First report of Asian rust of soybean caused by Phakopsora pachyrhizi in Cuba

Soybean (Glycine max) is of increasing importance in Cuban agriculture. Two species of rust attacking soybean, Phakopsora pachyrhizi and P. meibomiae, have been identified elsewhere (Ono et al., 1992). A severe Phakopsora sp. rust outbreak in soybean occurred in September 2006 and since then, minor outbreaks in legumes and beans (Phaseolus vulgaris) have also occurred. In April 2009, a severe rust outbreak in soybean developed at phenological stage R3 of the plants, leading to the complete defoliation of two 50 ha fields of soybean cv. Conquista in Jovellanos, Matanzas province. First symptoms in the field consisted of yellowing of the lower leaves of plants leading to defoliation and early maturation (Fig. 1A, B). Symptoms on the upper side of leaves consisted of yellow spots that turned brown then necrotic, surrounded by wide yellow areas (Fig. 2A, B). Uredinia developed on the undersides of the leaves. Anamorphic sori (Malupa-type) were amphigenous, circular, minute, pulverulent, whitish becoming pale cinnamon-brown, scattered or in groups on discoloured spots, subepidermal becoming erumpent, cone-like, 1-2 mm diameter, surrounded by paraphyses, with a central opening. Paraphyses were cylindrical to clavate, 25-45 µm (more frequently 37.5 µm) x 6-14 µm, slightly thickened at the apex, 7.5-17.5 (12.5) µm, colourless to pale yellowish-brown. Uredinospores were sessile, obovoid to broadly ellipsoidal, 25-35 (28) x17-22.5 (18.7) µm; wall 1-1.5 µm thick, minutely and densely echinulate, colourless to pale yellowish brown, sometimes pale cinnamon-brown with age (Fig. 3A, B).

A positive reaction was obtained when sap from rust-affected leaves was tested with immunological test sticks containing a specific antibody to P. pachyrhizi, (QuickStix, EnviroLogix, Portland, USA ). Real-time PCR was conducted on sap from affected leaves using SYBR green as label and the specific primers developed by Frederick et al. (2002) for the diagnosis of each species.The results of the PCR confirmed the presence of both P. pachyrhizi and P. meibomiae causing mixed infections within the same plant, although higher concentrations of DNA of P. pachyrhizi were noted, presumably due to the faster growing and more aggressive nature of this species. According to Bromfield (1984), P. meibomiae was previously reported in Cuba in 1926, but there were no further records of its presence in soybeans and other legumes since this time. This is the first report of Phakopsora pachyrhizi attacking soybeans in Cuba and the confirmation by advanced diagnostic procedures of the presence of P. meibomiae.