First report of foliar blight of castor bean caused by in Sinaloa, Mexico.

Castor bean (Ricinus communis) is cultivated agriculturally for oil and ornamentally for its bright foliage and seed. Ornamental castor bean has naturalized in many areas of the world, including the state of Sinaloa, Mexico, where it is not planted commercially. In a survey conducted in 2019 in Sinaloa, wild castor bean was found widely affected by a foliar blight with symptoms similar to Alternaria ricini previously described in the United States (Stevenson 1945) and in the state of Chiapas, Mexico (López-Guillén et al. 2015). Sampling was conducted from February to April along roadsides and irrigation canals at 10 sites that were 10-12 km apart. Symptoms consisted of dark lesions < 2 cm dia with occasional concentric rings and coalescing with age. Incidence of disease among sites ranged from 20 to 60%. The severity of disease was greater in the lower canopy where the foliage area diseased (FAD) ranged from 15 to 35%. Ten symptomatic leaves were collected from each site and leaf fragments were plated on potato dextrose agar (PDA) using standard microbiological techniques for fungal recovery. Isolates of long-beaked Alternaria were recovered from 60 to 70% of fragments from each site and representative isolates (1/site) were selected for further studies. Isolates were transferred to Petri dishes containing PDA for characterization of the colony and V8 juice agar (V8A) for examination of the conidia (Simmons 2007). On PDA, colonies were dark olive brown, cottony, subsurface microsclerotia production abundant, and no production of pigments in the medium. On V8A, conidia were produced singly, darkly pigmented, and broadly ellipsoidal. The length and width of mature conidia ranged 70 to 100 µm (avg 88 µm) and 17 to 26 µm (avg 22 µm), respectively, with 2 to 4 transepta and 1 to 3 longisepta. The apical beak length ranged 10 to 125 um (avg 72 µm). Morphological characteristics were consistent with those of A. ricini from commercial castor bean (Simmons 2007). DNA was extracted from each isolate and the GAPDH and calmodulin genes were sequenced using protocol previously described (Lawrence et al. 2013). For each locus, sequences were identical among fungal isolates and representative sequences were deposited in GenBank (accessions OQ868796 and OQ868803, respectively). When compared to the NCBI database by BLASTn, the sequences had 100% identities to A. ricini GAPDH JQ646331 (576/576 bp) and A. ricini calmodulin JQ646230 (772/772 bp). To confirm pathogenicity of recovered Alternaria, all ten isolates were tested on the commercial castor bean cultivar Chinatan in two independent greenhouse trials. Seed was sown one per pot and four plants per isolate were tested. Spore solutions were prepared from V8A plates and spore densities were adjusted to 4 x 104 conidia/ml. At 4 to 5 leaves stage, plants were sprayed with spore solution until runoff, placed in clear polyethylene bags for 48 hr. After 14 dpi, the percentage of FAD ranged from 19 to 80% with significant differences among fungal isolates. Control plants sprayed with water remained healthy. To fulfill Koch´s postulates, the fungus was re-isolated from representative inoculated plants and its identity was confirmed using morphological characteristics. This is the first report of A. ricini causing foliar blight of castor bean in the state of Sinaloa, Mexico. Strategies for managing inoculum from wild castor bean should be developed in advance of increasing commercial castor production in Sinaloa.