Fire Blight Prediction Models: Using Bloom and Weather Risk

Fire blight prediction models estimate the chance of blossom infection on susceptible hosts. They combine orchard observations with weather data so growers can recognize high-risk bloom periods. They do not diagnose symptoms, guarantee infection or replace a pesticide label.

What a model evaluates

• Host stage: susceptible blossoms must be open for blossom-infection forecasts to apply.
• Temperature: warmth influences multiplication of Erwinia amylovora on flower surfaces.
• Wetting: rain, dew or other wetting can move bacteria into floral tissues.
• Recent weather and local history: some tools incorporate accumulated heat, prior risk and orchard disease pressure.

CougarBlight and Maryblyt

CougarBlight and Maryblyt are established decision-support systems, but their inputs, versions and regional calibration can change. Use the model currently recommended by your local Extension or university tree-fruit program. Enter observations from your own planting and use the nearest representative weather station; a distant station may not reflect conditions in your orchard.

A practical workflow

1. Confirm that the tree is a susceptible host and record bloom stage.
2. Choose a locally supported model and reliable weather source.
3. Review forecast risk before a predicted infection period, not after symptoms appear.
4. Combine the model output with orchard history, cultivar susceptibility and current local advisories.
5. If a preventive material is being considered, verify that its current label lists the crop and fire blight, then follow all timing, rate, PPE, re-entry and harvest directions.
6. Continue scouting. A forecast is not proof that infection occurred.

What the risk level does not tell you

A high-risk reading is not a universal instruction to spray, and a low-risk reading does not rule out wound infections after hail or other injury. Models aimed at blossom blight may not address every shoot, rootstock or trauma-blight scenario.

After symptoms appear

Use the model to understand possible infection timing, then diagnose the damage and follow the management steps in our complete fire blight guide. Pear growers can use the pear-specific symptom guide.

Always use the current regional model documentation. Pesticide labels and local regulations control whether and how a product may be used.