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The grape vine downy mildew overwinters as Oospores in the fallen leafs. The infection of the vine starts from these Oospores in early summer (Recommendation of Agroscope Chänging- Wädenswill (Viret): starting in january: calculate the daily average temperature and substract 8°C of that. All positive values will be summed up. At a sum of 160-170°C the occurrence of first mature oospores is predicted)


Infections coming from the Oospores are called primary infections. This term is misleading since several primary infections can occur in early summer. In epidemiological events, the primary infection does not play an important role if there are sufficient oil spots in the vineyards and the infection potential of the summer spores (Sporangia) exceeds the Oospores.  

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Image Macrosporangia

The oospores which overwintered in the fallen leaves develop the so-called macrosporangies whenever there is sufficient relative humidity for about 24 hours. The macrosporangies release their zoospores into free water and heavy rain (as in a thunderstorm). Spores are carried to the vine leaves and/or green shoots. Primary infections, therefore, need longer rainy periods or several successive thunderstorms. The first rain causes the fallen leaves to be saturated with water and strong rain on the following days causes the macrosporangies to release their zoospores, which reach the vine leaves or shoots. For the successful infection, a sufficiently long leaf wetness period is necessary to allow the zoospores to reach the stoma of the leaves or shoots and infect them (the Illustration above shows the development cycle of the downy mildew of the vine (Plasmopara viticola).   

The FieldClimate.com model for the Downy Mildew primary infection checks first, if the weather is suitable for development of macrosporangies. This is the case as long as the leaves are wet, or the relative humidity after the rain does not fall below 70%. Depending on the temperature, the macrosporangies can develop within 16 to 24 hours. If mature macrosporangies are available, it will be shown graphically in the display of the Downy Mildew primary infection. If macrosporangies are present, a strong rainfall can spread their zoospores. FieldClimate.com interprets a continuous rain of 5 mm as a strong rain. If the zoospores are spread by strong rain, a primary infection is possible if the leaf wetness has lasted long enough. 

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The duration of leaf wetness, necessary to cause a successful infection depends on the temperature. At a temperature of 6°C the leaves have to be wet for 9 hours. At a temperature of 25°C, a leaf wetness duration of 2 hours is already enough for an infection.
FieldClimate.com shows the progress of infection, in situations where macrosporangia are present. When the infection line reaches 100% the infection is finished. With the start of infection it starts to calculate the incubation period for this infection. It calculates up to 6 incubation counters. In years and areas with frequent infection this 6 incubation counters might not be enough. In most years and places this counters will reach 100% of incubation before they will be stated from 0 by a new infection. When the incubation counter reaches 100% new sporulation can be expected from the lesions formed by this infections. Oil spots will be visible now or in the following morning.  
The model bases on the publications of MUELLER-THURGAU, ARENS, MUELLER and SLEUMER, BLAESER, HERZOG, GEHMANN and many other workers connected to research network on grape vine downy and powdery mildew epidemiology since the early nineteenth of the last century. 

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