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.

As the primary host of the stripe rust pathogen, Puccinia striiformis f. sp. tritici (Pst), wheat can be infected by both aeciospores and urediniospores, and later is the host that gives rise to urediniospores and teliospores. Barberry species (e.g., Berberis vulgaris) can be infected by basidiospores, produced from the teliospores of wheat plants, and later gives rise to pycniospores and aeciospores, which has been demonstrated through artificial inoculation (3). Oregon grape (Mahonia aquifolium), closely related to Berberis, is a native evergreen shrub that is also grown as an ornamental plant in the Pacific Northwest. To determine if M. aquifolium can also serve as an alternate host for Pst, we conducted artificial inoculations under controlled conditions. Seeds of M. aquifolium collected from Pullman, WA, were sown in pots filled with soil mixture, and plants were grown in a greenhouse under wheat-growing conditions (1). In the first experiment, conducted in May to June 2011, the inoculum was telia collected from artificially inoculated wheat cv. Avocet S with urediniospores of isolate 09-134 (race PST-127) from the greenhouse. In the second experiment, conducted in July to August 2011, the inoculum was telia collected from naturally infected wheat cv. Nugaines with urediniospores from isolate 11-292 (race PST-127) from an experimental field near Pullman. For each experiment, mature teliospores of 60 telia from a single wheat plant were suspended in 1.0 ml of distilled water and inoculated with a fine paint brush onto the leaves of seven or eight 10- to 15-day-old plants of M. aquifolium. Plants were incubated initially in a dew chamber at 10°C for 72 h in darkness, then transferred to a growth chamber with a diurnal temperature cycle of 10 to 24°C and a 16 h light/8 h dark cycle (1). Reddish pycnia with nectar appeared on adaxial surfaces of inoculated leaves at 12 days post-inoculation (DPI), and reddish aecia were produced on the baxial surface at 16 DPI. All 15 M. aquifolium leaves of the 15 plants inoculated with teliospores produced pycnia and aecia. Seedlings of Nugaines and Avocet S, wheat cultivars that are susceptible to all Pst races (1), were then inoculated with a water suspension of aeciospores of 30 aecia collected from the M. aquifolium plants. Wheat plants were incubated as described above for M. aquifolium. Uredinia appeared at 15 DPI, and telia were produced after an additional 15 days. From these uredinia that formed on inoculated wheat, a total of 30 single-uredinium isolates were obtained using the standard procedure (1). Virulence tests were carried out on 20 wheat differentials for 10 randomly selected urediniospore isolates, revealing six virulence patterns. When tested with four selected Pst SSR markers (PstP001, PstP003, PstP005, PstP029) (2) and compared to other race PST-127 isolates, all 10 progeny isolates were homozygous, as were the parental isolates (09-134, 11-292). The virulence tests and marker genotypes verified that the urediniospore isolates resulted from infection by aecia, produced by parental isolate 09-134 through its sexual cycle on M. aquifolium. The study exhibited the completed sexual lifecycle of Pst through the five spore stages on wheat and M. aquifolium in a controlled setting, and suggests that under appropriate weather conditions, M. aquifolium may serve as an alternate host for Pst. Due to the wide distribution of M. aquifolium, further studies are needed to determine if the species can be infected by Pst under natural conditions. References: (1) X. M. Chen et al. Can. J. Plant Pathol. 32:315, 2010. (2) P. Cheng et al. Mol. Ecol. Resour. 12:779, 2012. (3) Y. Jin et al. Phytopathology 100:432, 2010.