The FRY Model
W.E.FRY (1983) published his work done on infection of potatoes with different susceptibility levels at different durations of relative humidity higher than 90% or leaf wetness and temperatures. Derived from this results he developed an infection model for late blight in potato and in the next step a model to estimate the propriety spray interval for the fungicide cloranthonil (Bravo).
Susceptible cultivars can be infected within shorter moist periods and the disease severity will be higher. Whereas moderate susceptible and resistant varieties will need a longer moist period or warmer temperatures to be infected and the disease severity is lower.
For susceptible varieties the maximum rating of an infection period can be 7 whereas for moderate susceptible varieties it can be 6 and for resistant varieties it can be 5 only. In the same way the assessment of the spray interval is asking again for the level of susceptibility of the cultivar. A spray is needed if the last spray is longer than 6 days away and the accumulated blight Units are exceeding: 30 for susceptible varieties, 35 for moderate susceptible varieties and 40 for moderate resistant varieties.
This model is very useful to estimate if a new spray is needed. We can start to accumulate the fry units from the date of the last spray on. If the accumulated value excites the threshold we will have to spray again.Fry, WE, AE Apple & JA Bruhn (1983). Evaluation of potato late blight forecasts modified to incorporate host resistance and fungicide weathering. Phytopathology 73:1054-1059.