The incidence of blossom blight is one of the most sporadic aspects of fire blight epidemics. The MARYBLYT model was built on the assumption that there is an abundance of inoculum and that, for a blossom infection event to occur, four strict conditions must be met in sequence. These conditions are:
- flower must be open with stigmas and petals intact, stigmas have to be exposed for colonization, flowers in petal fall are resistant;
- accumulation of at least 110 °C hours > 18.3°C within the last 44 °C days > 4.4 °C defines the epiphytic infection potential for the oldest open and hence most colonized flower in the orchard
- a wetting event occurring as dew or 0.2 mm of rain or 2.5 mm of rain the previos day allows movement of bacteria from colonized stigmas to the nectarthodes
- an average daily temperature of >= 15.6 °C: This may influence the rate at which the bacteria migrate into the nectarthodes as well as the multiplication of bacteria needed to establish infections.
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