Soybean rust is caused by the fungus Phakopsora pachyrhizi. Symptoms are most common after flowering, beginning on lower leaves. Lesions start to form on lower leaf surfaces as small, gray spots that change to tan or reddish-brown. Lesions are scattered within yellow areas appearing translucent if held up to the sun. Mature lesions contain one to many small pustules, usually on lower leaf surfaces. The rust pathogen survives in southern states. Movement northward depends on dispersal of spores and disease establishment in new areas, a process that needs to be repeated several times in a growing season for rust to cause an epidemic in northern states. Cool, wet weather or high humidity favor epidemics. Dense canopies provide ideal conditions for disease development.