Records of Rust Caused by Tranzschelia mexicana on Black Cherry (Prunus serotina) in the United States

Black cherry (Prunus serotina Ehrh.), the largest of the native cherries and the only one of commercial value, is found throughout the eastern United States. Black cherry grows from Nova Scotia and New Brunswick to Texas and central Florida. Several varieties extend the range to Central America from Mexico to Guatemala and several South American countries (Marquis 1990). Rust disease on black cherry is commonly caused by Tranzschelia spp., primarily T. arthurii Tranzschel & M. A. Litv., and T. pruni-spinosae (Pers.) Dietel (Farr and Rossman 2015). However, T. mexicana M. Scholler & M. Abbasi has been reported on P. serotina subsp. capuli (Cav. ex Spreng.) McVaugh (as P. salicifolia Kunth) from Central and South America including Mexico, Ecuador, and Colombia (Scholler et al. 2014) and has very recently been reported from California on P. salicifolia (Blomquist et al. 2015). Study of newly collected and herbarium specimens (PUR 6818, collected in October 1900 to PUR N11113 collected in September 2011) revealed that T. mexicana has a much wider distribution range in North America than previously suspected. Studied materials were mostly infected by the uredinial state. Telia states were only present on two herbarium specimens collected from Texas (PUR 6843 and PUR 6818). The following morphological features were consistently observed: uredinia small, on lower side of the infected leaves, mostly on chlorotic spots, scattered on the entire leaf surface and sometimes in small groups; pulverulent, surrounded by remaining parts of host epidermis; with cylindrical or clavate paraphyses which were straight to incurved and hyaline to brown, paraphysis wall mostly 1.5 µm thick, slightly thicker (up to 3 µm) at apex. Urediniospores elongate-obovoid, clavate or fusiform, and mostly rounded at the apex; 27.2 to 52.8 (to 56) × 11.2 to 21 µm; urediniospore wall dark brown to yellowish, mostly darker in the supra-equatorial region, echinulate except for the apex, mostly 1.5 µm thick at side and up to 9.6 µm thick at apex; germ pores 3 to 5, varying from supra-equatorial to equatorial and rarely bizonate. Telia were hypophyllous and blackish brown. Teliospores two-celled, in fascicles, broadly obovoid, oblong or ellipsoid, weakly constricted at septum; 30.4 to 40 × 22.4 to 25 µm; teliospore wall light chestnut brown, 2.5 to 3.5 µm thick, verrucose, verrucae often less dense in the lower cell; germ pore of upper cell apical or sub apical, of the lower cell equatorial. These morphological features and measurements were compatible with those of T. mexicana (Scholler et al. 2014). All studied specimens were deposited in the Arthur Fungarium (PUR) at Purdue University as follows: on Prunus serotina from Texas (PUR 6843 and PUR 6818); from Louisiana (PUR N1078, PUR 89705, and PUR N11113) and one specimen on P. serotina subsp. capuli (as P. capuli) from Florida (PUR 54656). DNA was extracted from PUR N11113 (collected from Louisiana, East Baton Rouge Parish, LSU Campus, September 2011, MCA4542) and the nuclear ribosomal large subunit (28S) was amplified according to the protocol outlined by Aime (2006). The resulting 471-bp sequence was deposited in GenBank (Accession No. KR921880). One sequence of T. mexicana (KP308391) was available for comparison in GenBank with which it shared 100% sequence identity. All herbarium specimens studied here, viz. PUR N1078, 89705, 6843, 6818, and 54656 were previously identified as T. arthurii. Tranzschelia mexicana is distinguished from T. arthurii in having urediniospores with 3 to 5 equatorial to supra-equatorial germ pores and weakly constricted teliospores with thicker walls (Scholler et al. 2014). Moreover, T. arthurii has urediniospores, which are “usually acute at the apex” (Tranzschel and Litwinov 1939). This feature was not common in urediniospores of T. mexicana. Based on our results, it is likely that this species entered the United States more than a century ago and is common along the Gulf states.