Ethel M. Doidge, mycologist and plant pathologist, was the daughter of Henry Doidge, an analytical chemist, and his wife, Elizabeth Craven. In 1897 (or 1895) the family moved from England to Pietermaritzburg, where Ethel attended Epworth High School and matriculated in 1903. The next year she continued her studies at Huguenot College, Wellington, studying botany under Dr Bertha Stoneman*, with mycology as her main interest. She received the degree Bachelor of Arts (BA) from the University of the Cape of Good Hope in 1907. The next year she moved to Pretoria to take up an appointment as assistant to Dr I.B. Pole-Evans*, mycologist and plant pathologist in the Division of Botany, Department of Agriculture, of the Transvaal Colony. Continuing her studies privately, she was awarded the degree Master of Arts (MA) in botany by the University of the Cape of Good Hope in 1909, for her thesis on The flora of certain Kaffir beers. For this thesis she won the university's Cornwal and York Prize for 1909. It was published that same year in the Science Bulletin (No. 8) of the Transvaal Department of Agriculture.
After the formation of the Union of South Africa in 1910, Doidge was promoted (in 1912) to professional assistant in the Division of Botany and Mycology. Two years later she was awarded the degree Doctor of Science (DSc) by the University of the Cape of Good Hope for her thesis A bacterial disease of mango. This newly described disease had caused considerable damage to mango crops in South Africa during the previous few years. Her doctoral degree was the first ever to be awarded to a woman in South Africa. In 1919 she was promoted to assistant chief of the Division of Botany and Plant Pathology, and in 1924 attended the Imperial Botanical Conference and the first Imperial Mycological Conference in London. She was appointed principal plant pathologist to the mycological section of the newly created Division of Plant Industry in 1929, a post she held until her retirement in 1942 at the age of 55. After her retirement she was employed in the Division for several more years in order to complete the research for her most extensive and important publication, "The South African fungi and lichens to the end of 1945". This work was eventually published in Bothalia (1950, Vol. 5, pp. 1-1094) and for decades remained the most important publication on the topic. After its completion she retired to the KwaZulu-Natal south coast.
Doidge's work in the Division consisted mainly of pioneering research in taxonomic mycology and studies of the bacterial and fungal diseases of crop plants. The results contributed much to the success of South African agriculture, particularly of the citrus industry. In the course of her work she made extensive collections of fungi and the host plants on which they occurred, during the early years often with horse and cart. Most of her specimens are in the National Herbarium, Pretoria, the National Collection of Fungi of the Plant Protection Research Institute, Pretoria, and the Selmar Schonland Herbarium of the Albany Museum, Grahamstown. Her research was published in about 50 scientific papers between 1915 and 1950. Many of these appeared in the Transactions of the Royal Society of South Africa (including a series of six on plant parasites of the order Perisporales, of the group Ascomycetes, 1915-1921), the South African Journal of Science (including a series of four "Descriptions of some previously unnamed South African fungi", 1925-1928) and, from 1921 onwards, Bothalia. Her papers in the latter journal included a series of four with descriptions of South African Ascomycetes in the National Herbarium (1921-1922) and a series of six dealing with the South African rust fungi (1927-1948). In addition she wrote more than 100 semi-popular articles and pamphlets aimed mainly at farmers, including Potato diseases (Johannesburg, 1919, 49p), Some common diseases of the tomato (Johannesburg, 1919, 24p), and Diseases of the apple, pear and quince (Johannesburg, 1919, 50p). As a leader in her field she did much to inspire younger scientists by her example and with advice.
Doidge was elected a Fellow of the Linnean Society (England) in 1912. She was a member of the Royal Society of South Africa and in 1918 was elected one of its Fellows. In 1915 she became a member of the South African Association for the Advancement of Science, serving as president of Section C (which included botany) for 1918/9. Her presidential address dealt with "The role of bacteria in plant diseases". She became a member of the Transvaal Biological Society and read papers at its meetings in February 1909, July 1910 and April 1915. In 1916 she became a foundation member of its successor, the South African Biological Society, served on its council, and in 1920 was awarded its prestigious Senior Captain Scott Memorial Medal in recognition of her work in plant pathology. From 1920 to 1921 she was furthermore editor in chief of the society's serial publication, the South African Journal of Natural History. The plant species named after her included Aplanodes doidgeana, Crotalaria doidgeae and Nitella doidgeae, as well as the fungi Meliola doidgeae, Phyllachora doidgeae and Eutypella doidgeae.
Outside botany Doidge practiced gardening and bred Pekinese dogs. As a licentiate of Trinity College of Music (London), she was a competent pianist and singer, and was a member of the Pretoria Music Society for many years. She was a member of the council of the University of South Africa From 1918 to 1924.