First Report of Sclerotinia Fruit and Crown Rot Caused by Sclerotinia sclerotiorum on Short-Day Strawberry in Maryland.

Abstract

With over 1,000 producers in the Mid-Atlantic region, short-day strawberry (Fragaria x Ananassa Duch.) represents an economically important industry. In spring of 2016, a white rot was observed on strawberry stem, crown, and fruit tissue at two Maryland farms. Light to dark brown necrotic lesions, white mycelium and medium to large black sclerotia were present on petioles and fruit (Fig. 1). These symptoms were most consistent with Sclerotinia fruit rot and crown rot caused by Sclerotinia sclerotiorum (Lib.) de Bary (Maas 1998), not previously described in the region (Farr and Rossman 2019). Lesion margins were excised, surface disinfested (70% EtOH for 30 s and 0.1% NaClO for 1 min) and placed onto potato dextrose agar (PDA) amended with 0.03% tetracycline. Single hyphal tip cultures were obtained which formed fluffy white to tan mycelium, no asexual conidia, and numerous pigmented sclerotia resembling S. sclerotiorum (Bolton 2006). DNA was extracted from five isolates and the internal transcribed spacer (ITS) region was amplified by PCR using primers ITS1/ITS4; all five ITS sequences (533 bp) had 99-100% similarity with S. sclerotiorum (GQ375746) in Genbank. Sequences of SL1380 and SL1382 were submitted to NCBI under the accession numbers MK429976 and MK429977, respectively. These two isolates were evaluated for abilities to cause fruit and stem petiole rot. Pathogenicity on fruit was examined by placing a 5 mm plug from five-day-old cultures on surface disinfested fruits, with mock-inoculated (PDA plug only) and non-inoculated controls. Fruit were placed in incubators (~98% RH, 22°C, 10:14 hrs L:D) and arranged in a complete block design with three blocks and five fruit/treatment/block; the study was conducted twice. White hyphae and sclerotia formed on 41 to 53% of fruit inoculated with both isolates, necrotizing fruit within 6 days post inoculation (DPI); Sclerotinia fruit rot did not develop in controls. Pathogenicity on petioles was examined in a replicated study on three-month-old cv. Flavorfest using a petiole inoculation method described in (Peltier and Grau 2008); mock-inoculated stems were placed in sterile PDA. Plants were arranged in the greenhouse (~20-28°C; 12:12 hrs L:D) in a randomized complete block design with three blocks and three plants/treatment. Dark brown necrosis averaging 5.8 ± 0.2 mm in length developed on 100% of plants in both isolate treatments within 3 DPI and by 6 DPI had expanded to 42.1 ± 4.8 mm on average, spreading into the crown on some plants. Necrosis did not extend beyond 2 mm from the cut tip in controls. The pathogen was recovered from surface disinfested tissue for both isolates. In a subsequent study, stem rot susceptibility was compared among four short-day strawberry cultivars common to the region, cvs. Flavorfest, Chandler, Allstar and Honeoye (same method as above). Stem lesions were 2 to 3 times longer on cv. Flavorfest (41.78(ave) ± 6.73(se) mm) compared to all other cultivars (13.68(ave) ± 1.56(se) mm) (P < 0.0001, Tukey's test). To our knowledge, this is the first report of both Sclerotinia fruit and crown rot on strawberries anywhere in the mid-Atlantic region. Our results indicate that cv. Flavorfest, a recently developed cultivar with expanding acreage, may be more susceptible to Sclerotinia. These are both unusual diseases circumglobally, especially Sclerotinia fruit rot (Farr and Rosman 2019, Maas 1998); the new occurrence in this region may be associated with increased use of this cultivar.