First Report of <i>Bipolaris maydis</i> Causing Maize Stalk Rot in Henan, China

Maize stalk rot is one of the most prevalent diseases worldwide. In September 2022, 2023 and 2024, stalk rot was observed in 8 farmers' maize fields (1 ha per plot) in Xinyang, Zhumadian, Changge and Xinxiang cities, Henan Province, with disease incidence ranging from 2 to 8%. Observed symptoms initially appeared as chlorosis of upper leaves and discoloration of lower internodes, progressing to stalk rot soft and pith tissue browning. 39 symptomatic plants were collected, and the pith tissues were cultured as previously described (Shan et al. 2017). The cultures of 30 single-spore isolates exhibited grayish-brown centers with white outer regions. Conidia were curved, yellowish brown, fusiform, smooth, 5 to 11 septa, 35.57 to 102.22 µm × 10.36 to 22.89 µm (n = 100, avg. 68.23 µm × 14.55 µm). Four isolates (MR1369-1, MR1486-2, WC161, and WX98) were selected for molecular identification and inoculation tests. The internal transcribed spacer (ITS), glyceraldehyde-3-phosphate dehydrogenase-like (<i>Gpd</i>) and 1,3,8-trihydroxynaphthalene reductase (<i>Brn</i>) regions were amplified with primers ITS1/ITS4 (White et al. 1990), Gpd1/Gpd2 (Berbee et al. 1999) and Brn01/Brn02 (Shimizu et al. 1998), respectively. The sequences of ITS (PP068972, PP068973, PQ795810, and PQ795811), <i>Gpd</i> (PP077405, PP077406, PQ810323, and PQ810324), and <i>Brn</i> (PP077409, PP077410, PQ810331, and PQ810332) were deposited in GenBank and showed 100% (531/531 bp, 538/538 bp), 100% (570/570 bp), and 100% (867/867 bp, 919/919 bp) identity, respectively, with the <i>B. maydis</i> isolate 1-4-2 (MT130249, MT130279 and MT130208). All isolates were identified morphologically and molecularly as B. maydis (Sivanesan et al. 1987). Pathogenicity tests were performed on maize inbred lines HCL645, 798-1, NK178 and Zheng58 as previously described with modification (Zhang et al. 2016) and on seedlings in the greenhouse. In the experimental field of Yuanyang, ninety flowering plants per variety were drilled at the second or third stem nodes above the soil surface to make a 6-mm-diameter wound, of which twenty plants were inoculated with 1 ml of conidial suspension (105 conidia/ml) per isolate, and ten plants with sterile water. At 30 dpi (days post-inoculation), the stalk tissues above and below the injection sites exhibited obvious browning and rot, similar to those observed in the field. In the greenhouse, five seeds per pot were planted in 300-ml plastic pots (8 cm deep) containing a mixture of <i>B. maydis</i> inoculum (sorghum grains inoculated with each isolate, and ground into powder with a blender) and autoclaved vermiculite-soddy soil (1:2 v/v), 10:90 w/w, and grown at 20 to 25°C and 80% relative humidity for 20 days with 16/8 h of light and dark cycle. Each isolate was inoculated into six pots per variety, with three non-inoculated pots serving as controls. Seedlings were removed from the substrate and washed under tap water, and observed for the precence of root browning and rot. No symptoms were observed or detected in all controls. The pathogen was successfully reisolated from symptomatic tissues and identified as <i>B. maydis</i>. <i>B. maydis</i> is a common pathogen of Poaceae plants and has intense pathogenicity to <i>Zea mays</i> (corn), <i>Rottboellia cochinchinensis</i> (Macedo et al. 2015) and <i>Microstegium</i> <i>vimineum</i> (Huang et al. 2017). To our knowledge, this is the first report of <i>B. maydis</i> causing maize stalk rot in Henan, China. Given the substantial yield losses caused by this disease, further research and management strategies are needed.