First Report of Blight on Ipomoea purpurea Caused by Phytophthora ipomoeae.

Abstract

Several wild species of Ipomoea grow in the central highlands of Mexico. During the summer of 1999, in Metepec, Mexico, blighted leaves and petioles of Ipomoea purpurea were collected from diseased plants and placed in a moist chamber to induce sporulation. Sporangia that formed on the lesions were transferred with a piece of agar to selective rye agar medium (2). Phytophthora ipomoeae was consistently isolated. Species identification was based on sporangial and gametangial characteristics of five cultures grown on rye agar. Sporangia were mainly ellipsoid but occasionally ovoid, semipapillated, and deciduous with a short pedicel. All isolates were homothallic with smooth-walled and aplerotic oospores. Genotypic analysis for the allozymes Peptidase (Pep) and Glucose-6-phosphate isomerase (Gpi) indicated that all five isolates belonged to one genotype with alleles 78/78 (Pep) and 108/108 (Gpi). Morphological characteristics and the allozyme genotype correspond to the new, recently described species P. ipomoeae Flier & Grünwald (1) isolated from I. orizabensis (Pelletan) Ledeb. ex Steud. (I. tyrianthina) Lindl. and I. longepedunculata (Mart. & Gal.) Hemsl. Pathogenicity tests were carried out with leaves from greenhouse-grown I. purpurea plants. Detached leaves were inoculated with a suspension of 103 sporangia per ml and kept in a moist chamber at room temperature (17 ± 3°C). Lesions were observed between 7 and 15 days after inoculation and were characteristic of those observed in the field. The pathogen was reisolated from inoculated symptomatic tissue. To our knowledge, this is the first report of blight on I. purpurea caused by P. ipomoeae.