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Disease Forecasting Calendar

Plant diseases need three things at the same time: a susceptible plant, a pathogen and the right weather. Forecasting watches all three.

What is plant disease forecasting?

Forecasting uses weather data, crop stage and knowledge of how a pathogen behaves to estimate when infection is likely. The idea is old: plant pathologists described the disease triangle of host, pathogen and environment long ago. A forecast tells you when conditions favor infection, so you can scout harder, adjust irrigation or plan protective steps before symptoms show.

It does not replace scouting. A forecast gives the risk, and a field check shows whether disease is actually present.

For example, a grower with potatoes might receive a late blight advisory for the area and respond by checking lower leaves that same week, without waiting for the scheduled walk.

Which weather factors matter most?

Leaf wetness duration, temperature, relative humidity and rainfall are the key inputs for many foliar diseases. Fungi and bacteria often need water on the leaf surface for a minimum number of hours at a given temperature range to infect. Soilborne diseases respond to soil moisture and temperature.

The National Weather Service and state weather networks provide forecasts and observations, and many universities provide station data you can use for your own records.

• Hours of leaf wetness
• Temperature during wet periods
• Rainfall and splash events
• Humidity and air movement

Where do you find disease forecasts?

Many land-grant universities run disease advisory programs for specific crops. Examples are tools for potato late blight, apple scab, grape diseases and vegetable diseases in some states. Availability varies, so ask your county extension office which models exist for your crop and region.

Models differ in the inputs they need, the weather stations they use and the thresholds they apply. Use the model that has been calibrated for your region and crop, and read the instructions for how to interpret the output.

Forecast tools often include maps, station lists and email or text alerts, so sign up for any alert service your extension program offers for your crop.

How do you build your own risk log?

If no model exists for your crop, a simple log still helps. Record daily rainfall, high and low temperatures, hours of morning dew and the dates symptoms first appeared. Over a few seasons you will see which weather patterns preceded disease on your farm.

A leaf wetness sensor is helpful but not required. Observing dew and rain and logging the hours is a starting point.

Keep your log in a notebook or spreadsheet, with one row per day during the main growing season, and add a column for notes such as irrigation or dew.

How do crop stage and field history change the risk?

Some crop stages are more susceptible than others. Young leaves, flowering and fruit set may each be the most vulnerable time for a given disease. Field history matters too, since many pathogens survive in residue or soil from prior crops, and a field with a heavy disease year behind it starts with more inoculum.

Rotation, residue management, resistant varieties and spacing for air flow lower the baseline risk before any forecast is even read.

A planting made in a low spot or in the shade of a windbreak may stay wet longer than the rest of the field, so log risk by location, not only by farm.

How should you act on a forecast?

When a forecast shows high risk, increase scouting, check lower and inner canopy where humidity is higher and review what protective options are available in your state. Adjust irrigation timing so that foliage dries earlier in the day, where that is practical.

Any fungicide or bactericide use must follow the label, which is the law. Pre-harvest intervals apply, resistance management guidance should be followed and applicator licensing varies by state. Your extension office and a certified crop adviser can help interpret forecasts for your crop.

What are the limits of disease forecasts?

Forecasts are models, and models are simplified. Weather stations may sit far from your field, and the microclimate in a dense canopy can differ from the air above it. A model built for one crop and region may not fit another. Treat forecast output as a prompt for field checks, not as a verdict.

Compare the model's output with what you see in the field. If your fields consistently show disease earlier or later than the model suggests, tell your extension office, since they refine advice from grower feedback.

Finally, remember that forecasts typically address a single disease, and other problems can still occur while risk for that one is low.

Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.

More in Pest, Disease & Weed Timing Calendars

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• Bacterial Disease Timing Calendar
• Viral Disease Vector Calendar
• Root Rot Timing Calendar
• Blight Forecasting Calendar
• Mildew Timing Calendar