DOWNCAST model

Prediction Model

DOWNCAST was developed by de Visser (1998) in the Netherlands. It calculates infection risk of Peronospora destructor (Onion Downy Mildew) using weather data. An actual infection requires present inoculum. Criteria for sporulation and infection must be met for the model to issue a high infection risk (red alert).

Sporulation events are calculated based on high (over 92%) nightly relative humidity (RH). Sporulation may occur at temperatures between 4 and 26 °C (optimally 12-20 °C). A sporulation index of 1-3 is calculated based on the high RH and temperature duration, where an index of 3 indicates optimal sporulation conditions. Sporulation events are not expected to occur if it's been raining between 00:00 and 06:00 AM, or the previous day was very warm, with several hours above 27 °C.

Infection events are based on leaf wetness. A direct infection requires a minimum of 2 hours of leaf wetness the morning after sporulation (optimum temperature 6-16 °C). Delayed infections may occur as long as 72 hours after sporulation, given at least 144 minutes of leaf wetness during a 3-hour period. It is assumed that all the spores germinate simultaneously, thus only one infection event can occur after a sporulation event.

Short periods of leaf wetness (90-150 minutes during a 5 hour period) may cause the spores to germinate and die. Under these circumstances we have a failed infection, and the potential infection event is cancelled. A later infection event requires a new sporulation event.

A sporulation event triggers a yellow alert (possible infection risk). A sporulation event followed by an infection event within 72 hours triggers a red alert (high infection risk).

Literature

de Visser, C.L.M. 1998. Development of a downy mildew advisory model based on downcast. European Journal of Plant Pathology 104: 933-943.

Forecast interpretation (Norway)

In Norway, Peronospora destructor normally occurs in the late summer, if at all. A spraying decision should therefore be based on an assessment of present inoculum. Spraying more than every 10th day is normally not necessary, but with frequent alerts and inoculum detected, a spraying interval close to 7 days may be considered.