If sufficient inoculum is nearby, infection is very likely if the risk index exceeds 4. This is the Smith version of the tomato-potato late blight risk model and is documented at http://uspest.org/npdn/riskdoc.html#TPLB.
Below is an explanation of the risk index:
1 = conditions are favorable for fungal spore formation if there are lesions on plants in the area,
3 = Conditions are favorable for infection if there is inoculum (spores) in the area.
4= The latent period following infection is completed, you will see the lesions of the new infections in the field.
These models reflect only weather induced sporulation and infection conditions, not whether inoculum is actually present in the area. Scout regularly to determine if there is inoculum in the area. As with all weather-driven pest models, use with caution and be sure to verify the validity of weather data inputs for all sites. Plants are the most vulnerable to late blight after canopy
closure. Site specific irrigation events are not reflected in this weather data and may result in different risk levels.