Grub damage and fungal disease result in similar symptoms in a lawn. Patches of thinning, yellowing grass with poor growth eventually gives way to dead patches throughout the landscape. A close inspection of the turf grass, along with a consideration of the time of year and recent weather conditions, clearly points you in the right direction in most circumstances. Differentiating between grub damage and a fungus is important because treatment is entirely different.
Pull on the turf grass to see if it remains attached to its roots or easily comes apart. This is a sure sign of a severe grub infestation, according to University of Minnesota Extension.
Search for the grubs themselves in late winter or early spring to ensure a proper diagnosis. Do not look only in the patches of dead grass, because grubs have moved on from these areas to others, where it appears that healthy grass still grows. Instead, look at several soil samples taken throughout the yard. Dig these up with a shovel in square foot pieces and look through them for white grubs. It is normal to find some grubs in your turf grass. Treat for grubs if you find an average of eight or more grubs in each piece of sod.
Examine the pattern of damage across the lawn. While grubs leave patches of dead grass in their wake, they do so in an irregular pattern. Many fungi, on the other hand, create concentric patches of dead grass. Fungal diseases also cause rings of disease, with healthy grass growing both inside and outside the affected area. As the fungal disease progresses, circles enlarge until they hit one another, at which time they stop growing in that direction. It is easiest to see the difference between grub and fungal damage early in the disease, before a fungus overtakes most of a lawn, losing some of its orderly appearance.
Note the time of year at which the problem first begins. Grub damage appears in late summer or early fall. Spring damage is likely a fungal disease. Weather also plays a part in diagnosis, with fungus most likely in wet or humid weather.