Evaluation of Models for Alternaria Brown Spot of Mandarin Under Mediterranean Conditions by Partial Receiver Operating Characteristic Curve Analysis.

Alternaria brown spot (ABS) is a serious fungal disease of mandarin in the Mediterranean Basin. Due to the rigorous fruit quality standards, models for ABS should avoid false negatives. Experiments were conducted with susceptible 'Fortune' and 'Nova' inoculated at different temperatures and leaf wetness durations, including interrupted periods. Effects of temperature and time elapsed after inoculation were also studied. Disease incidence data were fitted to generalized additive models and a generic infection model. Exposure of trap plants in affected orchards was used for model evaluation, including the Alter-Rater and a simple rule system (SRS). The predictive ability of the models was analyzed using the partial area under the receiver operating characteristic curve in the high-sensitivity range between 0.9 and 1. Postinoculation temperature had a significant effect on disease incidence, with maximum symptom expression after 30 h on Fortune and 60 h on Nova. ABS incidence did not increase after a leaf wetness interruption of 1 h on Nova and 2 h on Fortune. All the models evaluated had high false-positive rates on Fortune. Only the SRS showed a substantial strength of agreement in Nova, with a true-positive rate of 0.93 and false-positive rate of 0.16.