Why Your Lawn Has Dead Yellow Patches (& What To Do About It)

For many homeowners, a core landscaping goal is having a pristine green lawn that's the envy of their neighbors. That's why it's so frustrating when thinning, discolored spots start showing up in your grass. It's easy to assume that sudden yellow patches are the result of not watering or mowing properly, or misunderstanding the minimum amount of times you can fertilize your lawn. But if your yard contains Kentucky bluegrass, perennial ryegrass, or tall fescue, among other species, the patches may actually be caused by melting out — a disease that affects cool-season turfgrasses.

Melting out typically occurs in cool, wet weather and spreads most easily in shaded areas. As such, it usually shows up in April or May, but some varieties appear during rainy summer months. Grass that's cut too short or receives too much nitrogen fertilizer is particularly vulnerable. While melting out can leave your lawn looking unsightly, there are ways you can combat it, including planting disease-resistant turfgrass cultivars and mowing your lawn less frequently or stopping completely while it's infected.

Melting out is a fungal disease caused by the Bipolaris sorokiniana fungus. It is very similar to another fungal disease, leaf spot, and until recently, the two were actually considered the same disease — Helminthosporium leaf spot. However, despite similar symptoms, they are caused by different fungi and thrive in different conditions. As previously mentioned, melting out typically infects grass in cool, wet weather, while leaf spot usually prefers the warmth.

The disease is spread by spores from fungi in the infected grass. The spores are typically carried throughout the lawn via the wind, but rain and water from irrigation systems can also help move them. Spores can even get stuck to your shoes, gardening equipment, or even a pet and be carried from one grassy area to another. Weather conditions can make your lawn susceptible to fungus, and this is when the disease can take hold.

The first signs of melting out on your grass are small purple or black spots on the blades. As the disease gets worse, the spots grow larger and develop a tan center. The disease eventually overtakes the entire leaf and moves down to the crowns and roots of the grass. When it has progressed to this point, the blades become weak and the leaves around the spots turn yellow. If the weather is hot and windy, the grass can die, leaving your lawn with thin, uneven patches.

The most effective option for long-term control of melting out at bay is planting a grass species that is resistant to the fungus. Some improved Kentucky bluegrass cultivars are resistant to melting out, including 'Adelphi', 'Bristol', 'Destiny', and 'Eclipse'. You can also overseed your lawn in affected areas with a resistant species or cultivar to help encourage better growth going forward.

If the disease is currently affecting your grass, avoid mowing it. The lawn mower can carry the spores across your lawn and spread the fungus further. When you do mow, make sure your blade is sharp to ensure clean cuts. Grass leaves with jagged edges are more vulnerable to melting out and other, similar diseases. It's also important that you don't cut healthy grass too short because doing so leaves it susceptible to infection. Cut grass to between 2½ and 4 inches high and never cut more than ⅓ of the blade at a time.

In early spring, avoid applying nitrogen-heavy fertilizer to your lawn help prevent melting out. A good rule of thumb is using ½ to 1 pound of fertilizer per 1000 square feet of lawn. You'll also want to keep the grass well-hydrated. Water your lawn daily, aiming for 0.1 to 0.2 inches of water at a time. But stick to watering only in the middle of the day. Irrigating in evening or at night leaves your lawn vulnerable to fungal diseases — the water doesn't evaporate, and fungi thrive in moist conditions.