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Pest, Disease & Weed Timing CalendarsBlight Forecasting Calendar
Blights can move quickly in the right weather. A forecasting calendar combines weather risk, crop stage and regular scouting.
Key points
- Blight is a symptom word covering several diseases with different weather needs.
- Use state advisories and the NWS forecast, and increase scouting in wet, cool or humid periods.
- Start with certified seed, residue cleanup, rotation and resistant varieties.
- Report confirmed late blight to extension and follow the label for any product.
What does blight mean?
Blight is a general term for diseases that cause rapid browning and death of leaves, stems or other tissue. Different blights have different causes. Late blight of potato and tomato is caused by a water mold, early blight by a fungus, and fire blight of apples and pears by a bacterium. Each has its own weather and timing, so identify the disease first.
Forecasting tools exist for some blights, and they use weather data to estimate when infection conditions occur.
The word is also used loosely, so a neighbor's report of blight may refer to a very different disease from the one on your plants. Confirmation by a lab is worth the small effort.
Which weather conditions drive blight risk?
Late blight is generally favored by cool, moist weather with long periods of leaf wetness, and it can spread over long distances by spores carried on wind. Early blight tends to appear in warmer weather, often on older leaves and stressed plants. Fire blight infection is associated with warm, humid conditions during bloom.
Check your extension service for the exact conditions and the thresholds for each disease in your area.
Wind can carry late blight spores a long way, so a case in a region near you can raise your own risk even if your local weather has been dry.
Where do you find blight forecasts?
Some university programs publish risk maps or advisory messages during the growing season, especially for potato and tomato late blight and for fire blight in orchards. Availability varies by state and year, so ask your county extension office which forecast or alert program covers your crop.
Use the National Weather Service for local forecasts and observations of rain, temperature and humidity, which help you interpret any advisory.
- State extension disease advisories
- Regional late blight alerts when available
- NWS forecasts for wet periods
- Your own on-farm weather record
How should you scout for blights?
Start scouting when plants are growing and increase frequency when advisories or weather indicate risk. For late blight, look for water-soaked lesions on leaves and stems that may show white growth on the underside in humid conditions. For early blight, look for dark spots with concentric rings on lower leaves. For fire blight, look for shoots that wilt and bend into a shepherd's crook.
Send suspect samples to your state plant diagnostic lab. For late blight, reporting a confirmed case often helps neighbors and extension programs track spread.
Check lower leaves and the inside of the canopy first for early blight, and check the whole plant for late blight, since it can start anywhere and spread fast.
What cultural steps reduce the risk?
Plant certified seed potatoes and healthy transplants, destroy cull piles and volunteer plants, rotate crops, space plants for air flow and irrigate in ways that let foliage dry. Choose resistant varieties where available, since resistance levels differ among varieties. In orchards, manage vigor and prune according to extension recommendations.
Good fertility balance matters too, since excess growth can increase susceptibility to some blights.
Dispose of cull potatoes and volunteers promptly, since a single infected tuber can start an outbreak in a field.
What about protective products?
Protective products are labeled for certain blights and work best before infection, which is why forecasts and timing matter. The label is the law, and it sets crops, intervals, pre-harvest limits and restrictions to protect pollinators and the environment. Resistance management guidance appears in extension publications, and applicator licensing varies by state.
A certified crop adviser or your county extension office can help interpret advisories and current recommendations.
Applying a protective product after symptoms are widespread generally gives poor results, so timing relative to the forecast matters.
How should you record and review?
Keep a log of rain, temperature, wet periods, advisory dates, first symptom dates and the varieties affected. Photograph symptoms with dates. For example, if first symptoms show about a week after a long wet spell in your log, you have a local lag time that helps with planning.
At the end of the season, review what worked and note changes for next year. Share confirmed findings with your extension office, since reports from growers help them warn others.
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.