First report of the aecial stage of a rust disease caused by Puccinia elymi on Thalictrum squarrosum in China    

During a plant disease survey in 2005 in Baicheng City, Jilin province of China, a rust disease caused by Puccinia elymi was observed on the leaves of Leymus chinensis (Li et al., 2008). At the same site in June 2006, serious rust infection was found on the leaves of Thalictrum squarrosum (Ranunculaceae) around the L. chinensis plants infected by P. elymi in the previous year (Fig. 1). The aecia were abundant, hypophyllous and aggregated on rounded or irregular thickened spots on leaves, or grouped on the thickened distorted stems, urn-shaped or more or less cylindrical, and yellow with a whitish laciniate margin. The aeciospores were angular, subgloboid or ellipsoid, minutely verruculose, orange when fresh, and 17-25×15-20 µm (Fig. 2). Based on the host and the morphological characteristics described, this rust pathogen was identified provisionally as the aecial stage of Puccinia elymi (Wilson & Henderson, 1966).

In order to confirm identification and to determine whether aeciospores of P. elymi from T. squarrosum could infect L. chinensis to cause rust symptoms, an inoculation experiment was performed. In June 2006, healthy leaves of L. chinensis grown in a greenhouse at 20^°C were sprayed with an aqueous suspension of aeciospores (10⁵ spores per ml) collected from rust symptoms on T. squarrosum. After inoculation, the plants were covered with a transparent plastic bag for 48 h in a growth room (20⁰C, 16 h photoperiod) before transfer to a glasshouse. Control plants sprayed with sterile water were treated similarly. Within 8-12 days after inoculation, uredinia and urediniospores developed on inoculated leaves of L. chinensis. Production of teliospores was observed in September 2006. The control plants remained healthy. The urediniospores and teliospores were found to be consistent with those of P. elymi, confirming that the aecial stage on T. squarrosum was also that of P. elymi.

P. elymi was first described on Elymus arenarius from Belgium in 1851. The aecia occur on Thalictrum spp. of Ranunculaceae, and the uredinia and telia have been recorded frequently on Elymus spp. (Cummins, 1971).

In China, a rust on T. foetidum collected in Tibet was provisionally identified as the aecial stage of P. elymi, but this has not been confirmed yet (Zhuang et al., 1998). The aecial stage of P. elymi, however, has never been recorded on T. squarrosum in China or in other parts of the world. This is the first record of the aecial stage of a rust caused by P. elymi on T. squarrosum. It also confirms that aeciospores of P. elymi from T. squarrosum can cause rust symptoms on L. chinensis in China.