In this experiment, we used foliar fertilizers containing phosphorus to study the effects on the development of red skin color in ´Fuji´ apples. We conducted this experiment in orchards of the company Evrosad d. o. o. in Krško, Slovenia. We used three different foliar fertilizers: Plonvit, Phostrade Ca and MKP in comparison with untreated control trees. The first treatment was applied 5 weeks before harvest. The second treatment was applied 14 days after the first. Harvesting took place on October 10, 2022. Each box was weighed and then sorted in a sorting facility. We separated apples with less than 50% red coloration and apples with more than 50% red coloration. Later, we performed tests for ripening parameters. There were no statistically significant differences between treatments for the measured parameters. In the laboratory, we continued colorimetric measurements, analyzed total phenolic content (TPC), and performed HPLC analyzes for anthocyanins, sugars, and organic acids. For the majority of the tests, we did not detect statistically significant differences. However, some data showed the effects of treatments on red skin color development. Apples treated with Plonvit had higher levels of cyanidin-3-arabinoside and cyanidin-7-arabinoside than other treatments; the difference was statistically significant. The year 2022 was very difficult for apple production due to weather conditions and the appearance of a pest. The trees were under stress and therefore did not respond to foliar fertilizer as expected. The trial should be repeated in a year with better conditions.
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