Peter Nielsen (28 July 1829 – 30 September 1897) was a Danish botanist and plant pathologist.
[1] He was born at a farm in Vonsbæk parish in the Duchy of Schleswig.
In 1859, he became a school teacher at Ørslev in Zealand, where he studied the local flora.
He undertook meticulous studies of rust fungi.
[2][3] [4] He was the first to describe the host alternation of Puccinia poarum between grasses and Tussilago farfara.