Evaluation of residual chlorothalonil levels on processing tomato foliage using the TOM-CAST spray program

Field tomatoes were sprayed with chlorothalonil on a fixed-interval spray program and a TOM-CAST spray program with disease severity value threshold of 18. Foliage samples from upper and lower canopy layers were collected prior to spray re-applications. Chlorothalonil residue data were compared to the chlorothalonil efficacy threshold (1.2 μg/cm²). Using a seven-day interval program, eight of the nine and seven of the nine spray intervals had chlorothalonil residues above the critical level for the upper and lower canopy layers, respectively. Using the TOM-CAST program, four of the five spray intervals had chlorothalonil residues above the critical level for both upper and lower canopy layers when the DSV threshold of 18 was reached. Persistence of chlorothalonil residues at effective concentrations could lengthen the spray interval beyond the DSV-based spray recommendation.