Forma specialis is a taxonomic

Taxonomy

Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

 grouping allowed by the International Code of Botanical Nomenclature, that is applied to a parasite (most frequently a fungus) which is adapted to a specific host (there being minimal or no morphological differences); this classification may be applied by authors who do not feel that a subspecies or variety name is appropriate. An example is Puccinia graminis

Stem rust

The stem, black or cereal rusts are caused by the fungus Puccinia graminis and are a significant disease affecting cereal crops. An epidemic of stem rust on wheat caused by race Ug99 is currently spreading across Africa, Asia and most recently into Middle East and is causing major concern due to...

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Forma specialis is abbreviated f. sp.,

formae speciales is abbreviated ff. spp..