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First Report of Sunflower Broomrape Wilt Caused by Fusarium brachygibbosum in China

Xia, B., Hu, J. Y., Zhu, X. F., Liang, Y., Ren, X., Wu, Y. H., Chen, D. X.
Plant disease 2018 v.102 no.11 pp. 2372
Fusarium, Helianthus annuus, Orobanche cernua, agar, air drying, antagonists, chlamydospores, conidia, crop production, culture media, ethanol, fungi, greenhouses, internal transcribed spacers, mercuric chloride, mycelium, nutrients, parasites, pathogens, photoperiod, pigments, relative humidity, ribosomal DNA, shoots, soil, sporulation, stems, China
Orobanche cumana Wallr. (sunflower broomrape) is an obligate root parasite that absorbs plant nutrients and water, thus causing severe crop production losses. In July 2016, symptoms were observed on the stems of parasitic broomrapes in sunflower (Helianthus annuus L.) fields in Liaoning, China (42.47403°N, 122.47169°E). Diseased broomrapes were unbent and displayed dry wilt with large brownish lesions. Fusiform to oval, hollow lesions (average 2.7 cm in length) were observed with a dark-brown zone in the center of lesions. Diseased shoots showing dry wilt with large brownish scales near the soil were collected for isolation. Small pieces of diseased tissue were surface disinfested (30 s in 75% ethanol and 1 min in 0.1% HgCl₂, followed by three rinses with sterile distilled water), air dried, and placed onto potato dextrose agar (PDA) medium. A fungus was isolated on all plates, abundant white to slight yellow aerial mycelium with red pigments on PDA. Mycelium from PDA was transferred to Spezieller Nährstoffarmer agar (SNA) to induce sporulation. Macroconidia were slightly curved, three marked septa, wide central cells, slightly sharp apexes, basal cells with foot-like shape, and measuring 30.2 ± 4.8 × 4.1 ± 0.6 µm (n = 30). Spherical chlamydospores were 10.5 ± 0.4 × 9.9 ± 0.5 µm (n = 30), intercalary or terminal, single, and in chains. Microconidia were absent or sparse on either PDA or SNA. Genomic DNA was extracted for amplification and sequencing of the internal transcribed spacer (ITS) region of the ribosomal DNA (GenBank accession no. MG267121) and translation elongation factor-1 alpha (EF1-α; MG267120) (White et al. 1990). The sequences had 99% similarity to the sequences of the ITS (KF985966) and EF1-α (JQ429370) regions of Fusarium brachygibbosum (Mirhosseini et al. 2014). Based on morphological and molecular characteristics, the fungus was identified as F. brachygibbosum (Al-Mahmooli et al. 2013). To satisfy Koch’s postulates, mycelial plugs excised from the margin of actively growing colonies on PDA (about 6 mm in diameter) were placed on the base of 10 1-week-old healthy broomrapes parasitized on sunflower in pots (diameter 15 cm) in a greenhouse at 25°C, 90% relative humidity, and 12-h light/dark photoperiod. Ten healthy broomrapes were treated with PDA plugs as a control. All PDA plugs were wrapped with Parafilm to maintain moisture. Brown to dark-brown lesions (average 1.3 cm) were observed at the inoculation sites 1 week postinoculation. Within 16 days after inoculation, all inoculated broomrapes wilted or died, and lesion length averaged 2.4 cm. The symptoms were identical to those observed on broomrapes in sunflower fields, whereas the control broomrapes remained asymptomatic. Pathogens that were successfully isolated from the inoculated stems again exhibited morphological characteristics identical to F. brachygibbosum. China is one of the most important sunflower producers and consumers in the world. Sunflower broomrape is widely distributed in the northern provinces of China. The fungus might be a potential antagonist agent to control broomrape in sunflower fields. To the best of our knowledge, this is the first report of F. brachygibbosum causing stem wilt of sunflower broomrape in China.