white rust
/ˌwʌɪtˈrʌst/noun
1A white coating or discoloration produced on the surface of various metals by exposure to moist air or other corrosive agents; especially (in early use) a coating of white lead (basic lead carbonate) formed on the surface of lead; (now more usually) a powdery white coating of zinc oxide, hydroxide, and carbonate formed by corrosion of zinc or galvanized steel.
2Any of several fungal diseases of plants characterized by the formation of white pustules or blisters on leaves or stems, especially (a) those affecting cultivated brassicas or salsify and scorzonera, caused by oomycetes of the genus Albugo; (b) that affecting chrysanthemums, caused by the rust fungus Puccinia horiana.
Origin
Late 17th century; earliest use found in Walter Harris (1647–1732), physician. From white + rust.