Net blotch of peanutes

Didymosphaeria arachidicola

Generally minor but can flare up in some years. Causes rapid defoliation (and subsequent yield loss) during cool, showery weather.
A network of very fine brown lines develop on the top surface of the leaf. These join together to form brownish blotches, which may go through the leaf.
The fungus survives on peanut residues from the previous season.