Cercospora leaf spot disease on lettuce (Lactuca sativa L.) caused losses in Tabanan Regency, Bali, Indonesia. Cercospora leaf spot in lettuce results from the pathogenic fungi Cercospora canescens. The preventive research aimed to identify the fungi on the morphological and molecular basis by using universal primers for PCR of ITS 1 and ITS 4. The study used Koch’s Postulates method to isolate the pathogenic fungal isolates from lettuce plants and test for pathogenicity. Identification of fungi continued macroscopically, microscopically, and molecularly. The macroscopic and microscopic observations showed that the cause of cercospora leaf spot on lettuce is the fungal disease caused by pathogenic fungi Cercospora spp. The isolate creates a dull white and robust mycelium structure, sideways growth, branched and septate hyphae, branched conidiophores, and dark lanceolate conidia. Through molecular identification, it helped recognize that fungi Cercospora canescens isolate Cer11-18 (Accession Number: MN400290.1) is the foremost cause of a withered lettuce crop. It is also the first research on the cercospora leaf spot disease on lettuce caused by Cercospora spp. in Tabanan Regency, Bali, Indonesia.

Keywords: Cercospora canescens, DNA barcoding, lettuce (Lactuca sativa L.), leaf spot disease, molecular study

Key findings: This research identified the pathogenic fungi Cercospora canescens being a prime cause of the cercospora leaf spot disease on lettuce crops morphologically and molecularly.