The effect of number of applications and application date on phosphonate residue in nutmeats of pecan

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 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 pecan nutmeats. In both 2016 and 2017 we compared how number of applications, and timing of those applications affected phosphonate residue in nutmeats. Phosphonate residue increased with the number applications during the season. The relationship between the number of phosphonate applications and the log ppm was linear. Residue accumulation varied depending on experiment, with a percentage increase for each additional spray 94, 26 and 45% per additional 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, although nutmeats from trees receiving later applications tended to have the highest phosphonate residues. The vast majority (96.2%) of nutmeat samples from individual trees in the six experiments receiving a range of 1–9 sprays per season had phosphonate residue <500 ppm. Although up to 9 sprays were tested, 5 is 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 83.5, 150.8 and 145.2, respectively. Thus, even with the recommended usage for phosphonate type products the risk of exceeding the EU MRL is small.