Up through the 1960's GLS was confined to continuous corn on river bottoms in the eastern corn belt. The spread of GLS during the past 25 years has paralleled the use of conservation tillage practices where large amounts of crop residue remain on the soil surface. Two lesions may appear as one broader lesion if adjacent interveinal areas are infected. GLS lesions fill the area between the two leaf veins, but do not cross over. The disease cycle can begin in the spring as warmer temperatures enable spores to develop on infected crop residue.
Spores mature and are splashed or blown on to the lower leaves of the growing crop.
Leaves lose this natural resistance as they age, allowing the disease to progress. The disease may cycle and reproduce infection spores every two to three weeks. Gray Leaf Spot will not develop if one of the three ingredients of the disease triangle is not present. Disease inoculum is directly related to the amount of infected corn residue remaining on the soil surface. Pathologists attribute the recent increase of GLS pressure to the expansion of acres farmed with no-till and conservation tillage practices in the last five years.