Best weather to spray potatoes

Potatoes are often on a regular fungicide programme through the season, especially against late blight, which thrives in warm, humid weather. That creates a tension: the conditions that increase disease pressure — humidity, leaf wetness and rain — are the same ones that make a good spraying window hard to find.

The weather window

• Delta T: 2–8 °C. See Delta T.
• Humidity: high humidity is common in blight weather; the concern is wet foliage and calm, near-saturated air. See humidity and dew point.
• Wind: a light, steady breeze within the label's range. See wind speed for spraying.
• Inversion: calm, humid mornings and evenings are prone to inversions. See thermal inversion.
• Rain: the rainfast period is critical in a protective programme. See rain and spraying.

Crop-specific considerations

Blight programmes. Protective fungicides need to be on the leaves before infection. Many growers use disease-risk forecasts to time sprays, then look for the best weather window within the interval set by the programme and the label. A dry window long enough for the product to become rainfast is often the deciding factor.

Canopy coverage. Once the canopy closes, the lower and middle leaves are harder to reach. Water volume, droplet class and a Delta T in range all affect coverage.

Aphids and virus. Where aphids spread viruses, insecticide timing is driven by pest activity and the label.

Haulm desiccation. Before harvest, the foliage is often killed off chemically or mechanically. Uniform coverage matters, and neighbouring crops that are still green need the usual drift precautions.

Tramlines and wet soil. Heavy, wet soils limit when ground sprayers can enter, which can push applications into marginal weather. Plan ahead using the forecast.

Typical applications

Fungicides (often on a regular programme), herbicides before emergence, insecticides where needed, and desiccants before harvest.

Setting it up in PulveCast

There is no potato preset, so use Custom. It starts from Delta T 2–8 °C, humidity 55–95 % and a Medium droplet class. Conservative starting points:

• Keep Delta T at 2–8 °C.
• Set the droplet class to what your nozzles produce, or leave it on automatic.
• Lower the maximum wind from the 25 km/h default towards 15 km/h (about 9 mph), or to the label's limit.
• Lower the precipitation probability limit from 50 % if your fungicide needs a long rain-free period.

Other crops

• Best weather to spray soybeans
• Best weather to spray corn (maize)
• Best weather to spray sugarcane
• Best weather to spray cotton
• Best weather to spray wheat
• Best weather to spray coffee
• Best weather to spray rice
• Best weather to spray citrus
• Best weather to spray vineyards
• Best weather to spray pastures