Secondary infections of Plasmopara viticola are only possible if already mature oil spots exist in or nearby your vineyard. Oil spots are mature when they are able to sporulate (produce sporangia). Sporangia are only produced by night. Sunlight inhibits the sporulation. Sporangia are produced if it is warmer than 12°C and the relative humidity is very high. The rate of sporangia production increases with temperature up to 24°C. The optimal temperature for sporulation for European grape varieties (vitis vinifera) is about 24°C. If temperatures exceed 29°C, no sporulation can take place. FieldClimate.com checks to see if humidity of more than 95% occurs during the night. If this condition lasts for acccumulated hourly temperature of more than 50°C, the sporulation is concluded and new sporangia of the Downy Mildew exist in the vineyard. 50°C hours corresponds to 4 hours with 13°C or 3 hours with 17°C.
|The following graphs show the relations between temperature and wet conditions (relative humidity, leaf wetness, precipitation) which are used to model the infection events in Fieldclimate.
Sporangia of Plasmopara viticola have a limited lifetime. The warmer it is and the dryer the air, the faster they die. They definately die in the next dew or leaf wetness period which is to short for infection. The fitness of older sporangia is quite limited therefore. FieldClimate.com assumes the lifetime of sporangia to be limited to one day.
In order to cause new infections, the sporangia must be spread. There are two ways this happens. If it rains immediately after the forming of sporangia, they spread with the rain splashes. If the vine leaves stay long enough wet, a high level of Plasmopara viticola infections takes place. If the next morning begins without rain and with decreasing humidity, the dried up sporangia abscise from their branches. Even a slight air movement will carry them to healthy leaves. Unless it rains soon, they will die.
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