Evaluation of hop varieties using spectral and thermal characteristics derived from UAV images

Abstract

Hops are a traditional crop grown in the Czech Republic. Hop varieties have different qualitative characteristics not only in terms of harvest, but also in terms of phenological development and diseases resistance. The presented study focused on varieties grown in both conventional and organic systems. Since hops are a crop grown in relatively small blocks in rows where bare soil shows through canopy in the early stages of growth, the use of UAVs is a convenient way for crop monitoring. The advantage is that the resulting images have a very high spatial resolution, and it is thus possible to separate green plants from the soil to derive accurate results. Normalized Difference Vegetation Index (NDVI), Leaf Area (LA) and Crop Water Stress Index (CSWI) were calculated to determine varieties characteristics and their response to weather, diseases pressure and agronomic interventions during the whole growing season 2023. The results showed that NDVI and LA had the same development trend for all varieties, but organic varieties had significantly lower values compared to conventional. The reason was a different response to diseases pressure. The CWSI values showed a different trend depending mainly on management system. However, it was clear from the coefficient of determination that, CWSI for organic Premiant was explained by 65% using NDVI (structure and health), while only 50% for organic Saaz. Leaf area affected CWSI in organic Premiant by 50% and in Saaz by 43%. For conventional hops, NDVI explained CWSI from 53% (Agnus), 60% (Premiant) to 63% (Sládek). In conventional hop plants, the leaf area was higher, and the diseases pressure was not significant. CWSI was affected by LA on average by 23%. The achieved results can be applied in determining the effectiveness of cultivation management and agrotechnical interventions, including hop irrigation.