Fusarium oxysporum f. sp. lycopersici
Fusarium wilt is distributed world wide; several biological forms (or races) of the pathogen are known. Leaves or whole branches wilt, turn yellow, then brown and die still attached to the plant (described as a yellow-flagging appearance). Sometimes half of a leaf or branch will be affected, with the other half seemingly unaffected. The fungus can be observed as brown discoloration in the vascular tissue of affected branches.
Infection enters the plant through the roots, and may be spread rapidly between plants in a field by irrigation. The best control in this case is prevention, by selecting varieties that are resistant to the disease.