Identification of Races and Mating Types of Cochliobolus carbonum from Corn in the Yunnan Province in China

Northern corn leaf spot, a foliar disease caused by Cochliobolus carbonum, has become prevalent in southwestern China, especially in the Yunnan Province. Races and mating types were identified for 169 isolates collected from 13 prefectures of Yunnan by artificial inoculation using six hybrid corns as differential hosts and by crossing with three standard mating strains: CC092 (MAT1‐2), CC120 (MAT1‐1) and CC026 (MAT1‐1). Results showed the existence of three races: CCR1 (one isolate), CCR2 (43 isolates) and CCR3 (125 isolates). Most isolates were moderately or weakly virulent with only five being highly virulent. CCR3 was widely distributed and significantly more virulent than CCR2 that coexisted with CCR3 in many locations. On Sach's nutrient agar, 20.71% of the Yunnan isolates self‐mated, forming sterile perithecia. Fully developed perithecia could be formed between isolates of different geographic origins, but only 15.98% strains mated successfully with CC092 and 5.33% formed mature perithecia with 4–6 ascospores per asus. Similar results were obtained in crossing with CC026 or CC120. Mating could also occur between CCR3 and CCR2. Both mating types were found in Yunnan with 84 MAT1‐1 strains (one CCR1, 10 CCR2 and 73 CCR3) and 85 MAT1‐2 strains (33 CCR2 and 52 CCR3) and they coexisted in most areas. To identify the mating type rapidly, three specific primers were successfully developed and employed to amplify the mating‐type genes, with stable patterns of 1627 and 876 bp fragments obtained from MAT1‐1 and MAT1‐2 isolates, respectively. The ratio between MAT1‐1 and MAT1‐2 was 1 : 1, indicating that the mating‐type genes segregated randomly in the field naturally.