The foliar fungal disease Septoria leaf spot is economically important for sunflowers. It develops most intensively during the wet and warm season. However, the genetic nature of sunflower resistance to Septoria leaf spot is still not clear. The purpose of our work was to identify some inheritance patterns of sunflower resistance to this disease based on the assessment of lines and hybrids of the first and second generations. The studied material was sown in 2020 and 2021 and subsequently grown on a stationary artificial infectious plot enriched with Septoria helianthi pathogen. The frequency of infection and the severity of infection of each sample were taken into account. The two-factor variance analysis revealed a significant influence of genotype on the frequency of infection in six sunflower lines and confirmed the effect of individuality of the response of individual genotypes to disease damage. It was shown that about 65 % of the total variation of the trait is due to the genetic nature of the line, and only a little more than 20 % and about 11 % – to the influence of the environment and the “genotype × environment” interaction respectively. Based on disease damage data in 2020 and 2021, the lines were classified as susceptible, moderately susceptible, moderately resistant and resistant. Differentiation of lines according to sensitivity to Septoria leaf spot made it possible to obtain hybrids with different manifestations of the trait in their parents. The nature of inheritance of infection frequency in F1 sunflower hybrids was determined by the degree of dominance. As a result, it was revealed intermediate inheritance, as well as positive and negative overdominance of the trait. The severity of infection by S. helianthi pathogen in F1 hybrids was inherited according to the type of parental form with greater resistance to the disease. F2 populations were found to inherit infection frequency from the more resistant parent, showing a proportion of infected plants similar to that of the more resistant line from a given cross combination. It has been shown that the frequency of infection of the F2 population with Septoria leaf spot can be affected by the severity of infection of F1 plant with the pathogen S. helianthi.
In recent years, Septoria leaf spot has taken a strong place in the list of economically important diseases of sunflower. It may be a favorable factor for the development of other diseases on sunflower plants too. The aim of the research was to evaluate the frequency and degree of infection of sunflower plants with Septoria helianthi as well as possibility of simultaneous infection with this pathogen and other fungi which cause foliar spots, such as downy mildew, Alternaria leaf spot and Phoma black stem under conditions of stationary infectious nursery. The year 2021 turned out to be extremely favorable for the development of fungal leaf spots on sunflower. Assessment of fungal diseases of sunflower plants was performed in F2 families obtained after crossing ZL22A, ZL58A, ZL70A, ZL78A, and ZL169A lines of Zaporozhye breeding (Ukraine) with HAR7 line (originating from the USA). Each F2 family (sample) was obtained from self-pollination of one F1 plant with varying severity of Septoria disease. It was found that in the conditions of 2021, sunflower was quite affected by Septoria disease. The percentage of injured plants in F2 samples ranged from 41.0 to 100.0%. The severity of disease was different. Less resistant genotypes included plants with lesions on sunflower leaves of lower, middle, and upper tiers in approximately equal numbers, while more resistant genotypes had almost no plants with injured leaves of upper tiers. It was established that the development of Phoma disease occurred both on plants with Septoria leaf spot and on healthy plants. On average, among the plants infected with Phoma macdonaldii, the proportion of plants jointly affected by Phoma and Septoria diseases was 2/3. However, the frequency of simultaneous infection of plants with S. helianthi and P. macdonaldii in genotypes with different resistance to Septoria leaf spot differed significantly and ranged from 80% in less resistant to 40% in more resistant accessions. Most plants (about 90%) infected with Plasmopara halstedii were simultaneously infected with S. helianthi. That is, both of these pathogens can simultaneously develop on the same plant and lead to a decline in its basic physiological functions. The rate of simultaneous damage of sunflower by Alternaria and Septoria was quite low compared to plant damage by Alternaria alone and was less than 30%. This may mean that the pathogen of Alternaria leaf spot avoids plants with existing S. helianthi fungus.
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