Strawberry Powdery Mildew – fruitweb forecast model

The fruitweb model for strawberry powdery mildew (Podosphaera aphanis) is used to estimate the daily infection risk of this fungal disease based on weather data. The model makes it possible to identify phases with an increased risk of infection and thereby better plan plant protection measures to the actual conditions in the crop.

Required weather parameters:

• Air temperature
• relative humidity
• Optional weather parameters
• wind speed
• Leaf wetness duration
• Area of application

The model was developed for use in polytunnels or greenhouses. It can also be used outdoors. However, the rain parameter is not used. Since this has a major influence on the development of powdery mildew on strawberries, the statements in the field are subject to restrictions.

How the model works

Infectious conditions

In the model, the risk of infection is calculated for each day based on the temperature and relative humidity (red line). The higher the red line rises in a day, the greater the risk of infection. The calculation always begins at 6 p.m. The prerequisite for an infection is a relative humidity of at least 60%. The higher the relative humidity, the more spores begin to germinate. The same applies to temperature. Most spores germinate in the optimal temperature range between 18 and 20°C. Relative humidity has no effect on the rate of spore germination. This is solely determined by the temperature. The optimum here is also around 20°C.

As a rule, the relative humidity and the temperature behave in opposite directions at night. As a rule, the relative humidity increases in the evening and at night and often moves into the optimal range of over 90%. In contrast, the air temperature usually drops, which slows down spore germination. Optimal infection conditions therefore exist with high relative humidity and temperatures around 20°C.

In addition to the daily infection risk, a moving average over six days is calculated (blue line). This makes it possible to estimate what the risk of infection will be over a certain period of time.

Consideration of spore flight

An infection can only occur if there are spores on the leaf and they germinate. If infections with powdery mildew have already occurred in the crop and sporulating fungal tissue has formed, this can release spores again under favorable conditions. This happens particularly at temperatures between 20 and 28°C. At temperatures above 30°C and at temperatures below 15°C, however, there is hardly any sporulation.

If you select the setting “With spore flight” – “yes”, then a relative amount of spores is calculated (light brown area). Since the model cannot take into account whether sporulating tissue is already present in the population, the significance of this parameter remains limited.

If you choose the setting “With spore flight” – “no”, a fixed number of 1000 spores is assumed at 6 p.m.

Wind speed also has a major influence on the amount of spores. The spores can only spread throughout the crop if they are detached from the sporulating tissue by wind and then blown onto other plants. Since most devices for collecting weather data in the foliar tunnel do not have a wind sensor, this value was not taken into account when calculating the amount of spores. However, if the device has a wind sensor, the wind speed will be displayed (brown line) and can be used to draw further conclusions.

Latency and lesions

If an infection occurs, it usually takes between 6 and 8 days for the fungus to develop to the point where it can sporulate again. This time is called the latency period (bottom gray line). If the latency reaches 100, the fungal tissue can form new spores and a new infection cycle can begin. For the sake of clarity, a new latency curve always starts when the infection value (red line) has exceeded the value of 50.

As soon as the latency period has ended, the fungal tissue begins to grow. This is expressed by the upper gray line. The value scale from 0 to 3 represents the diameter of the visible fungal tissue in millimeters. In this way, the approximate time of infection can be determined.

Overall view of the graphic