Reaction of forage sorghum lines and hybrids to target leaf spot

IntroductionTarget leaf spot of sorghum, caused by fungal species Bipolaris sorghicola, is considered as one of the most important leaf diseases in sorghum, so that occurrence before flowering, can lead to damage and reduction of more than 50% of grain yield. This disease is also significant in Iran and usually occurs in most of the humid regions of the country, where sorghum is cultivated. Since there are very few researches on leaf spot diseases in sorghum especially on the resistance of sorghum cultivars in the country, it seems necessary to carry out research in this field. Therefore, the present research was carried out with the aim of evaluating the response of different sorghum genotypes to leaf spot disease. The resistant genotypes identified in this research can be used to transfer favorable characteristics to the host plant in future breeding programs.Materials and methodsTo study the reaction of different forage sorghum genotypes to target leaf spot, 25 sorghum genotypes, including ten commercial hybrids and 15 promising lines were evaluated in a randomized complete block design with three replications in two regions, Karaj and Gorgan. Artificial inoculation of genotypes in the field was carried out with a mixture of several pathogenic isolates of the disease-causing fungus in two stages. The first, at the 3-4 leaf stage by injection of spore suspension (3 ml per each whorl) using a syringe stage, and the second, at the 6-8 leaf stage using the Bazooka method by placing 2-3 fungus infested sorghum grains in the whorl of each plant. To evaluate the reaction of genotypes and progress of the disease, characteristics such as percentage of disease incidence (DI%) and disease severity on each plant (DS%) were recorded two weeks after the last inoculation. In addition to the field experiment, the reaction of the genotypes to the disease was investigated in a greenhouse experiment based on a completely randomized design with three replications.Research findingsThe results of this experiment showed that there was a significant difference between the studied sorghum genotypes in terms of resistance or susceptibility to leaf spot. Comparison of genotypes showed that the disease severity of the studied lines and hybrids varied from 0.7% to 51.7% in the Karaj region, and from 1.7% to 66.3% in the Gorgan region. Hybrid No. 8 (FGCS110) with 0.7% and 1.7% disease severity in the Karaj and Gorgan regions, respectively, had the lowest disease severity and was recognized as the most resistant genotype in both regions. In contrast, line No. 2 (FGS109) with disease severity of 51.7% and 66.3% in Karaj and Gorgan regions, respectively, was the most susceptible genotype. Furthermore, a statistically significant difference was observed among the studied regions for disease severity of all genotypes, so that the recorded disease severity in Gorgan was higher than in Karaj. Also, there was a positive and significant correlation (r = +0.74, p< 0.01) between disease severity (DS%) and disease incidence (DI%) measured in different sorghum genotypes in two studied locations. The results of greenhouse experiment also confirmed the results of field experiment.ConclusionThe results of the current research led to the identification of resistant and semi-resistant sorghum genotypes to leaf spot among the studied genetic materials, which can be used in future breeding programs. The results obtained from the Karaj region showed that genotype No. 8 (foreign hybrid FGCS110) was highly resistant, line No. 13 and foreign hybrids No. 1, 6, 7, and 10 were resistant, and the number of 11 lines along with a foreign hybrid were semi-resistant. In the Gorgan region, foreign hybrids No. 7, 8, and 10 were resistant, and seven lines and five foreign hybrids also had a semi-resistant reaction to the disease, while no resistance reaction was observed in any of the studied lines in this region.