First Report of Basal Drop and White Mold on Lettuce, Broccoli, and Mustard Caused by Sclerotinia sclerotiorum in Tennessee, U.S.A.

Abstract

Sclerotinia sclerotiorum causes basal drop and white mold diseases on a wide range of vegetable crops, resulting in significant economic loss (Purdy 1979). Diseased plants of various vegetables with black irregular to cylindrical sclerotia were observed in high tunnels of an organic farm near Knoxville, TN, in February of 2016 and 2017. Leafy mustard (Brassica juncea cv. Purple Osaka) had necrotic leaves and stems, with black sclerotia inside stems. Chinese broccoli (Brassica oleracea var. alboglabra cv. Te You) had water soaked lesions on stems and leaves with white mycelium and black sclerotia. Two kinds of lettuce (Lactuca sativa); butterhead cv. Mirlo and red leaf cv. Cherokee, had symptoms of lettuce drop, i.e., water soaked leaves and crown with white mycelial mats and black sclerotia. Infected plants (1- to 3-cm pieces) and sclerotia, collected in both years, were surface sterilized in 10% commercial bleach for 1 min, followed by 1 min in 10% ethanol. Samples were plated on potato dextrose agar amended with 10 mg/liter rifampicin and 3.45 mg fenpropathrin/liter (aPDA) and incubated at room temperature. After 3 days, white fluffy mycelia grew from infected tissues, and after 7 to 10 days, black sclerotia (1.5 to 10.0 mm × 2.0 to 6.0 mm) were formed. Apothecia from sclerotia were induced on vermiculite according to Pratt (1992). Asci contained eight uniform, binucleate, elliptical ascospores (Kohn 1979). The fungus was identified as S. sclerotiorum (Lib.) de Bary based on morphology. For molecular identification, genomic DNA was extracted (Extract-N-Amp, Sigma-Aldrich, MO) from mycelia. PCR amplicons were produced and sequenced using internal transcribed spacer (ITS) region primers ITS1 and ITS4. Comparison with reference sequences in GenBank (e.g., accession nos. KP340898, KX184720.1, JQ618848) resulted in 100% identity to ITS sequences of S. sclerotiorum. The ITS sequences (520 to 531 bp) were deposited in GenBank (KY859157 [broccoli], KY859158 [lettuce], KY859159 [mustard]). To complete Koch’s postulates, 7-day-old mycelial plugs from aPDA were placed on two 55-day-old lettuce cultivars (PIC 714 and New Red Fire) and 35-day-old broccoli cv. Te You and mustard cv. Purple Osaka grown in soilless growing medium. For each cultivar, the stem at the soil line and newly expanded leaves and petioles of four healthy plants were each inoculated with three to five mycelial plugs (1-cm²-diam.) of the pathogen grown on aPDA, and four more were inoculated with aPDA only as the control. Plants were covered with plastic, and sprayed with water (under the plastic) twice daily for 3 to 5 days to maintain high humidity, and kept in a growth chamber at 25°C. After 7 to 10 days, all pathogen-inoculated plants had symptoms identical to those observed in the field, with or without sclerotia; control plants showed no symptoms. The fungus was reisolated from infected plants, and identity was confirmed with ITS sequence analysis. To our knowledge, this is the first report of S. sclerotiorum on broccoli, mustard, and lettuce in Tennessee. For organic growers where crop rotation is the primary control option, occurrence of this pathogen on multiple crops could cause significant loss.