Lawn & Turf Problem

Summer Patch

Magnaporthe poae — root-infecting fungus

Summer Patch is a serious, root-destroying turf disease in Colorado, primarily affecting Kentucky bluegrass and occasionally annual bluegrass. It thrives in hot soil temperatures, poor drainage, and compacted or overwatered lawns. Damage is often mistaken for drought stress, but watering does not revive affected patches.

Seasonal Disease Activity Calendar

Problem	Type Active Season
Summer Patch	Fungus Spring infection; symptoms peak Summer

Symptoms (Homeowner-Friendly)

Symptom	Description
Frog-eye or ring patterns	Patches of dead grass with green centers, similar to NRS but caused by a different fungus
Irregular yellow or brown patches	6"–3 ft wide, expanding during heat
Grass wilts and dies despite watering	Roots are damaged, so water cannot help
Root rot	Roots appear dark, rotted, brittle, with poor depth
Symptoms peak in hot weather	Especially July–September

Primary Causes (Colorado Conditions)

Compacted clay soils
Overwatering or daily irrigation
Excessive thatch
High soil temperatures (>70°F at root zone)

Poor mowing height (cutting too short)
Shallow root systems

Treatment & Management

Summer Patch is difficult because the pathogen infects roots in spring, but symptoms appear weeks later. Management focuses on soil improvement + correct irrigation.

1. Cultural Practices (Most Critical)

Action	Why it Helps
Raise mowing height to 3–3.5"	Encourages deeper rooting
Deep, infrequent watering	Prevents shallow, vulnerable roots
Core aeration 1–2× per year	Reduces compaction and improves drainage
Reduce thatch (<½”)	Removes fungal habitat
Topdress with compost	Boosts beneficial microbes
Avoid excessive spring nitrogen	Produces weak, disease-prone roots

2. Overseeding (Recovery)

Use turf-type tall fescue or disease-tolerant KBG varieties.

Overseed thin areas in late summer or early fall.
Consider replacing chronic Summer Patch areas with tall fescue entirely.

3. Fungicide Options (Timing Is Everything)

Fungicides must be applied preventively in spring, BEFORE symptoms appear. They do not cure active Summer Patch.

Fungicide Class	Timing	Notes
QoI fungicides (azoxystrobin)	Late May–June	Most effective option
DMI fungicides (propiconazole, myclobutanil)	Late May–June	Often tank-mixed
SDHI fungicides	Late May–June	Professional-only options for severe cases

Apply when soil temps reach 65–70°F at 2–3" depth.