First Report of <i>Sclerotinia minor</i> Causing Soft Rot of <i>Scabiosa atropurpurea</i> in California

In May of 2019, Scabiosa atropurpurea samples with brown discoloration, soft rot of the crown and lower stem, with presence of white mycelium and black sclerotia (Supp. Fig. 1A, B) were collected from a 0.10 ha open field diversified cut flower production in San Luis Obispo County, CA. Approximately 30 to 40% of the scabiosa crop planted in a quarter of the field, exhibited symptoms. Symptomatic crowns and lower stems from five plants were surface disinfested by rinsing in 0.1% Tween 20, soaking in 70% ethanol for 30 s, 1% sodium hypochlorite for 2 min and sterile water. Disinfested tissue was placed in 1/10 potato dextrose agar (PDA) and incubated at 20°C (12 h photoperiod). Resulting colonies (n = 5) formed abundant white mycelia, with black sclerotia formed on the outer edge of the plates after two weeks (Suppl. Fig. 1C). Sclerotia (n = 50) had an average size of 1.6 (± 0.19) mm in diameter. Morphological identification resulted in Sclerotinia sp. (Hao et al., 2003). The pathogen was further identified by DNA extraction of two hyphal tipped isolates, followed by amplification and sequencing of the rDNA internal transcribed spacer (ITS) region, ITS1/ITS4 (White et al. 1990), calmodulin (CaM), CAL-228F/CAL-737R (Carbone and Kohn, 1999), and DNA replication licensing factor Mcm7 Mcm7-709for/Mcm7-1348rev (Schmitt et al., 2009). NCBI BLAST searches with consensus sequences for each maker revealed 99 to 100% identity with S. minor ex-types for all loci (Supp. Table 1). A maximum parsimony multilocus phylogenetic analysis clustered Californian isolates with reference strains of S. minor (Supp. Fig. 1F). Sequences were deposited in GenBank (Supp. Table 1). Pathogenicity tests were conducted with isolate CS435, which was transferred onto PDA plates and incubated at 20°C for one week. Inoculum consisted of CS435 infested PDA plugs (1 cm3). In the greenhouse, the experiment was set as a complete randomized design and observed for six weeks. Fourteen-week-old scabiosa 'Merlot Red' grown in 3.78 L pots (n = 6), were inoculated by wounding plants at 0.5 cm above the crowns with a 1 mm probe. Inoculum was placed directly on top of the wound and was secured with parafilm. Negative control plants (n = 6) were wounded as above and inoculated with PDA plugs. In experiment 1(19.4 (± 3) °C, RH 46.9), 83% of plants exhibited yellowing of the lower leaves and wilting at one week post inoculation (wpi). Symptoms progressed over time until wilting, major leaf and stem necrosis, was observed in all inoculated plants (Supp. Fig 1E, D). Plant mortality incidence at five wpi was 83%. Pathogen signs including white mycelia and black sclerotia were also observed. In experiment 2 (20.0 (± 10) °C, RH 39.6), 66% of the total plants were symptomatic at five wpi: 33% exhibited yellowing of the lower leaves and wilting, and 33% of plants died. Disease did not develop in non-inoculated plants in either experiment. S. minor was successfully reisolated from surface disinfested tissue of at least 50% (Exp. 1) and 100% (Exp. 2) of inoculated plants, yielding identical sequences to those of the inoculated isolate. S. minor is a soil-borne pathogen that infects tomato, lettuce, brassica, and sunflower crops in California. This study stablishes S. minor as a new soil-borne pathogen of Scabiosa in California. Soil-borne pathogens are a recurrent issue in the cut flower industry, characterizing new pathogens in these crops can inform crop advisors and disease diagnostician to improve disease management in the ornamental industry.