Necrotic Ring Spot Fungicides

Necrotic ring spot is a root- and crown-infecting disease of Kentucky bluegrass and several other cool-season grasses. Injury often recurs in the same areas for years.

What to Look For

Rings or arcs of yellow, bronze, or dead turf may contain healthy grass in the center, creating a frog-eye pattern. Roots and crowns are dark and weak, and plants pull easily from affected areas.

Why It Develops

Cool-season root infection is followed by visible decline during heat or drought. Compaction, excessive thatch, high spring nitrogen, low mowing, and drought stress increase damage.

Reduce Disease Pressure

• Aerify and topdress to reduce thatch and compaction.
• Avoid excessive spring and summer nitrogen.
• Maintain at least about a 2-inch mowing height where the turf use permits.
• Prevent severe drought; light frequent irrigation can temporarily protect shallow roots during an outbreak.

What University Research and Extension Guidance Says

Kentucky recommends preventive spring timing when the pathogen is active, often in April or May, rather than waiting for summer rings. Fungicides should be moved into the crown and root zone as directed. Recovery still depends heavily on reducing root stress and encouraging new growth.

Choosing and Using a Fungicide

Use the products below as a starting point, then match the active ingredient, FRAC group, turf species, use site, timing, rate, interval, and watering directions to the current product label. Preventive or very early applications are usually more reliable than rescue treatments after extensive injury. Rotate effective FRAC groups and combine fungicides with the cultural changes above.

Research and label note: University research may identify useful activity even when a current product label does not name this disease. That is research evidence, not a label direction. Apply a product only where the turf or application site and use pattern are permitted, and follow the complete label and applicable state requirements.

University sources: University of Kentucky: Chemical Control of Turfgrass Diseases 2024.