Phosphite residue in nutmeats of pecan

Authors: Bock, Clive; Hotchkiss, Michael - Mike; BRENNEMAN, TIM - University Of Georgia

Submitted to: Pecan Grower
Publication Type: Trade Journal
Publication Acceptance Date: 6/10/2019
Publication Date: 6/20/2019
Citation: Bock, C.H., Hotchkiss, M.W., Brenneman, T.B. 2019. Phosphite residue in nutmeats of pecan. Pecan Grower. 31:21-30.

Interpretive Summary: Scab on pecan is caused by the plant pathogenic fungus Venturia effusa, and epidemics of the disease can cause substantial yield loss and associated cost of control. Phosphonate type fungicides have been widely used since 2011 to manage this disease, and are a valuable class of fungicide due to their low risk of resistance development, and the fact that V. effusa has developed reduced sensitivity to at least four classes of the seven used to control scab in the southeastern USA. As with many other pesticides, use of phosphonates can result in residues in produce destined for human or animal consumption. Indeed, in 2013, the Maximum Residue Limit (MRL) for phosphonate in pecan and other agricultural produce was set to 2 ppm by the EU, based on new legislation. Recently the EU revised legislation pertaining to nut group crops and granted a permanent MRL of 500 ppm for this commodity group. Even though phosphonate fungicides are labelled for use on pecan in the USA, it is not known how foliar (canopy) application affects residue in in pecan nuts 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 17.1, 30.5 and 57.7 ppm per spray, depending on experiment. The maximum residue recorded was 928.0 ppm and was from a tree receiving 9 sprays of phosphonate. In a further three experiments, timing of single sprays did not appear to have a profound or consistent effect on residue level at harvest. The vast majority (96.2%) of individual tree samples in the six experiments receiving a wide range of different spray numbers (from 1 to 9 sprays per season) had phosphonate residue <500 ppm. 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.

Technical Abstract: Scab on pecan is caused by Venturia effusa, and epidemics of the disease can cause substantial yield loss and associated cost of control. Phosphonate fungicides have been widely used since 2011 to manage this disease, and are a valuable class of fungicide due to their low risk of resistance development, and the fact that V. effusa has developed reduced sensitivity to at least four classes of the seven used to control scab in the southeastern USA. As with many other pesticides, use of phosphonates can result in residues in produce destined for human or animal consumption. Indeed, in 2013, the Maximum Residue Limit (MRL) for phosphonate in pecan and other agricultural produce was set to 2 ppm by the EU, based on new legislation. Recently the EU revised legislation pertaining to nut group crops and granted a permanent MRL of 500 ppm for this commodity group. Even though phosphonate fungicides are labelled for use on pecan in the USA, it is not known how foliar (canopy) application affects residue in in pecan nuts 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 17.1, 30.5 and 57.7 ppm per spray, depending on experiment. The maximum residue recorded was 928.0 ppm and was from a tree receiving 9 sprays of phosphonate. In a further three experiments, timing of single sprays did not appear to have a profound or consistent effect on residue level at harvest. The vast majority (96.2%) of individual tree samples in the six experiments receiving a wide range of different spray numbers (from 1 to 9 sprays per season) had phosphonate residue <500 ppm. 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.