First report of a rust disease on Suaeda californica in California

Authors: Bruckart, William; Thomas, Jami; Frederick, Reid; AIME, MARY - Purdue University; ABBASI, MEHRDAD - Purdue University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2019
Publication Date: 5/20/2019
Citation: Bruckart, W.L., Thomas, J.L., Frederick, R.D., Aime, M.C., Abbasi, M. 2019. First report of a rust disease on Suaeda californica in California. Plant Disease. https://doi.org/10.1094/PDIS-09-18-1690-PDN.
DOI: https://doi.org/10.1094/PDIS-09-18-1690-PDN

Interpretive Summary: A fungal pathogen, Uromyces salsolae, has shown promise for biological control of Russian thistle. California seablite is an endangered, native plant that is found at Morro Bay and San Francisco Bay in California, and as a result of being listed as an endangered plant, California seablite has been included on the host range determination for U. salsolae. In a survey around Morro Bay in 2015, a few California seablite plants were found to have rust disease symptoms. A fungus isolated from diseased California seablite plants, was characterized using morphological and molecular techniques and found to to be a new fungal species and not U. salsolae. This information is important in addressing the host range of the endangered plant, California seablite, in efforts to utilize U. salsolae as a potential biological control agent for Russian thistle.

Technical Abstract: Suaeda californica (Amaranthaceae, California seablite) is an “Endangered” halophyte that grows at the tide line of Morro Bay and parts of the San Francisco Bay estuary in California. Because of its listing, S. californica has been included in the host range determination of Uromyces salsolae, which is under consideration for biological control of Salsola tragus (Amaranthaceae, Russian thistle). In August, 2015, a few plants (<10) were found with uredinia at two sites around Morro Bay. Disease was easily established on S. californica following greenhouse inoculations with urediniospores, resulting in production of dark brownish-orange uredinia, only on leaves. Dark brown to black telia developed on stems several weeks after the appearance of, and apart from, uredinia and were very similar to those illustrated by Trotter (1904). New disease resulted from reinoculations by greenhouse-grown urediniospores, and inoculations by greenhouse-grown teliospores resulted in production of spermagonia. Urediniospores (field and GH) are golden, globose or obovoid, have multiple germ pores, and measure [l x w; lsmean (± CI, P = 0.05), range]: 20.9 (± 0.4), 20.3 – 27.2 x 16.7 (± 0.2), 14.2 – 16.7 µ. Teliospores are single-celled, brown, globose, ellipsoid or obovoid, measuring 22.5 (± 0.4), 18.5 – 28.5 x 18.2 (± 0.2), 16.0 – 23.2 µ, and pedicels are variable in length, 25.1 (± 4.5), 3.0 – 110 µ. Spore measurements were significantly smaller (P = 0.05) than those of specimens from Suaeda taxifolia identified as either U. giganteus (Cummins 1979); and seven specimens from California at the Arthur Herbarium, Purdue University (Arthur 1934), all considered as S. taxifolia) or U. chenopodii (Farr and Rossman, nd). LSU sequence, amplified as described in Aime (2006), was compared with those of Uromyces species from related hosts also suggests this fungus is different and likely a new species. This is the first report of a rust disease on S. californica in California. Incidence of disease in the field is low and not likely to affect recovery programs managed by the F&WS. From the perspective of weed biological control, differentiation of this fungus on this listed, native species is possible, particularly in comparison with a candidate pathogen under consideration for biological control of Russian thistle.