Specimen Spotlight - Downy Mildews

April 14, 2021
Thumbnail - Downy Mildew

The downy mildews are serious pathogens of plants. Although they were once considered to be fungi they are now classified with the stramenoples – organisms with bi-flagella zoospores, cellulose cell walls and other features. They are part of a group of algae including the diatoms. The downy mildews are placed in the class Oomycetes along with other fungus-like organisms the Hyphochytridiomycetes and Labyrinthulomycetes. The Farlow Herbarium collections have an array of downy mildews. One important historical connection involves William G. Farlow’s early studies of the grape downy mildew. His was the first comprehensive overview of the disease caused by Plasmopara viticola. Farlow’s study was done before the pathogen was disastrously introduced into Europe. The other significant collection of downy mildews is that made by William H. Weston (Cap). Weston taught biology and mycology at Harvard for many years but early in his career he worked on tropical plant diseases, particularly the downy mildews on grasses. As a young investigator he had the title “Plant Pathologist in Charge of Downy Mildew Investigations” in the Office of Cereal Investigations in the USDA. His mandate was to study the downy mildews on grasses, particularly sugar cane and maize, caused by Sclerospora species and those now knows as Peronosclerospora. This work took him to the Philippines. There he undertook life history studies, infection studies and observations of the discharge and dispersal of the sporangia. He described the nocturnal production of sporangia and their forcible discharge. This work was all in the aid of improving the production of these important crops. 

plasmopara viticola
Grape leaves infected with Plasmopara Viticola, collected in Cambridge, MA by W.G. Farlow in 1874. Courtesy Farlow Herbarium.