First Report of Panicle Rot Caused by Fusarium asiaticum on Foxtail Millet in China.

Abstract

Foxtail millet [ Setaria italica (L.) P. Beauv.] is one of the most important nutritious food crops. It is used for wine and health products in China. In August of 2019, panicle rot symptoms with up to 85% of panicles infected were observed on foxtail millet (cultivar Chaogu 8) in a commercial field located in Chaoyang city of Liaoning Province, China. Typical disease symptoms included brown spots on spikelets at early stages and brown-colored withering and rot of whole panicles at late stages, with the symptoms being more severe at the tip of the panicles. Under high humidity conditions, pink or salmon-colored molds developed on panicles. Symptomatic spikelet pieces were surface-disinfested with 70% ethanol for 1 min followed by 2% NaOCl for 3 min, rinsed with sterilized water for three times, and placed on potato dextrose agar (PDA) medium at 25°C. After 5 days, colonies turned pink to dark red with fluffy aerial mycelium and pigmentation with the age. Ten pure cultures were obtained from single conidia of mycelium grown on carnation leaf agar (CLA) medium at 25°C under a 12-h light-dark cycle using an inoculation needle under stereomicroscope. Macroconidia were hyaline, falcate with foot cells, 3-5 septate and size: 28.5- 44.0 μm × 3.8 - 4.9 μm. Chlamydospores were globose to subglobose (5.4 to 13.8 μm). No microconidia were produced on CLA. Black, ostiolate subglobose perithecia were formed on CLA after one month of incubation at 20°C under a 12-h light-dark cycle. Morphological characteristics of the fungus were in agreement with the description of Fusarium asiaticum (O'Donnell et al. 2004; Leslie and Summerell 2006). To validate this identification, partial translation elongation factor 1 alpha (TEF1-a) gene, and rDNA internal transcribed spacer (ITS) region of five isolates were amplified and sequenced (O'Donnell et al. 2015; White et al.1990). Identical sequences were obtained, and the sequence of one representative isolate (JGF-3) was submitted to GenBank. BLASTn analysis of both TEF sequence (MW685833) and ITS sequence (MW423687), revealed 100% sequence identity with F. asiaticum KT380120 and MT322117, respectively. Pathogenicity test were conducted on cultivar Chaogu 8 of foxtail millet. Inoculum was prepared from the culture of JGF-3 incubated in 2% mung beans juice on a shaker (140 rpm) at 25°C for 48 h. Conidial suspension (5 × 105 conidia per ml) was prepared and sprayed onto the panicles of 20 plants at the initial flowering stage and 20 additional plants that were sprayed with distilled water served as the non-inoculated controls. Treated plants were covered with plastic bags for 48 h and maintained at a greenhouse with day and night temperatures of 26 and 24°C, respectively. Two weeks after inoculation, all inoculated panicles exhibited symptoms similar to the syptoms observed in the field. No symptoms were observed in the non-inoculated control plants. The experiment was repeated twice with similar results. F. asiaticum was reisolated from the inoculated plants and its morphological characteristics matched those of the original isolates; the fungus was not reisolated from the non-inoculated plants. To our knowledge, this is the first report of F. asiaticum causing panicle rot of foxtail millet in China. To date, the disease has been observed to be present in Fuxin and Tieling city of Liaoning Province. Panicle rot can become an important disease in foxtail millet in China.