Rainfastness of mancozeb on apple seedlings determined through deposition quantification of mancozeb residue and a fluorescent pigment

Abstract Mancozeb spray deposition and the persistence thereof to rainfall are important factors influencing the efficacy of apple scab control caused by Venturia inaequalis . Fungicide deposition can be assessed through quantification of fungicide residue, or fluorescent pigment quantification. Fluorescent pigment quantification is a more cost effective and less labour intensive method, since it is assessed using only photomacrography and image analyses. The yellow fluorescent pigment used in this study was shown to be a suitable tracer for five different mancozeb formulations on apple leaves when evaluated at different concentrations (0.5, 1.0, 1.5 and 2.0). Pearson's correlation was significant ( P r  = 0.896) between fluorescent particle coverage (FPC%) and mancozeb residue (mg/kg Dry Weight ) for all five formulations. The particle size ranges of two wettable powder (WP) formulations were significantly smaller than those of the other WP formulations, but this did not result in differences in mancozeb residue on apple leaves. The persistence of mancozeb to different rain volumes applied to apple leaves was determined for three mancozeb formulation treatments: Dithane M-45 800 WP NT, Ventum 800 WP, and Ventum 800 WP combined with the sticker-spreader adjuvant Nu-Film P. There were no significant differences ( P  > 0.495) between the three treatments based on FPC% and mancozeb residue quantity when simulated rain was applied to apple seedlings at a constant rainfall intensity of 5 mm/h at five different rainfall volumes (0, 1, 5, 10 and 15 mm). A fair to good correlation ( r  = 0.697 and 0.995) existed between FPC% and mancozeb residue, and their percentage loss at different rainfall volumes. However, the response of FPC% and mancozeb residue differed based on exponential regression curves. This was due to a markedly larger predicted loss by each model's asymptote, 51.67% for FPC% and 40.76% for mancozeb residue. Based on actual mancozeb residue values, a significant percentage loss in residue already occurred after applying 1 mm (32.90%) rain. When rain volumes were increased to 5 and 10 mm rain, the percentage losses (37.88 and 41.08%) did not differ significantly from 1 mm rain. The same was true for percentage loss in FPC%, except that a significant higher loss occurred at 10 mm (52.36%) than at 1 mm rain (41.13%).