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What Are the Causes of Light Green Spots on a Lawn?

Homeowners often find patches of light green and yellow on their lawns. Areas of discoloration are often due to pests or turf diseases, which can ruin the appearance of your lawn. Diagnosing lawn diseases is best accomplished by taking note of the time of year the disease develops, searching for lesions on grass blades and determining if a recent fertilization has contributed to the disease.

Necrotic Ring Spot

Necrotic ring spot is a disease caused by the fungus Ophiosphaerella korrae, which is found in the soil. Surviving each year on dead grass roots, this disease spreads when soil temperatures range between 65 and 80 degrees F. According to Colorado State University Extension, necrotic ring spot affects Kentucky bluegrass, annual bluegrass and red fescue. This disease is capable of causing extensive damage to grass and recovery from this disease is a slow process. The symptoms associated with necrotic ring spot include light green or tan colored rings on grass, dead grass at the margins of the rings, root discoloration and crown rot. Light patches in grass are often referred to as frog eye spots, which are used to diagnose this disease when present with root discoloration. Prevent necrotic ring spot in your grass by choosing a resistant grass variety, avoid overwatering grass and keeping grass at a height of 2 1/2 to three inches. Avoid applying fertilizers with a high nitrogen content, as they may exacerbate this disease.

Rhizoctonia Yellow Patch

Rhizoctonia yellow patch is a disease characterized by patches of light green and yellow on grass. North Dakota State University states that Rhizoctonia yellow patch is a serious and destructive disease of Kentucky bluegrass. New sod is especially susceptible to this disease. The symptoms commonly associated with Rhizoctonia yellow patch include patches of light green and yellow that measure two to three inches, turn brown and increase in size. Frog eye patches are common with this disease as well. Light green patches of grass often have a sunken or depressed appearance and leaf blades may take on a red or purple tint. Black strands of fungus are not present on grass roots with Rhizoctonia yellow patch, as they are with necrotic ring spot. Applying fungicides do not reduce the effects of this disease, but reducing wet conditions in grass is often helpful.

Rough Bluegrass

Rough bluegrass is a weedy grass condition that occurs in some grass. Rough bluegrass causes light green grass spots that turn brown when temperatures exceed 80 degrees F. According to Ohio State University, rough bluegrass is commonly confused with annual bluegrass. Rainy, wet weather and shady areas favor the development of rough bluegrass, which is usually not tolerant to drought. Applying glyphosate to the grass followed by reseeding is the best means of control for this condition.

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