Optimization of the method for determining nitrogen balance in apple leaves when assessing the effectiveness of fungicides for scab

Under conditions of apple scab ( Venturia inaequalis ) epiphytotics, optimizing instrumental approaches adopted to assess the physiological state of plants becomes an urgent task. The study aims to develop and test a method for non-destructive rapid assessment of nitrogen balance in apple tree leaves ( Malus domestica Borkh .) using the nitrogen balanceindex (NBI) to analyze the effectiveness of various fungicide protection schemes. The experiment began in 2024 as part of a three-year study into the biological effectiveness of fungicides for apple scab in the apple orchard of Solnechnogorsky District of Moscow Oblast on the Mantet, Melba and Lobo cultivars. The experiment included four variants: two proposed schemes under study with sequential triple administration of a contact fungicide, followed by double application of a systemic preparation. Scheme No. 1 involved using Merpan, WP (500 g/kg captan) – 3 kg/ha + Znatok, WDG (500 g/kg trifloxystrobin) – 0.14 kg/ha. Scheme No. 2 included Shrapnel, WSG (700 g/kg dithianon) 0.7 kg/ha + CipAgro, WDG (750 g/kg cyprodinil) – 0.2 l/ha, commercial fungicide protection system (with a predominance of difenoconazole) and control (water treatment). NBI index measurements were performed on 3 July and 3 September using a Dualex device on leaves of three age groups. Comparison of the methods showed that the 30 × 1 approach (30 points on one leaf) provides less variability (SD ranges from 1.99 to 3.62) compared to the 6 × 4 approach (6 points on each of 4 leaves). Leaf age had the greatest effect on NBI (24.7% of variance, p < 0.05). The study established reliable inverse correlations between the NBI level and the spread (ρ = -0.615, p <0.05) and development (ρ = -0.601, p <0.05) of scab. It was observed that the protection scheme with captan and trifloxystrobin enabled the maximum increase in NBI during the growing season (up to +5.25 units). The obtained results confirm the effectiveness of using NBI as an objective criterion for assessing fungicidal protection under conditions of high infection pressure.