A tobacco farmer is walking through her fields. On a tobacco plant leaf, she notices several small brown spots surrounded by what appears to be necrotic or dying tissue. What likely caused these spots and necrotic patches?

This is a fungal disease which is caused by Alternaria alternata. This is responsible for causing brown spot disease.

Explanation:

This is a most economically devastating disease which is caused by a pathogen called Alternaria alternata that mostly affects the mature leaves.

They survive in the debris and disperse through asexual spores. Their germination requires moisture and they attack the host plant through colonization and later propagate to another tissues.

They create irregular shaped dark brown spots which have yellow halos.

The pathogen tear away the narcotic tissues.

The spots get spread and creates curling leafs and moldy growth.