Bottle gourd (Lagenaria siceraria (Mol.) Standl.) is an important rootstock in watermelon production in several countries such as Japan, China, and Israel where 60 to 70% of watermelons are grafted (2). We are evaluating bottle gourds for their ability to improve disease resistance when used as rootstock for watermelon (3). In the summer of 2007, symptoms of wilting and crown necrosis appeared on bottle gourd seedlings 1 month after transplanting in a field in Charleston, SC. Infection was observed on commercial cv. Emphasis and four advanced breeding lines. In October of 2007, 35 of 85 plants examined (41%) had stem rot at the crown area just above the soil line where coarse, white mycelia and abundant sclerotia were observed. The fungus tentatively identified as Sclerotium rolfsii produced sclerotia that were white or light to dark brown and measured 0.6 to 2.5 mm in diameter (mean = 1.1 mm). Diseased tissues with sclerotia from four plants were disinfested for 1 min in 0.5% sodium hypochlorite and plated on acidified potato dextrose agar (APDA). Fungal colonies that produced white mycelia and tan-to-brown sclerotia were isolated from four wilted plants. A single PCR product of approximately 680 bp was amplified from DNA extracted from two isolates using the primers ITS1 and ITS4 (4). One PCR product was cloned into the TOPO TA cloning vector (Invitrogen, Carlsbad, CA) and sequenced (GenBank Accession No. EU338381). BLASTN analysis of the sequence in the NCBI databases revealed 99% similarity to the internal transcribed spacer (ITS) sequences of S. rolfsii and Athelia rolfsii (perfect stage of S. rolfsii), confirming that the pathogen was indeed S. rolfsii. Two S. rolfsii isolates were used to test pathogenicity. Each isolate was used to inoculate five young seedlings and five adult (10-week-old) bottle gourd plants. For inoculation, 10 sclerotia obtained from the APDA plates were placed on the surface of the potting soil 0.5 to 1 cm from the collar region of each bottle gourd plant growing in 10-cm pots. Inoculations were done carefully to ensure that the plants were not injured. After inoculation, the plants were maintained at high humidity and 25°C for 3 days and then transferred to laboratory benches. Four young seedlings and three adult noninoculated plants kept under the same conditions served as controls. The pathogenicity test was repeated once with similar results. All inoculated plants developed symptoms of southern blight. The inoculated plants developed symptoms of wilting 4 to 5 days after inoculation and completely wilted within 7 to 10 days. Symptoms of wilting were soon followed by the appearance of white-to-light brown sclerotia on the collar region. No symptoms were observed on the noninoculated plants. S. rolfsii was reisolated from the inoculated plants on APDA. Although southern blight caused by S. rolfsii has been reported on many crop plants in the southern United States, to our knowledge, this disease has not been reported previously on bottle gourd in North America. However, the disease has been reported on bottle gourd in India (1). Identifying sources of resistance to southern blight in bottle gourds may be necessary to make them suitable as rootstocks in areas where S. rolfsii is present. References: (1) K. S. Amin. Indian Phytopathol. 34:253, 1981. (2) R. Cohen et al. Plant Dis. 91:916, 2007. (3) K. S. Ling and A. Levi. HortScience 42:1124, 2007. (4) T. J. White et al. PCR Protocols: A Guide to Methods and Amplifications. Academic Press, San Diego, 1990.
Southern Blight (Sclerotium rolfsiiSacc.) of Cowpea: Genetic Characterization of Two Sources of Resistance
