Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2016
Publication Date: 1/14/2016
Citation: Kousik, C.S., Ikerd, J.L., Mihir, M. 2016. First report of fruit rot of ridge gourd (Luffa acutangula) caused by Sclerotium rolfsii. Plant Health Progress. 17:13-14.
Interpretive Summary: Ridge gourd is a specialty vegetable in the cucumber family (known as Cucurbitaceae) that is cultivated in the United States on a small scale for select markets. During September of 2014, 19 days of rainfall produced over 27 cm of rain that resulted in rot of over 25% of the fruit of ridge gourd in a field in Charleston, SC. The microbe causing the fruit rot was identified using microscopy and molecular tools. Prior to this study, it was not known if this microbe, a fungus known as Sclerotium rolfsii caused fruit rot of ridge gourd. Knowledge of the microbe causing the rot will be useful to suggest appropriate strategies to growers and extension agents to help manage the rot in areas where such gourds are grown for select markets.
Technical Abstract: Ridge gourd is a specialty cucurbit vegetable cultivated in the United States on a small scale for select markets. Ridge gourds are generally grown on a trellis which prevents the fruit from curving and lets it grow straight for the market. However some growers cultivate these on raised beds to lower production costs. Our research program has been planting specialty cucurbits for the past 5 years to monitor pathotypes and races of the cucurbit powdery mildew pathogen. Nineteen rainy days in Charleston, SC, during the month of September in 2014 resulted in 11 inches of rainfall that led to rot of 25% of the fruit of the ridge gourd variety Surekha in our research plots. Most fruits in contact with the soil exhibited symptoms of rot. It has been known that Luffa fruit touching or close to the ground can rot, however, the causal organisms are generally not specified. Visual examination of rotting fruit revealed presence of sclerotia on the fruit surface and the pathogen was identified as Sclerotium rolfsii (teleomorph: Athelia rolfsii). The pathogen was isolated on acidified potato dextrose agar (APDA) plate. The ITS region from the pathogen mycelium and sclerotia was amplified using ITS1 and ITS4 primers described before.The fragment was cloned using the TOPO cloning kit and sequenced. The sequence (submitted to GenBank: KU128903) was 99-100% identical to Athelia rolfsii sequence in the NCBI data base (GU080230.1, HQ420816.1, and KJ546416.1). In addition primers were designed using known A. rolfsii sequences in the data base for the lectin gene and translation elongation factor 1-alpha (tef1). The primers were used to PCR amplify and sequence the fragments from the S. rolfsii isolate from ridge gourd. The sequences were 99-100% identical to A. rolfsii tef1 (GU187681.1, JF267817.1 and KP982854.1) and 98% to lectin gene (JN811676.1 and FJ211419.1) in the NCBI data base further confirming the identity of the pathogen. The sequences for S. rolfsii isolated from ridge gourd have been submitted to NCBI GenBank with the accession numbers KU128904 (lectin gene) and KU128905 (tef-1). Symptomless ridge gourd fruits were surface sterilized and inoculated with four sclerotia per fruit placed at the blossom end of the fruit as most of the infection in the field was observed in this area. The sclerotia were obtained from an actively growing colony on APDA plate. There were six fruits per replication with four replications. After inoculation the fruits in each replication were placed on a shelves in a humid chamber (>95% RH, 26±2°C). Five days after inoculation 87% of the fruit were infected and symptoms similar to what was observed in the field was noticed. The experiments were repeated one more time with similar results. Southern blight and fruit rots caused by Sclerotium rolfsii has been reported on cucurbits such as melon and watermelon. To the best of our knowledge we have not come across reports of Sclerotium rot of ridge gourd in the U.S. or elsewhere. In fields with history of S. rolfsii the gourds should be grown on trellis to prevent the fruit from coming in contact with wet soil. Several fungicides are available for managing S. rolfsii and may have to be applied when and if necessary.