Mystery rust (?) on Quercus ilex

Mystery rust (?) on Quercus ilex

Observation - Mystery rust (?) on Quercus ilex. Description: A rust (?) like patches of brown felt on underside of evergreen oak, Quercus ilex - can anyone put a name to this please?

A rust (?) like patches of brown felt on underside of evergreen oak, Quercus ilex - can anyone put a name to this please?