Levytska K. Peculiarities of the manifestation and inheritance of sunflower resistance to Septoria leaf spot in the conditions of the Southern Steppe of Ukraine.

Sunflower is one of the most important and profitable crops in the world. In Ukraine sunflower is the most cost-effective among other oil crops. The soil and climatic conditions of our country make it possible to successfully grow sunflowers. One of the important problems in the cultivation of this crop is a decrease in productivity due to the influence of various factors. Violation of crops rotation is one of these factors, which contributes to the spread of diseases and depletion of soil fertility in many regions of the country. Foliar diseases have become widespread lately, among which Septoria leaf spot prevails. The action of Septoria leaf spot leads to a decrease in the content of chlorophyll, the destruction of chloroplasts, a violation of the processes of respiration, photosynthesis and can reduce the yield of sunflower. The important methods are the protection of sunflower crops against Septoria leaf spot. For this purpose, it is necessary to conduct constant monitoring of the infection, to study the biological features of the pathogen and the genetic nature of resistance to the disease in order to create resistant hybrids and varieties. The aim of the research was to study the biological features of the pathogen Septoria helianthi in vivo and in vitro, to assess the manifestation of sunflower Septoria leaf spot, inter alia jointly with other foliar fungal diseases, in the conditions of the Southern Steppe of Ukraine, and to establish patterns of the inheritance of resistance to this disease. The scientific novelty of the obtained results is that for the first time F1 and F2 sunflower hybrids obtained from crossing the lines contrasting in terms of damage were assessed for Septoria leaf spot resistance in the conditions of an artificial infectious background. It was identified that about 65 % of the total variation in Septoria leaf spot damage indicators of sunflower line samples is due to the genetic nature of the line, slightly more than 20 % is due to the influence of the environment, and about 11 % is due to the ―genotype×environment‖ interaction. The analysis of first-generation hybrids in most cases revealed intermediate inheritance, as well as positive and negative overdominance of the trait ―affected by Septoria leaf spot‖. It was found that the inheritance of the degree of damage by the pathogen S. helianthi in the first generation of sunflower hybrids follows the type of parental sample that is more resistant to the disease. It was discovered that the F2 populations inherited Septoria leaf spot resistance close to the more resistant parental components from the appropriate crossbreeding combination. It was established that the level of Septoria leaf spot damage in F2 families can be influenced by the presence or absence of Septoria leaf spot infection of the F1 plant. For the first time, the possibility of joint development of Septoria leaf spot and other foliar diseases, such as downy mildew, black stem and Alternaria leaf spot on one plant with different frequency in the conditions of the Southern Steppe of Ukraine, was revealed. For the first time, a high level of sunflower seeds infestation with seed pathogens was established against the background of plant damage by Septoria leaf spot. New scientific data were obtained regarding the particulars of the development of the pathogen S. helianthi in vitro conditions. The method of artificially infecting sunflower with Septoria leaf spot pathogen in phytotron conditions has been developed. The ―Method of artificially infecting sunflower with Septoria leaf spot pathogen in phytotron conditions‖ was developed, and a patent for a utility model was received (Patent № 2022 01998 dated 22.03.2023).