First Report of Sclerotinia sclerotiorum Infecting Peanut in Texas

Abstract

Peanut (Arachis hypogaea L.) is grown extensively in several counties in West Texas. Sclerotinia blight, caused by the soilborne fungus Sclerotinia minor Jagger, is an increasingly important disease throughout this region. In September of 2007, diseased peanut plants (cv. Tamrun OL02) exhibiting symptoms of Sclerotinia blight (2,4) were collected from a commercial farm in Gaines County during a regional survey. Infected stem sections were surface disinfested in 0.5% sodium hypochlorite for 1 min, air dried, and placed in petri dishes containing water agar. Hyphae were subsequently transferred to petri dishes containing potato dextrose agar (PDA) after 3 days of incubation at room temperature. Pure cultures formed abundant, aerial, white mycelia, which later became darkly pigmented. Black, spherical to elongated sclerotia, 3.8 ± 0.8 mm, formed on the outer edge of petri dishes after 1 week. These characteristics are consistent with S. sclerotiorum (Lib.) de Bary (1,2). Pathogenicity tests were conducted by inoculating stems of greenhouse-grown peanut plants (cvs. Tamrun OL02, n = 4 and Flavorrunner 458, n = 4) with PDA plugs containing S. sclerotiorum. Mounting pins were used to create a shallow wound and affix inoculum plugs to stems. Control plants (n = 4) were inoculated with noncolonized PDA plugs. Plants were placed in a dew chamber at 20°C and 95% relative humidity in a randomized complete block design. The experiment was conducted two times. Symptoms identical to those observed on the original plant specimens were observed after 3 days on all inoculated plants; the controls remained healthy. Darkly pigmented cultures of S. sclerotiorum were consistently reisolated from all symptomatic stem sections. Fungal DNA was extracted from mycelia and sclerotia with a Qiagen DNeasy Plant Mini kit (Valencia, CA) and amplified by PCR using three S. sclerotiorum-specific primer pairs (3). PCR products for three replicates (two from mycelia and one from sclerotia) were sequenced and subjected to a NCBI BLAST search of highly similar sequences (megablast). The BLAST search revealed that our sequences are highly similar only with reported sequences of S. sclerotiorum. Sequences generated from three primer pairs in this study were 99, 95, and 95% homologous to S. sclerotiorum Accessions Nos. AF377925.1, AF377919.1, and AF377904.1 over 373, 376, and 300 bp of aligned sequence, respectively. Results from the pathogenicity tests and molecular study confirm that the S. sclerotiorum isolate obtained in this study is capable of inciting Sclerotinia blight of peanut. While S. minor is the primary causal agent of the disease, recent reports indicate that S. sclerotiorum is becoming more prevalent throughout the peanut-producing regions of the United States (2,4), and must therefore be considered in disease diagnosis. References: (1) L. M. Kohn. Phytopathology 69:881, 1979. (2) S. Sanogo and N. Puppala. Plant Dis. 91:1077, 2007. (3) C. Sirjusingh and L. M. Kohn. Mol. Ecol. Notes 1:267, 2001. (4) J. E. Woodward et al. Plant Dis. 90:111, 2006.