Biofix prediction

Apple scab Biofix

The biofix corresponds to the moment when pseudothecia (the sexual structures containing asci and ascospores) reach maturity.

In the absence of data on pseudothecia maturation, the recommendations for setting the biofix were as follows: set the Biofix at the date of the first ascospore release or at the “green tip” stage, whichever occurs first.

However, in some years, mild and dry weather conditions (even if the soil remains moist) can accelerate the maturation of scab spores. In such cases, waiting for the first ascospore release to set the biofix can be risky, as significant ascospore discharge may already have occurred.

We have therefore developed a new module to predict the biofix date based on winter conditions.

Background on the sexual reproduction of apple scab:

Apple scab, caused by Venturia inaequalis, primarily overwinters on fallen leaves. In autumn, after leaf fall, the fungus begins its sexual reproduction.
It gradually forms survival structures called pseudothecia within the decomposing leaf tissues. During winter, these pseudothecia develop slowly under the influence of moisture and temperature. Inside, asci form, each containing ascospores produced through sexual reproduction. In early spring, the pseudothecia mature, and the ascospores can be released.

Model for estimating the biofix:

Our model estimates pseudothecia maturation starting from the average date of 50% leaf fall in your orchard. By default, this date is set to November 1 of the previous year for the Northern Hemisphere and May 1 of the same year for the Southern Hemisphere.

From this date, the model calculates all stages of pseudothecia maturation. These stages are described by several plant pathologists, with Sutton’s stages being primarily used.
Each stage (1 to 12) is characterized by a temperature response curve. Maturation progresses only when the soil is moist. When the soil is dry, maturation slows progressively and can even stop completely.

Model Validity

The model has been validated using extensive observational data across different climates (with date of 50% leaf fall on 1st november). It accurately detects early and late years, with a margin of error of approximately ±4 days in most cases.

The model also includes a “snow” module that estimates whether the soil surface is covered by snow. This is a simplified model that uses temperature data to determine whether precipitation falls as snow and then estimates the rate at which the snow cover disappears through melting or sublimation.
When snow is present on the ground, the soil temperature is adjusted to recalculate the maturation rate of pseudothecia.
In the regions where snowfall is not detected, the model will still tend to run late!

Important:
For accurate predictions, the model requires complete and accurate weather data from the time of leaf fall up to the present date. The “Biofix Prediction” tab will prompt you to check your weather data and, if gaps are detected, fill them with virtual weather data. Calculations using virtual weather data might be less accurate!

If the biofix date has already been reached, you will have the option to use it to replace the default biofix date. Otherwise, you will need to run the model again in a few days to see whether the biofix has been reached.