Cooperative Extension: Insect Pests, Ticks and Plant Diseases

The page provides some background to help understand how the NoBlight model uses site-specific weather data to estimate the optimum fungicide application frequency to prevent potato late blight infections.

The recommended maximum safe fungicide spray intervals to prevent potato late blight for today and later dates are based on forecast temperature, relative humidity, and rain values for the specified location.  As observation and forecast values change, the recommended intervals and NEXT spray dates can also change.

Dates subject to change from forecast shifts are shown in red font.

Today’s date in the Spray Date or NEXT Spray Date columns is highlighted by bright yellow cell color.  The recommended date for the first late blight fungicide application is in green font.

The date in the first row is set by the potato emergence date provided for this site.

Fungicide protection against late blight infection should begin when there have been 18 cumulative DSV points after emergence.  The first spray date may occur before 18 DSV have accumulated if there is an extended wetting period right after emergence.

Protection end date estimates assume that a fungicide effective against potato late blight was applied at full dose and with good coverage before any rain observed or forecast for that date.  Protection status does not account for late blight spore load at this location or for potato variety late blight susceptibility.

Exceeding the maximum number of days for a respray interval causes the model to estimate that there was a lack of continuous protection with increased potential for late blight infection.  The actual infection risk on unprotected dates depends on inoculum load and weather conditions.  If active LATE Blight infections are in the local area producing spores, symptoms of infection on unprotected plants can appear when there have been 70 DSV points after emergence.

Respray interval thresholds:

If the 7-day total precipitation is < 1.2 inch, then if the 7-day DSV total is:
>=6, the maximum spray interval is 5 days.
>=5 and <6, the max. spray interval is 7 days.
>4 and <5, the max. spray interval is 10 days.
<=4 and 7-day rain total is > 0, the max. spray interval is 12 days.
<=4 and 7-day rain total is 0, the max. spray interval is 14 days.

If the 7-day total precipitation is >= 1.2 inch, then if the 7-day DSV total is:
>=5, the max. spray interval is 5 days.
>=4 and <5, the max. spray interval is 7 days.
>3 and <4, the max. spray interval is 10 days.
<=3, the max. spray interval is 12 days.

Formatting note for the Daily ‘Disease Severity (DSV)’ column:  Days with gray shading and no value shown were part of a multi-day wetting period.  DSV points for a multi-day wetting period are assigned to the end of the period if there is no rain during the period (if all the wet hours are due to high relative humidity).  If there is rain during a multi-day wetting period, DSV are assigned to the first date within the period that had rain.  Some gray-shaded multi-day wetting periods are too short or too cool to generate DSV points.

EARLY Blight is a different disease, spray regime, and model.  Early Blight control should begin at 300 cumulative P-Days (shown in the rightmost column).