The utilization of a standard 5-today weather unit in the correlation and forecasting of plant diseases has several advantages, some of which already have been noted (4). A 5-day unit, or multiple of it, conicides very closely to the incubation and developmental period of most plant disease organisms. For example, 3 to 10 days are requiere to produce a generation of the potato and tomato late blight fungus (Phytophthora infestans), i.e. for sporangia or zoospores to germinate, cause infection, and again sporulate. Forecasts of initial and secondary outbreaks during the season are based usually on 15, 10, or 5 days of continuously favorable temperature and moisture conditions. The 5-day unit also greatly facilitates calculation of the moving mean temperatures, necessitating only the pointing off of the sum of the ten figures (maximun and minimum for each day). Recently, Hyre (5) in comparison found the 5-day mean as accurate as the widely employed (1-3, 6, 7) 7-day mean and adopted it in the correlation and forecasting of downy mildew (Phytophthora phaseoli) in lima bean.

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