Lettuce (Lactuca sativa L.) is the world's most popular leafy salad vegetable. One of the major challenges facing lettuce producers are fungal diseases that could, under favorable conditions, devastate the harvest (Raid, 2004). During February 2021, poor growth, plant stunning and blanching of leaves of lettuce plants, cultivated in unheated plastic tunel in Potočanje (Zlatibor region), Serbia, were observed. The crowns were softened with spreading decaying lesions covered with white mycelium, particulary on the leaves near the soil surface. Approximately 2 to 3 weeks before harvest, diseased plants began to wilt and collapse. Estimated disease incidence was 50-55%. In order to identify the causal agent, symptomatic tissues from diseased plants were cut into small pieces, surface sterilized with 70% ethanol for 1 min, rinsed three times in sterile distilled water and placed on potato dextrose agar (PDA). Five isolates with uniform morphology were derived from infected tissue. The colonies had fast-growing, white, cottony aerial mycelium, producing profuse numbers (184 sclerotia/ Petri plate in average) of small, black, irregularly shaped sclerotia, less than 2 mm in diameter. Based on morfological features, the isolates were identified as Sclerotinia minor Jagger (Kohn, 1979). To confirm the species identity, the internal transcribed spacer region of nuclear rDNA of a representative isolate 15-2 was amplified using the primer pair ITS1/ITS4 (White et al. 1990). Sequence analysis of ITS region revealed 100% nucleotide identity between the isolate 15-2 (GenBank Accession No. OL423632.1) and 14 isolates of S. minor from different parts of the world (e.g., accession Nos. MK356551.1, KY707828.1, JF2798801.1). Pathogenicity tests were conducted by artificial inoculation of 55-day-old lettuce plants cv. 'Majska kraljica', grown on commercial growth substrate in l L pots. The obtained isolates were grown on PDA for 7 days and mycelial plugs, 5 mm in diameter, were cut from the margin of the colony and placed mycelium-side down on undamaged ground-level leaves of lettuce plants. Two plugs per isolate were placed onto five plants each for a total of ten replicates per isolate. Negative control plants (5) were inoculated similarly with sterile PDA plugs. Inoculated plants were covered with transparent plastic bags, sprayed with water (under the plastic) twice a day for 3 to 5 days to maintain high humidity, and kept in a growth chamber at 22°C (13 h light). After 7 to 10 days, all pathogen-inoculated plants developed lettuce drop disease symptoms, whereas the control plants remained symptomless. The pathogen was reisolated from symptomatic leaves and Koch's postulates were completed by confirming the identity of the isolates. To our knowledge, this is the first report of S. minor on lettuce in Serbia. More research is needed to better understand this disease, establish control strategies and minimize the spread of the pathogen to other lettuce-producing areas of the country. References: Kohn, L. M. 1979. Delimitation of the economically important plant pathogenic Sclerotinia species. Phytopathology 69: 881-886. White, T. J., et al. 1990. PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA. Raid, R.N., 2004. Lettuce diseases and their management. In Diseases of Fruits and Vegetables: Volume II (pp. 121-147). Springer, Dordrecht. Founding: This work was funded by the Ministry of Education, Science and Technological Development (contract 451-03-9/2021-14/200214).
Keywords: Causal Agent; Pathogen detection; Subject Areas; lettuce drop.