Pseudomicrostroma juglandis

Large white patches of small spherical acervuli form on the undersides of living leaves of Walnut, emerging from the stomata.   There is a corresponding yellow patch on the leaf upperside, eventuialy causing the leaf tissue to die.  This species is not listed as a gall-causer by the British Plant Gall Society.   

yellow patches on Walnut leaves can be caused by other speces, including the galls Aceria erinea and A brachytarsus

Photograph upper and lower sides of leaf, and the patches of acervuli in detail

Walnut trees

July

Plant Parasites of Europe web-page

Taxonomy and taxonomic relations are difficult.  It is not closely related to rusts and smuts, but is currently (2026) 'Incertae sedis'.  It is listed in the subclass Exobasidiomycetidae and order Microstromatales, but has not been assigned to a family in the UK Species Inventory.  Some sources assign it to a family Microstromataceae so we have placed it here until there is more clarity about its proper place in the taxonomomic hierarchy.