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Model Cougar Blight for Pears

Fire Blight Symptoms on Pear

Fb1 The model requires the user to recognize specific and ever-changing local events and aspects of their orchard that may increase or decrease fire blight risk relative to other orchards in the region. The model requires the user to assume there is a risk of fire blight infection whenever blossoms are present on the trees, especially during the petal fall and "post bloom" period, when scattered blossoms may remain on many apple and pear varieties. The model user is asked to carefully assess the situation on their specific site and to initiate control measures if blossoms are present, risk levels are "High" or "Extreme," and blossom wetting is likely to occur sometime during the next 24 hours.

Model Structure: Temperatures and Wetness: The key Fire Blight process that must be modeled is the potential for bacterial growth on the stigmas of apple and pear flowers. This growth is temperature dependent, so dependable prediction of infection risk requires the use of a measurement method that most accurately reflects the growth of Erwinia amylovora colonies. The main disagreement among models is how this should be done.

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Fire Blight Model Output in FieldClimate.com

The Cougarblight model estimates bacterial growth rate with degree hours based on a specific growth rate curve. This growth curve is based on the growth rate of E. amylovora bacteria in laboratory tests. The degree hour values are accumulated each hour of the day that temperatures are over 15 °C. The hourly values increase as temperatures rise from 15 °C to 29°C, decline at higher temperatures, and reach zero for any hour with temperatures over 40°C.

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Table: Bacterial growth in depending on the temperature.