Weather Warnings for Crops

For an outbreak to occur, fungal spores must be in contact with liquid water on the fruit or leaves for a period of time. That time varies with temperature - the higher the temperature the less time it takes for an outbreak to occur.

For a light infection (brown rot) to occur, the fruit or leaves must be wet for 140 degree hours - that is, the number of hours of wetness multiplied by the mean temperature (degrees Celsius). This is known as the Mills period. The Mills period for a moderate fungal infection is 200 degree hours, while for a heavy infection (black spot) it is 300 degree hours.

In deciding whether to issue a weather warning for fungal outbreaks, the mean temperature is relatively easy to forecast. Estimating how long the fruit or leaves will stay wet is more difficult, partly because trees will dry unevenly. Some latitude must be allowed in estimating the period of wetness.

It can, however, be assumed that after an initial wetting, fruit or leaves stay wet while the relative humidity is near or more than 80 per cent. There is a good chance that the relative humidity will stay near 80 per cent into the night if clouds persist after sunset.

For example, the forecast is for rain from 3pm with cloud cover until after sunset. There is a reasonable possibility that the relative humidity will stay around 80 per cent until at least midnight. If the forecast mean temperature for the period is 16 degrees, then the forecast Mills period is 16 x 9 = 144 degree hours. This would justify a brown rot warning.