Turf Disease or Herbicide Injury?
A golf course superintendent shows you a 2,000 sq ft putting green with irregular yellow-to-brown patches that appeared over five days. The green was treated eight days ago with a pre-emergence herbicide and a foliar fertilizer. Irrigation runs at night, and the green has a thatch layer about 0.75 inch thick. The superintendent wants to apply a fungicide immediately and also asks whether to increase the herbicide rate next time for better control. You must diagnose before recommending.
1. How do you distinguish disease from herbicide injury before any treatment?
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Diagnostic steps:
- Pattern - disease patches often have defined margins and may follow drainage/mowing patterns; herbicide injury often follows spray swath overlaps or spreader patterns and can appear on a gradient.
- Timing - symptoms eight days after a pre-emergence application fit injury; disease timing depends on weather and pathogen.
- Plant inspection - look at roots, crowns, and leaves. Herbicide injury often shows distorted growth, bleaching, or root stunting; disease shows lesions, mycelium, or rotting crowns.
- Distribution - injury may cross species lines and appear on adjacent non-target plants; disease is often host-specific.
- Thatch and moisture - 0.75 inch thatch plus night irrigation favors disease (e.g., fungal activity) and also affects rooting.
- Send a sample to a diagnostic lab if uncertain.
Conclusion logic: do not spray a fungicide until the cause is identified; a fungicide will not fix herbicide injury and may mask nothing while costing money.
2. What would make you suspect herbicide injury, and what would you check in the spray records?
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Suspect injury if:
- Symptoms appeared 8 days after the pre-emergence application and follow a spray pattern.
- Distortion, bleaching, or stunting rather than classic disease lesions.
- Adjacent areas or non-target plants show similar symptoms.
Check in records:
- Product, EPA reg. number, rate, and carrier volume - was the rate correct for the green area?
- Calibration and overlap - was the sprayer calibrated; did overlaps double the rate?
- Tank mix partners and order - any antagonism or incompatible mix?
- Weather at application - temperature and moisture can increase injury.
- Fertilizer rate and timing - foliar fertilizer burn is also possible.
Judgment point: the superintendent's request to increase the herbicide rate is the wrong instinct if injury is suspected - it would worsen the problem and may violate the label.
3. What do you recommend short-term and long-term, and how do you respond to the rate-increase request?
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Short-term:
- Do not apply fungicide blindly; confirm diagnosis first (lab sample if needed).
- If injury: leach/irrigate lightly, avoid further stress, and do not reapply the herbicide.
- If disease: select a labeled fungicide matched to the pathogen, with correct rate and interval.
- Address thatch (0.75 in) and night irrigation - reduce thatch and adjust irrigation timing to reduce disease pressure.
Long-term:
- Recalibrate the sprayer, verify rate for the green's square footage, and train on overlap avoidance.
- Use labeled rates only; if control is poor, investigate cause (resistance, timing, coverage) rather than raising rate.
Response to rate increase: No. Increasing the rate above the label is illegal and increases injury risk. The correct fix is diagnosis, calibration, and application timing.
Exam point: the label rate is a ceiling, not a starting point; diagnosis precedes treatment.