How do the nitrogen level and form and crop density influence the progression of alternaria leaf spot disease on hemp crop in Northern Baltic region?

The cultivation of oilseed hemp (Cannabis sativa) has been growing in Northern Europe during the last decades, and so far, the crop has been relatively pest-free. Monitoring of Alternaria-like symptoms was started on the experimental field of hemp (cv ‘Finola’) in 2019. The aim of the research was to evaluate the resistance of hemp to Alternaria leaf spot and to assess the effect of hemp sowing rate (20–40 kg ha−1), nitrogen application rate (0–150 kg ha−1), fertilization method (at sowing; top dressing), and fertilizer type (complex fertilizer; ammonium nitrate) on the variation in disease progression within and among growing seasons. Across the years the maximum disease severity was 27.8 ± 4.2% (on average 12.7 ± 0.8% at the end of the growing season) with the lower degree of infection in the drier and warmer year 2021. There was a strong negative relationship with the nitrogen application rate throughout the study. The rAUDPC of the N150 treatment was on average 75% lower compared to the control. Furthermore, fertilization at the start of the growing season resulted in a higher disease occurrence throughout the study compared to the top dressing method. Higher sowing rates were related to higher disease values, but this effect was year-dependent. Although warmer and drier seasons might suppress Alternaria occurrence, the pathogen is progressively dispersing in the Northern climates. To avoid the increased need for pesticide use in the future, further monitoring, along with agronomic and microbiome studies is urgently needed to improve hemp cultivation in the Northern Baltic region.