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TECHNICAL SESSION: Chemical Control of Fungal Diseases

Number of applications and application date affect phosphonate residue in nutmeats of pecan
Clive Bock - USDA ARS, Southeastern Fruit and Tree Nut Research Laboratory. Timothy Brenneman- University of Georgia, Michael Hotchkiss- USDA-ARS-Southeastern Fruit and Tree Nut Research Laboratory

Scab (caused by Venturia effusa) can cause substantial yield loss in pecan. Phosphonate fungicides have been used since 2011 to manage scab, and remain a valuable class of fungicide due to their efficacy and low risk of resistance development. Phosphonate use can result in residues in produce destined for human or animal consumption. Recently the EU legislated that nut group crops could not exceed a Maximum residue limit (MRL) of 500 ppm. In the USA, it is not known how foliar (canopy) application affects residue in in pecan nut meats. In both 2016 and 2017 we compared how number of sprays applied, and timing of those sprays affected phosphonate residue in nutmeats. Phosphonate residue increased with the number of foliar sprays applied during the season. The relationship was linear, but the rate of accumulation varied showing an increase of up to 57.7 ppm per spray, depending on experiment. The maximum residue recorded in a sample was 928.0 ppm and was from a tree receiving 9 sprays of phosphonate. In spray timing experiments, later sprays tended to result in higher residues, although this was variable. Although up to 9 sprays were tested, 5 are the maximum currently recommended in the state of Georgia. When 5 sprays were applied, the mean phosphonate residues in nutmeats in the three experiments were 83.8, 152.33 and 146.33, respectively. Thus, even with the recommended usage for phosphonate type products the risk of exceeding the EU MRL is small.