also: しゅうきん
noun
rust fungus
A plant disease caused by rust fungi (order Pucciniales), appearing as rust-colored spots on leaves and stems.
さび菌は小麦に大きな被害を与えることがある。
Rust fungus can cause serious damage to wheat.
この葉のオレンジ色の斑点はさび菌のせいだ。
These orange spots on the leaf are due to rust fungus.
さび means 'rust' (metal oxidation), while さび菌 specifically refers to the rust fungus that causes plant disease.
Compound of さび (rust) and 菌 (fungus, bacterium), referring to the rust-like appearance of the fungal infection on plants.