Yellow Centipedegrass with High Soil pH and Root Rot
In April, a 6,000 sq ft centipedegrass lawn in Marion County is thin and yellow-green despite 1 inch of irrigation per week. Patches are irregular, not distinct rings; stolons are sparse, roots are short, dark, and rotted, and no leaf spots are visible. Soil test: pH 7.4, Mehlich-3 P 45 ppm, K 60 ppm, Ca 1,200 ppm, Mg 150 ppm, Mn 3 ppm, Fe 20 ppm. The homeowner limed last October at 50 lb/1,000 sq ft and fertilizes with 16-4-8 in spring and fall. Irrigation is from a well high in bicarbonate. A neighboring lawn with similar symptoms had large patch last year, so the homeowner wants azoxystrobin immediately. Thatch is 0.5 inch; mowing height 1.5 inches; no chinch bug activity found in five samples. The lawn is in partial shade from live oaks, and drainage is slow after heavy rain. The homeowner asks whether the problem is disease, nutrition, or both, and whether lime should be added again.
1. What is your leading diagnosis, and what diagnostic steps would you take to separate disease from nutritional and abiotic causes?
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The leading diagnosis is take-all root rot, caused by Gaeumannomyces graminis var. graminis, complicated by manganese and possibly iron deficiency induced by high pH. The key clues are short, dark, rotted roots; sparse stolons; irregular yellow-green patches; slow drainage; and a soil pH of 7.4 after a recent lime application. The absence of leaf spots and the lack of distinct rings make large patch less likely, although large patch can also occur in spring and the neighbor's history is relevant.
Diagnostic steps:
- Dig and wash roots from the margin of a declining area. Look for dark, rotted roots and stolons. Under magnification, take-all root rot shows dark, ectotrophic hyphae on roots; this is not a field diagnosis.
- Check stolon bases and leaf sheaths for rot typical of large patch. Large patch usually produces more circular patches and rot at the base, while take-all root rot is primarily a root and stolon disease.
- Repeat the soil test and include pH, lime history, and manganese. A Mehlich-3 Mn of 3 ppm is low, and availability drops sharply at pH 7.4. Iron may also be less available, even at 20 ppm.
- Test irrigation water for bicarbonate and salts, and check drainage. Bicarbonate and poor drainage stress centipedegrass and favor root rot.
- Submit a nematode assay if symptoms persist, because sting nematodes can cause similar thinning on sandy Florida soils.
- Rule out insects and shade effects. Five negative chinch bug samples help, but sample again at the margins. Live oak shade and poor airflow slow drying and increase disease pressure.
The pattern is probably both disease and nutrition: high pH and poor drainage favor take-all root rot, and the same high pH reduces manganese availability, producing the yellow-green color.
2. The homeowner wants to apply azoxystrobin immediately and add more lime. How would you respond, and what corrective program would you recommend for the next 6–12 months?
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I would advise against adding lime and against relying on azoxystrobin as a rescue treatment.
Do not add lime. At pH 7.4, centipedegrass is already above its preferred range, roughly pH 5.5–6.5. More lime would further reduce manganese and iron availability and worsen take-all root rot pressure. The October lime application was likely a major contributing factor.
Fungicides are not a cure. Azoxystrobin and related strobilurins may suppress some root-rot fungi, but they will not repair rotted roots or correct high pH and poor drainage. If a fungicide is used, it must be labeled for the site and disease, applied at the correct timing, and combined with cultural correction. It should not be applied as a one-time rescue without diagnosing the cause.
A 6–12 month program:
- Stop liming. Re-test pH in the fall. If pH remains above 6.5, consider acidifying nitrogen, such as ammonium sulfate, at modest rates based on soil test, rather than repeated elemental sulfur, which is slow and easy to overapply on sandy soil.
- Correct manganese and iron. Use a labeled foliar manganese sulfate or chelated micronutrient program, and possibly iron sulfate, based on tissue or soil test. Foliar feeding gives temporary greening while the pH is corrected.
- Reduce stress. Improve surface drainage, aerate if appropriate, reduce thatch, and irrigate deeply but less frequently. Avoid evening irrigation that keeps foliage and roots wet.
- Fertilize moderately. Use small, balanced applications rather than heavy spring and fall doses. Avoid excessive nitrogen, which pushes weak growth.
- Reassess in fall. Re-test soil, roots, and nematodes. Overseed or replug only after the underlying pH and drainage problems are corrected.
I would tell the homeowner that the neighbor's large patch history is a reason to check, not a reason to treat blindly.