Is it safe to dry a wet leather bag with a hair dryer?

No. Heat dries leather too fast and it stiffens or cracks. Let it dry in the shade, stuffed with tissue.

Should I condition leather before the monsoon?

A light leather conditioner before and after the season helps. Avoid household oils and creams, which darken leather and can turn rancid.

Surface mould can usually be cleaned and treated. Where it has marked the colour, the piece may need recolouring as well.

Leather does not mind rain nearly as much as it minds staying damp. Almost everything below is about letting it dry, and letting it breathe.

Wipe it the day it gets wet, with a soft dry cloth, and let it dry in the shade
Cotton dust bag, never plastic. Plastic holds damp against the leather
Silica gel inside, changed when it stops feeling dry
Open the cupboard. A closed cupboard in July is where mould starts
Carry it. The bag that is used gets aired; the one that waits for an occasion does not

Do not wear the same pair two days running in the monsoon — a shoe needs a day to dry out
Stuff wet shoes with plain tissue and change it once
Dry in moving air, not in the sun and not near heat

Wipe with a dry cloth weekly through the monsoon
Let air move — a car parked closed for weeks grows mould on its seats
Keep them out of direct sun, which dries leather into cracks once the rains end

When home care is no longer enough

White or green spots that return after you wipe them — read mould on your leather bag
A finish that has turned sticky
A smell that stays after airing
Colour that has gone patchy

The first three are dry cleaning, with antifungal treatment where mould has set in. Patchy colour is recolouring.