The rain falls, generally, in heavy and sudden storms, and frequently washes away the surface soil.
It washes out easily when you do your laundry.
The river Jumna, which washes the walls of its fort, was the natural highway for the traffic of the rich delta of Bengal to the heart of India, and it formed, moreover, from very ancient times, the frontier defence of the Aryan stock settled in the plain between the Ganges and the Jumna against their western neighbours, hereditary freebooters who occupied the highlands of Central India.
He is sent to a place where he washes himself clean from impurity.
Rain then washes spores from the leaves down into the soil where they can infect the tubers.