August Friedrich Adrian Diel (4 February 1756 – 22 April 1839) was a German physician and founder of pomology at the turn of the 19th century.
Adrian Diel’s father Kaspar Ludwig was a surgeon and apothecary. August Friedrich Adrian studied in Giessen and Strasbourg. He earned a Doctorate in Medicine and Surgery in 1780, and became a physician in Gladenbach. In 1786, Diel served as practitioner of the Count of Spaur, who was Judge at the Imperial Court in Wetzlar, and dealt mainly with Balneology. In 1790 he took over the office of spa doctor in Bad Ems (Nassau) and, at the same time, was the district physician at his residence in Diez. In the same year, Diel became Aulic Councillor, then Private Councillor of the Duchy of Nassau. He married a native of Diez, Maria Altgelt, but she died soon afterwards. In Diez, in the property known up to this day as "House Eberhard", which his wife had brought into the marriage, Diel planted extensive orchards with reportedly up to 12,000 apple trees.
Diel gained importance as a breeder of fruits and as an author of pomological writings, and laid the basis for the rise of that science in the 19th Century. In addition to his work on the systematics of the fruit trees, Diel also exerted political influence to promote fruit cultivation in the Duchy of Nassau. After the vineyards had been largely destroyed in the immediate area of Diez because of pest infestation, Diel also created large plum orchards. The introduction of cherries at higher elevations in the Lahn area, and the Nassau government regulation on planting fruit trees by the roadsides and on creating orchards in schools goes back to his initiative.
From his marriage with Adrietta Dorothea Christine, born Scriba, were born seven children.
In 1925, the city of Diez named a newly created road as Dielstraße. In his native city of Gladenbach there is an Adrian-Diel-Straße.