Yes, fungal diseases can be another cause of a lawn blighted by unsightly discolored patches. Often occurring in cooler season grasses, such as fescue, fungal infections can develop during warm, damp conditions and can spread pretty fast if not checked and treated.
Newly laid sod lawns are also susceptible to these infections as the stress of the harvesting process and relaying the turf, can result in fungal spores being disturbed and re-activated.
Dealing with any fungal grass infection is best left to the lawncare professionals. They will be able to identify the specific cause and administer a professional fungicide.