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First Report of Late Blight Caused by Phytophthora infestans on Cape Gooseberry (Physalis peruviana) in Colombia

Late blight caused by Phytophthora infestans is the most limiting disease for several species of the Solanaceae family in Colombia. A potential host for P. infestans is Cape gooseberry (Physalis peruviana), a species belonging to the Solanaceae family. Its center of origin is the highlands of Peru and it is grown at approximately 1,500 to 3,000 m above sea level. Cape gooseberry has become an important export fruit in Colombia. Consequently, in the last few years, the area cultivated with Physalis peruviana has increased dramatically. P. infestans was isolated from this crop in the province of Cundinamarca, Colombia. Symptoms caused by this oomycete appeared initially on the leaf margins as small, irregular, necrotic spots that expanded and merged, increasing the necrotic area. These spots had a soft texture resulting from the degradation of plant tissue by the pathogen. On old lesions, white mycelia and sporangia were observed. Affected plants were rarely killed, but under favorable conditions, severe symptoms were observed in leaves and yield was reduced. Ten isolates were obtained from infected tissue by placing a lesion directly on a potato slice in a moist chamber (2). Mycelia grown on the potato slice were then transferred to rye agar. Identification of the pathogen was performed based on morphological characteristics, specifically, sporangiophores of P. infestans are compoundly branched and develop sympodially, with swellings at the points where sporangia were attached (1). Further confirmation was obtained by sequencing the internal transcribed spacer (ITS) regions (GenBank Accession Nos. EF173467-EF173476). Koch's postulates were completed in the laboratory by spray inoculating detached leaves of Cape gooseberry with a zoospore suspension obtained from each of the 10 isolates. Inoculum was prepared by flooding 10-day-old cultures with sterile distilled water to obtain a 10⁴/ml sporangial suspension followed by zoospore induction at 4°C. Leaves were sprayed with this suspension, placed in moist chambers, and incubated at 20°C in the dark. Control leaves were sprayed with sterile distilled water. Two separate leaves were inoculated with each isolate. The pathogen was reisolated from leaf lesions in all cases. The period between infection and the appearance of symptoms ranged from 5 to 7 days. To our knowledge, this is the first report of P. infestans causing damage on Cape gooseberry in Colombia. Chemical control measures are to some extent successfully applied in most regions where solanaceous crops are grown in Colombia. Nevertheless, suitable disease management for Physalis peruviana has not been achieved and further studies on the epidemiology of the disease on this new host are needed.

References: (1) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society. St. Paul, MN, 1996. (2) G. A. Forbes et al. Phytopathology 87:375, 1997.