheat stripe rust in the PNW, further studies are needed to determine the role of Mohania spp. in the stripe rust disease cycle.