The uncontrolled spread of mistletoe in fruit trees jeopardises the orchards typical of Germany. With the help of photogrammetric analyses of multispectral images, an early, automatic detection and assessment of mistletoe infestation in orchards is to be made possible. This should enable actions against to be taken for the sustainable preservation of the tree population and the microclimates prevailing in orchards.
Many people associate mistletoe with a secret ingredient for the magic potion of some comic book heroes, or the romantic custom of standing under a sprig of mistletoe as a couple in the run-up to Christmas.
In total contrast to this positive image of mistletoe, however, is its parasitic nature. What sometimes looks like an evergreen apple tree in orchards in winter is actually a doomed apple tree whose parasitic mistletoe has taken over the tree.
Mistletoe is a parasite that nests in the branches of a few tree species - including apple trees in particular - and damages the tree by depriving it of water and nutrients until it dies.
The spread of mistletoe has increased significantly in recent years: Due to climate change, trees are under "drought stress" and are therefore more susceptible to diseases and parasites such as mistletoe. The climatic conditions for mistletoe growth itself have also become more favourable.
The increase in mistletoe infestation jeopardises the tree population in Germany's typical orchards. This results in crop failures and the loss of entire trees in fruit orchards. The infested and dying trees are then also lost as nesting sites for birds, habitats for other animals and as guarantors of microclimates (shade providers, foliage for microorganisms, etc.).
Municipalities, as well as fruit and horticultural associations, are therefore very interested in containing the uncontrolled spread of mistletoe through targeted countermeasures, for example by removing the mistletoe in good time (!).
Two master's theses in the geomatics department are already working on developing a method to enable early and comprehensive detection of mistletoe in orchards using drone flights and a multispectral camera.
Die mit der Multispektralkamera aufgenommenen „Spezial-Bilder“ werden dabei photogrammetrisch ausgewertet. Die in unterschiedlichen Frequenzbereichen (z.B. Nahinfrarot) des Lichts aufgenommenen Bilder werden analysiert, um beispielsweise geschädigte von gesunder Vegetation zu unterscheiden: Im Nahinfrarotbereich unterscheiden sich Bereiche mit hoher photosynthletischer Aktivität deutlich von Bereichen mit niedriger photosytnthetischer Aktivität durch die Reflexionseigenschaften. Mit weiteren Analysen versucht man letztendlich die Misteln zum Einen schon im Frühstadium, und zum Anderen auch bei leicht belaubten Bäumen im Herbst zu erkennen. Ziel soll die flächenhafte Kartierung eines Streuobstwiesengebiets mit Information über die Verbreitung der Misteln und ggfs. auch mit dem Standort der im Frühstadium der Entwicklung empfindlichen Misteln sein.
False colour images of a multispectral image in the near infrared range (black/white) and in the infrared range calculated from 3 colour channels - red, green, blue - (black/red). In the images, areas with different levels of photosynthetic activity can be distinguished and conclusions can be drawn about the condition and age of vegetation / biomass.
The image shows two trees in a meadow from an aerial perspective: The tree structures are recognisable as white (black/white image) or black structures (black/red image). The mistletoe is recognisable as a dark spot or deep red area.
The analysis and combination of different false colour images allows the targeted detection of mistletoe.
In addition to the techniques and methods of surveying, geoinformatics and photogrammetry, the increased use of artificial intelligence (AI) for the automatic detection of mistletoe in multispectral images is also being considered.
Further theses and possibly a research project are planned on this topic in order to further develop and validate the method for the early, area-based detection of mistletoe.
Orchard meadows are part of our habitat. We are making a contribution to their preservation with the extensive mistletoe detection as a basis for maintenance measures. We will continue to fly "above the mistletoe"!
