Mysterious Annual Yellow Circle in Zoysiagrass: Investigation Guide

A precise 6-7" yellow circle appearing same spot, same dates (mid-May to August) for 5 years in zoysiagrass, with no chemical use, defies typical disease patterns. This precision suggests a subsurface cause.

Systematic Investigation

1. Excavate the circle (12" diameter × 18" deep) in early May before appearance
2. Look for:
   • Buried construction debris (concrete, pipe, wire)
   • Tree root / stump remnant
   • Utility line (gas, electric, irrigation)
   • Septic component / dry well
   • Soil texture change (sand lens, clay ball)
   • Chemical residue pocket (old spill)
3. Test soil from circle vs. 3 ft away: pH, EC, nutrients, compaction
4. Check irrigation — head-to-head coverage? Pressure issue at that radius?
5. Gas leak test — utility company can check (free)

Zoysia-Specific Considerations

• Zoysia greens up late (May) — circle may be delayed green-up zone
• Shallow root zone over obstruction = early dormancy
• Large patch fungus (Rhizoctonia) typically irregular, not perfect circles

If Excavation Reveals Nothing

• Mark circle; apply horticultural corn meal (10 lbs/1,000 sq ft) in April
• Core aerate circle + 2 ft radius
• Topdress compost; overseed with zoysia sprigs
• Document with dated photos annually

Perfect geometric recurrence = physical cause. Dig. The answer is underground.