First report of root rot caused by Fusarium nygamai on foxtail millet (Setaria italica) in China.

Abstract

Foxtail millet [Setaria italica (L.) P. Beauv.] is one of the most important nutritious food crops in China. In August 2020, plants of the foxtail millet cultivar Xiao Huang Miao were found that were wilted and root rot symptoms of 25-75% incidence in a field production area of about 3000 m2 near Tongliao of Inner Mongolia and Chaoyang cities of Liaonning province. The wilted plants showed yellowing, stunting, and the lower stalk became straw colored, softened, with gray-whitemould on the surface of the stem nodes. The root system was poorly developed, brown and rotted. Symptomatic roots were surface-disinfested with 70% ethanol for 1 min and in 2% sodium hypochlorite (NaOCl) for 3 min, rinsed with sterilized water three times, and placed on potato dextrose agar (PDA) and incubated at 26ºC for 5 days. Ten pure cultures were obtained from single conidia with an inoculation needle under stereomicroscope. The cultures were transferred to carnation leaf agar (CLA) medium and incubated two weeks in the dark at 26ºC for microscopic observation. Macroconidia had one to four septa (three septa dominated), and were slender and straight with curved apical cell and foot-shaped basal cell, 25.5 - 30.5 × 2.5 - 4.5 μm (n=50). Microconidia were non-septate, oval, and were formed in short chains or false heads on monophialides, 2.5 - 15 × 2.75 - 4.0 μm (n=50). Chlamydospores were singly or in chains, circular or subcircular, 5.25 - 11.5 μm in diameter (n=50). Morphologically, the fungus was identified as Fusarium nygamai Burgess & Trimboli (Klaasen and Nelson,1998; Leslie and Summerell, 2006,). To validate this identification, rDNA internal transcribed spacer (ITS), partial translation elongation factor 1 alpha (TEF-á) gene, and RNA polymerase II second largest subunit (rpb2) of the ten isolates were amplified and sequenced (White et al.1990;O'Donnell K. et al. 2015,2010). Identical sequences were obtained and the sequence of the isolate GZGF23 was submitted to GenBank. BLASTn analysis of the ITS (OL964384), TEF-á (OL961517) and RPB2(ON756204) sequence of isolate GZGF23 revealed 99.86% (MH862671, 557/565bp), 100% (MT011009, 713/1770bp) and 100% (MT010976, 1002/3907bp) sequence similarity respectively with F. nygamai (CBS749.97). Pathogenicity studies were conducted on outdoor potted ground and with the foxtail millet cultivar "Xiao Huang miao". Five 12-L pots were filled with sterilized field soil mixed with 300ml conidial suspension at 3 × 105 spores/ml. Another five 12-L pots were filled with sterilized field soil mixed with 300ml sterilized water that served as controls. About twenty seeds per pot were surface disinfected in 2% NaOCl for 3 min, and rinsed with sterilized water. The foxtail millet seeds were sown the same day as soil inoculation and 6 plants were left in each pot when seedling emerged. Five weeks after seedling emergence, all inoculated plants exhibited symptoms similar to the syptoms observed in the field but control plants had no symptoms. The same results were obtained when pathogenicity tests were repeated two times in the same manner. Fusarium nygamai was reisolated from inoculated plants and its morphological and molecular characteristics matched the original isolate, but the fungus was not reisolated from control plants. This is the first report of root rot caused by F. nygamai on foxtail millet in China. The disease might bring a threat to foxtail millet production and effective control measures should be identified to reduce losses.