First Report of <i>Sclerotinia sclerotiorum</i> Causing Stem Rot on Caucasian Pincushion Flower ( <i>Lomelosia caucasica</i> ) in Korea

In April 2025, abnormal pincushion flower (Lomelosia caucasica) with brown discoloration, lower stem soft rot, white mycelium, and black sclerotia were collected from a commercial greenhouse in Waegwan-eup, Gyeongbuk province, Korea (35°55'35.8"N, 128°24'30.2"E). Approximately 5–6% of 5,000 surveyed plants in the affected area exhibited the symptoms above. Diseased tissues were surface-sterilized with 1% NaClO and 70% ethanol for 30 s each, rinsed three times with sterile distilled water, and plated on potato dextrose agar (PDA). After 14 days at 25°C, white mycelia with dark and irregular sclerotia (1–6 mm in diameter) were observed, similar to the morphology of Sclerotinia sclerotiorum (Kohn et al. 1979). As strains KNUF-WG01, KNUF-WG02, KNUF-WG03 exhibited similar morphological characteristics, KNUF-WG01 was selected as the representative strain for pathogenicity tests. For molecular analysis, internal transcribed spacer (ITS) regions with calmodulin (CaM) and minichromosome maintenance complex component 7 (Mcm7) genes were analyzed using primer pairs ITS1F/ITS4, CAL-228F/CAL-737R, Mcm7-709for/Mcm7-1348rev, respectively (White et al. 1990; Gardes and Bruns 1993; Carbone and Kohn 1999; Schmitt et al. 2009). Sequences were deposited in GenBank with accession numbers LC901940, LC901943, LC901946 for the ITS regions (541 bp); LC901941, LC901944, LC901947 for the CaM (430 bp); LC901942, LC901945, LC901948 for the Mcm7 (713 bp). The ITS regions of three isolated strains showed 100% similarity with S. sclerotiorum strains SS1 (PV029721) and SS5 (MG516605), while the CaM and Mcm7 sequences showed 99.7–100% similarity to those of strain SS1 (KF545279 and KF545469) and strain SS5 (KF545280 and KF545468). A maximum likelihood and bayesian inference phylogenetic tree based on concatenated sequences of three loci from the three studied strains showed that these strains clustered with S. sclerotiorum SS1 and SS5. Based on these results, three strains were identified as S. sclerotiorum. Pathogenicity tests were conducted with three replications by applying KNUF-WG01 mycelial plugs to healthy one-year-old L. caucasica plants at the stem and incubating them at 25°C with 80–90% relative humidity under a 16 h/8 h light/dark cycle. Control plants received PDA plugs and were incubated under the same conditions. Two weeks after inoculation, wilting and discoloration symptoms were observed on inoculated plants, whereas control plants remained asymptomatic. The pathogen was successfully re-isolated and showed morphology identical to the original inoculum, fulfilling Koch’s postulates. This study provides the first report of S. sclerotiorum causing stem rot and wilting on L. caucasica in Korea. As soil-borne pathogens are a persistent problem in the cut-flower industry (Yanez et al., 2024), continued monitoring and field investigations are needed to understand pathogen distribution and improve disease management.