Bacterial top rot is one of the important challenges in corn (Zea mays) production, causing the heart leaves of early-stage plants to turn yellow, curl, and rot in recent years. Between 2021 and 2023, a new devastating disease exhibiting top rot-like symptoms was observed in the corn fields of Wuwei City, Gansu Province, China. The leaves showed signs of yellowing and necrosis, with many leaves curling. In severe cases, this prevented proper leaf formation from the trumpet, subsequently preventing cob development. The incidence of the disease in the main affected fields exceeded 20%. Heart leaf samples with typical top rot symptoms were collected from an 8-ha diseased field in a central corn-growing area. The margin between healthy and diseased parts of the leaf tissues was disinfected (Fang 1997), cut aseptically into small sections, and immersed in sterile distilled water for 10 min. The suspension was spread evenly across the surface of nutrient agar (NA) plates using a sterile ring and incubated at 28°C for 48 hours. The predominant bacterium on the plates was re-streaked on NA plates for further purification and repeated three times using the streak plate technique. Pure cultures with uniform morphology were obtained, which were gram-negative, short rod-shaped, and appeared milky white, raised, and round on NA. DNA was extracted from the pure culture grown for 24 h on NA using the protocol described by Dashti et al. (2009). The universal 16S rDNA gene and the stable, specific gyrB gene were amplified using primers 27F and 1492R (Marchesi et al. 1998), and 1S and 2rS (Brady et al. 2008). The PCR products were then sequenced. The 16S rDNA sequence (OQ678150, 1390 bp) and gyrB sequence (OR051569, 1145 bp) were submitted to GenBank and compared with published nucleotide sequences using BLAST search in the NCBI database. The 16S rDNA sequence had remarkable 99.57 to 99.71% similarity with the K. michiganensis from soil in China (OR459811), from river water in Australia (CP172761), and from homo sapiens in the USA (CP114156). The gyrB sequence exhibited 100.00% similarity with that of K. michiganensis from human body fluids in China (CP089448), soil in South Korea (CP041515), and fecal in the United Kingdom (LR607367). These 16S rDNA and gyrB sequences were clustered with 100% confidence through phylogenetic analysis. The strain isolated from diseased corn was identified as K. michiganensis according to morphological and molecular features. A total volume of 100 μl of the bacterial suspension (108 CFU/mL) was inoculated into the stem of each 20-day-old seedling of 4 healthy corn plants (5288) through a sterile syringe for the pathogenicity test. Control seedlings were inoculated with sterile distilled water, and all plants were kept in a greenhouse under controlled conditions (28±2℃, >80% RH). After 8 days, all inoculated heart leaves exhibited yellowing and necrosis, similar to the initial symptoms observed in the field, whereas the control plants grew normally and remained asymptomatic. Bacteria were isolated from symptomatic leaf tissues and identified as K. michiganensis. It has been reported to cause bacterial blight of mulberry in China (Luo et al. 2022). Besides, K. pneumoniae was the pathogen of corn top rot disease (Huang et al. 2016). To our knowledge, this is the first report of K. michiganensis naturally causing top rot on corn in China and worldwide. This report will enhance growers' awareness of this disease and take effective measures to control the economic losses.
Xu et al. (2025) studied this question.