Disease Risk

Sprigg continuously monitors environmental conditions to assess the risk of common lawn diseases. Disease risk scoring directly influences irrigation decisions — when risk is elevated, Sprigg reduces watering to avoid creating conditions that promote fungal growth.

How Disease Scoring Works

Disease risk is calculated using a combination of temperature, humidity, and leaf wetness duration. Each disease has a specific environmental profile that describes the conditions under which it thrives.

Scoring Factors

• Temperature — Most fungal diseases have an active temperature range. For example, brown patch thrives when nighttime temperatures stay between 60-85°F.
• Humidity — High relative humidity (>80%) creates favorable conditions for spore germination and spread.
• Leaf wetness duration — Extended periods of wet foliage (from dew, irrigation, or rain) are a key trigger. Sprigg estimates this from forecast data.

Temperature-Gated Scoring

Sprigg uses an important refinement: humidity is only counted as a risk factor when temperatures are within the active range for a given disease. High humidity at 40°F doesn't increase brown patch risk because the pathogen isn't active at that temperature. This prevents false positives during cool, humid nights.

Score Calculation

Each disease receives a score from 0-100 based on how closely current conditions match its ideal growth conditions. The overall disease risk for your lawn is determined by the highest individual disease score, since you only need one active disease to have a problem.

Risk Levels

Common Diseases by Grass Type

Cool-Season Grasses (Fescue, Kentucky Bluegrass, Ryegrass)

• Brown Patch — Warm, humid nights (65-85°F). Circular patches of wilted, brown grass.
• Dollar Spot — Moderate temps (60-85°F), high humidity, nitrogen-deficient lawns. Small, silver-dollar-sized spots.
• Pythium Blight — Hot, humid conditions (>86°F). Greasy, water-soaked patches that spread rapidly.
• Snow Mold — Cold, wet conditions. Pink or gray patches after snow melts.
• Red Thread — Cool, humid weather (60-75°F). Red or pink threads on grass blades.

Warm-Season Grasses (Bermuda, Zoysia, St. Augustine)

• Large Patch — Cool, wet transitions (soil temps 50-70°F). Similar to brown patch but in warm-season turf.
• Gray Leaf Spot — Hot, humid conditions (85-90°F). Common in St. Augustine. Gray lesions on blades.
• Take-All Root Rot — Cool, wet soils (50-70°F). Yellowing and thinning patches.
• Spring Dead Spot — Appears in spring after dormancy. Circular dead areas in Bermuda.

How Disease Risk Affects Irrigation

When disease risk is elevated, the AI advisor adjusts its recommendations:

• Moderate risk — Slightly reduces watering amounts and favors early morning watering to minimize leaf wetness duration.
• High risk — Significantly reduces watering. May skip non-essential zones. Prioritizes deep, infrequent watering over light, frequent sessions.
• Critical risk — Minimizes irrigation to only what's necessary to prevent turf death. Strongly recommends fungicide if not recently applied.

If you've logged a fungicide application in your care plan, the AI factors that into its disease management strategy, potentially allowing slightly more normal irrigation.

Adjusting Disease Thresholds

You can customize the score thresholds for each risk level in Settings → Property under disease thresholds. This lets you tune sensitivity based on your lawn's history:

• If your lawn is disease-prone, lower the thresholds to trigger warnings sooner
• If you rarely have disease issues, raise them to avoid unnecessary watering reductions

The default thresholds work well for most lawns and are a good starting point.

14-Day Weather History

Disease risk isn't just about today's weather. Sprigg considers the past 14 days of temperature and humidity patterns to detect sustained conditions that favor disease development. A single hot, humid night is different from a week of them — the latter is far more concerning for disease pressure.