Branched broomrape is a holoparasitic weed present in some areas of central and southern Chile (33°S to 38°S), which is often found parasitizing tomato and tobacco crops. During an extensive survey conducted in different tomato-growing areas during the summer of 2010, branched broomrape plants with stem rot symptoms were detected in a commercial tomato crop located in the central zone (34°14′S, 71°1′W). Rotten stems were observed with white mycelia and approximately 1-mm-diameter spherical sclerotia on affected tissue below the soil surface. Parasitized tomato plants showed no symptoms. Sclerotia were taken directly from affected stems with a dissecting needle under a stereoscopic microscope in a flow chamber and placed on potato dextrose agar (PDA) medium. Germinating sclerotia consistently produced colonies similar to Sclerotium rolfsii with new sclerotia formed within 6 to 7 days. Mycelia produced hyphal clamp connections under the light microscope (2). DNA was extracted from one fungal culture. The ITS1 region, 5.8S rRNA gene, and the ITS2 region of the nuclear-encoded ribosomal RNA gene were amplified with primers ITS1 and ITS4 (4). The sequence was deposited in GenBank (Accession No. HM222638) and showed ≥99% identity values with sequences of similar regions from Athelia rolfsii (anamorph S. rolfsii; Accession Nos. AB075304, DQ0595578, AF499018, and AB075305). Different pathogenicity tests were performed. Inoculum was prepared by placing mycelia plugs from a PDA-grown, 6-day-old colony in a flask with sterilized wheat seeds and incubated for 2 weeks. Three Orobanche plants, each one with 10 to 15 shoots at different aerial stages (starting emergence, flowering, and formed capsules) were gently planted separately in 35-cm pots. Inoculum (10 g per pot) was placed in the soil surrounding the plants. One pot was used as a control. Forty-five-day-old tomato and tobacco plants were additionally inoculated by a similar procedure. After 12 days, inoculated Orobanche plants showed reduced vigor and stem decay. After 9 days, tomato and tobacco plants showed wilt. In all cases, the fungus was reisolated on PDA from all inoculated plants. To our knowledge, this is the first report of S. rolfsii on O. ramosa (1). A previous report of S. rolfsii parasitizing O. cernua has also been made (3). The high susceptibility of tomato and tobacco plants to this isolate of S. rolfsii precludes the use of this pathogen as a biological control agent against broomrape.
References: (1) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved May 14, 2010, from http://nt.ars-grin.gov/fungaldatabases. (2) Z. K. Punja and A. Damiani. Mycologia 88:694, 1996. (3) C. A. Raju et al. Phytoparasitica 23:235, 1995. (4) T. J. White et al. Amplification and direct sequencing of fungal ribosomal RNA genes for phylogenetics. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, New York, 1990.
