Assessment of the intensity of damage to sunflower caused by artificial infection with septoria in phytotron conditions

The aim of our study was to analyze the development of Septoria leaf spot in sunflower seedlings artificially infected with the mycelium suspension of the pathogen in the phytotron and to determine the degree of damage caused by this disease in the lines of different origins. The materials for the experiment were self-pollinating sunflower lines ZL22A, ZL58A, ZL78A (all of Zaporizhzhya breeding the Institute of Oilseeds Crops of NAAS) and line HAR7 (originating from the USA). The research was conducted in the phytotron facility at the Department of Genetics and Plant Resources of Zaporizhia National University. The seeds of selected sunflower samples were sown into the soil in the specially prepared pots containing drainage and a mixture of chernozem and sand in equal proportions to a depth of 3 cm in two rows of 10 seeds each, with a distance between seeds of 0.5-1 cm. The plants grew and developed on a photoperiod of 16/8 hours (day / night) at a temperature of 23-25oC and a relative humidity of 65%. All samples were watered when needed and received equal amounts of water. Sunflower plants were inoculated in accordance with generally accepted methods of phytopathology. Next, the infected plants were placed in a humid chamber. The plants were infected with the inoculum of a 30-day old mycelium culture of the fungus Septoria helianthi, grown by cultivating the pathogen on a solid nutrient medium. The isolation of pathogen in pure culture as well as preparation of inoculum for infection were carried out using methods which were generally accepted in phytopathology and mycology, supplemented by author's own methodology. The degree of damage to the plants was assessed by visual examination of all leaves, using a 5-point scale, modified for our studies. The affected lines were determined by the percentage of plants with a certain degree of damage. The results of this research showed that sunflower lines sustained varying degrees of damage and, accordingly, varying degrees of resistance to Septoria. The least affected was line HAR7, with 70% of the plants in this line having damaged leaves. The most damaged of those were cotyledons and the first pair of true leaves, whereas just 11.7% of plants had the second pair of true leaves damaged. Completely unaffected plants made up 29.5% of this line. Line ZL22A was affected by 75%. Almost all plants in this line had damaged cotyledons and the first pair of leaves, with 70%. Besides, there were 5% of plants that had all their leaves affected. The number of unaffected plants was 25%. It should be noted that the ZL22A line was close in degree of damage to that of the HAR7 line and showed relative susceptibility to Septoria under phytotron conditions. ZL58A and ZL78A lines were the most affected by Septoria, with 100%. In the ZL58A line, there were 56.2% of plants with damaged cotyledons and the first pair of true leaves, and 43.8% of plants whose second pair of true leaves showed the symptoms of the disease. The plants of the ZL78A line with lesions on the cotyledons and the first pair of true leaves made up 60%, whereas the number of plants with the symptoms of Septoria leaf spot on the second pair amounted to 40%. In general, these two lines were characterized by the same degree of damage and can be considered susceptible to Septoria leaf blight. The disease progression on the HAR7 line is 38.2%, on the ZL22A line is 40%, on the ZL78A line is 60%, on the ZL58A line is 60,9%. The findings show that sunflower plant lines that were artificially infected with an aqueous suspension of fungal mycelium containing Septoria leaf spot pathogen in the conditions of phytotron developed the disease rather rapidly and exhibited varying degrees of damage. The proposed method provides a reliable infection of sunflower samples with the pathogen S. helianthi, a rapid assessment of the intensity of damage to sunflower lines by septoria.