Orobanche cumana Wallr. (broomrape; synonym Orobanche cernua Loefl.) is a parasitic plant that causes severe yield losses on important crops such as sunflower (3) and tomato (1). It has become a serious threat to the survival of sunflower in Xinjiang Province, China. In July 2010, a stem rot disease was observed on sunflower broomrape plants in fields near Shihezi, China. Disease incidence was approximately 30% of the plants observed in a field. Symptoms started as localized necrosis of the stems that quickly expanded, girdling of stems, and finally plant wilt and death. Symptoms were accompanied by the development of a white, cottony, mycelial growth over injured tissues. Later, whitish aggregates of mycelia appeared and developed into dark rounded to elongated sclerotia that were up to 1 cm long. A fungus was consistently isolated from surface-sterilized fragments of diseased stem tissues on potato sucrose agar (PSA) at 25°C. Thirty-six isolates were obtained and grown on PSA at room temperature for several days. The fungus formed a white colony with a fairly flat sheet of aerial mycelia and subsequently produced black sclerotia on the colony surface mainly near the edge of the plate. In March, sclerotia were put in soil in plastic pots in a field and subsequently produced apothecia in April 2011. The one to three apothecia that emerged from each sclerotium were pale brown to dark brown, saucer shaped, and lacked epithecia. Hymenia consisted of asci and paraphyses; asci were unitunicate, cylindrical, narrower at the base, 95 to 135 × 6 to 8 μm, and lacking in apical structure. Ascospores were hyaline, smooth, aseptate, wide ellipsoid to ovate, and 8 to 12 × 3.5 to 5 μm. On the basis of morphological characteristics, the fungi were identified as Sclerotinia sclerotiorum (Lib.) de Bary (2). DNA sequences of internal transcribed spacers of the ribosomal RNA genes, including 5.8S genes of both single-mycelium isolates XJSHZ-1 (GenBank Accession No. JN012605) and XJSHZ-3 (GenBank Accession No. JN012606), from the plants had 99% homology with that of S. sclerotiorum (AAGT01000678). To demonstrate pathogenicity, mycelial blocks of three isolates grown on PSA were placed on the base of the stems of 10 2-week-old healthy plants grown on sunflower. Ten healthy plants were treated with PSA plugs as a control. Inoculated plants were kept in a moist chamber for 2 days and then transferred to a greenhouse at 25°C. After 2 days, the initiation of stem necrosis was observed, and 7 days after inoculation, the plants collapsed and died. S. sclerotiorum was recovered from all inoculated plants, but not from control plants, which remained asymptomatic. S. sclerotiorum has a wide host range and has been recognized as the causal agent of sclerotinia rot in many vegetable plants and fruit trees (2). There has been a previous report of S. sclerotiorum causing disease in tomato from China and in sunflower from Liaoning, China (4). To our knowledge, this is the first report of sclerotinia rot of broomrape in China.
References: (1) Y. El-Halmouch et al. Crop Prot. 25:501, 2006. (2) J. E. M. Mordue et al. No. 513 in: Descriptions of Pathogenic Fungi and Bacteria. Commonwealth Mycological Institute, Kew, UK, 1976. (3) D. Rubiales et al. Crop Prot. 22:865, 2003. (4) F. L. Tai. Science Press, Academica Sinoca. Peking, 1979.