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First Report of Vinca Blight Caused by Phytophthora nicotianae in Northwestern Mexico

Vinca (Catharantus roseus (L.) G. Don) is a common ornamental landscape plant. From July to September 2012, blighted and wilted vinca plants were found in retail stores, commercial nurseries, and urban landscape areas of Culiacan, Sinaloa, in northwestern Mexico. In several commercial nurseries and a retail store, incidence of the unknown disease on vinca plants ranged from 20 to 50%, resulting in significant economic losses. Symptoms of the disease started with a foliar blight, and if warm and wet conditions were present, the disease progressed, causing plant wilting and death. Surface-sterilized (0.5% NaOCl 1 min) diseased plant tissue was plated on V8 agar medium, and after 72 h of incubation at 25°C, white colonies of coenocytic mycelium were developed from the plated tissues. Isolates produced cottony colonies on V8 agar medium, grew well between 7 and 30°C (optimum of 25°C), and produced spherical, intercalary, and terminal chlamydospores (17 to 30 μm) and non caducous, papillate, spherical to ovoid sporangia of 30 to 39 × 21 to 31 μm. Based on these morphological characteristics, Phytophthora isolates were identified as Phytophthora nicotianae Breda de Haan (1,3). The identity of two representative isolates OV4 and OV11 was confirmed by sequence analysis of the rDNA internal transcribed spacers (ITS; GenBank Accession Nos. KC248202 and KC248201), and of the β-tubulin (β-tub; KC248404 and KC248403) and translation elongation factor 1-α (EF1-α; KC248206 and KC248205) genes. Comparative sequence analysis against the NCBI nucleotide database showed a high degree of identity with reference sequences of P. nicotianae (ITS, 99%; β-tub, 99%; EF1-α, 100%) (2). A pathogenicity test with a representative isolate of P. nicotianae was performed on 10-week-old healthy vinca seedlings (n = 10). An aliquot of 10 ml of a zoosporic suspension (10⁴ zoospores/ml) was sprayed onto the seedlings' leaves. An equal number of non-inoculated control seedlings were sprayed with sterile distilled water. Seedlings were maintained in a moist chamber at 25°C with 80 to 90% relative humidity and watered as needed with sterile water. Inoculated plants showed initial symptoms of foliar blight after 4 days, whereas control plants remained healthy. Ten days after inoculation, inoculated plants showed severe wilting. P. nicotianae was reisolated only from inoculated plants, thus fulfilling Koch's postulates. To our knowledge, this is the first report of P. nicotianae attacking annual vinca in northwestern Mexico.

References: (1) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996. (2) L. P. Kroon et al. Fungal Genet Biol. 41:766, 2004. (3) F. N. Martin et al. Plant Dis. 96:1080, 2013.