Infection conditions: late blight and apple scab
Published criteria relating temperature and wetness to the possibility of infection. What is computed is the fulfilment of the conditions, not the occurrence of infection.
Quantities
| Symbol | Quantity | Unit |
|---|---|---|
T_min | minimum daily air temperature | °C |
RH | relative humidity | % |
n₉₀ | hours in the day with RH ≥ 90 % | h |
W | leaf wetness duration | h |
T̄_W | mean temperature of the wetness period | °C |
Formulas
Hutton criteria (Phytophthora infestans)
two consecutive days with T_min ≥ 10 °C and n₉₀ ≥ 6 h
Mean temperature of the wetness period (Revised Mills Table)
T̄_W = (T_min,W + T_max,W) / 2
The half-sum of the extremes over the period rather than the mean over hours; the rule is given in the notes to the table.
Joining wetness periods
periods are joined if separated by no more than 8 dry hours
Infection condition for scab
W ≥ W_req(T̄_W) from the table
W_req falls from 40.5 h at 1 °C to 6.0 h at 17…23 °C and rises again to 11.3 h at 26 °C.
Input data
Temperature and relative humidity are taken from the open-meteo hourly series or from a sensor on the bed. The series are not mixed: a day is formed entirely from one source. Days for which fewer than 24 hours are available are excluded.
Substitute for wetness duration
Leaf wetness duration is not published by forecasts and is not measured by the available sensor. An hour with RH ≥ 90 % or recorded precipitation is taken as its estimate. This is an approximation and is stated alongside the result. The scab criterion requires a precipitation series, so sensors without a rain gauge are not used for it.
Applicability and limitations
- Fulfilment of a criterion indicates conditions favourable to infection and does not indicate infection.
- Inoculum pressure, cultivar resistance and treatment history are not part of the criteria.
- Powdery mildew and grey mould models are not implemented: the published forms require a probability threshold to be chosen, or were calibrated on crops absent from the catalogue.
Sources
- AHDB & James Hutton Institute, 2017. Development and implementation of a new national warning system for potato late blight in Great Britain (Hutton Criteria). Agriculture and Horticulture Development Board — full text open Open
- OMAFRA. Revised Mills Table. Adapted from the table of W. D. Mills, revised by W. E. MacHardy. Ontario Ministry of Agriculture, Food and Rural Affairs — full text open Open
- Open-Meteo. Weather forecast and historical weather API. open-meteo.com, CC BY 4.0 — full text open Open