Evaluation of Septoria leaf spot (Septoria helianthi) alone and in combination with other foliar fungal spots on sunflower

Abstract 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.