Update on phosphite residues in pecan

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

Submitted to: Meeting Abstract
Publication Type: Other
Publication Acceptance Date: 2/23/2019
Publication Date: 2/23/2019
Citation: Bock, C.H., Brenneman, T.B. 2019. Update on phosphite residues in pecan. Meeting Abstract. 1-25.

Interpretive Summary: Scab on pecan is caused by the fungal pathogen, Venturia effusa, and epidemics of the disease can cause catastrophic yield loss. Phosphonates are a valuable class of fungicide due to their low risk of resistance development, but they can result in nutmeat residues. We present results of experiments to 1) compare the effect of different rates of phosphonate products in reducing scab, and 2) compare how number of sprays applied, and timing of those sprays affected phosphonate residue in nutmeats. Treatment with higher rates of phosphonate product most often significantly or numerically reduced severity of scab compared to the lower rates of the products applied on foliage and fruit. Maximum Residue Limit (MRL) for phosphonate in pecan was set at 500 ppm by the EU. 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. The vast majority (96.2%) of individual tree samples in the experiments receiving from 1 to 9 sprays per season had phosphonate residue <500 ppm. A total of 5 sprays 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. 1) Our results demonstrate that higher rates contribute to more efficacious control of this yield-limiting disease, and 2) even with the recommended usage for phosphonate type products, the risk of exceeding the EU MRL of 500 ppm is small.

Technical Abstract: Scab on pecan is caused by the fungal pathogen, Venturia effusa, and epidemics of the disease can cause catastrophic yield loss. There is also 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. Higher rates of phosphonate fungicide may provide superior control. But use of phosphonates can result in residues in produce destined for human or animal consumption. We present results of experiments to 1) compare the effect of different rates of phosphonate products in reducing scab, and 2) compare how number of sprays applied, and timing of those sprays affected phosphonate residue in nutmeats. Rates were compared using cultivar Desirable in four experiments, and cv. Wichita in two experiments. Four rates of ProPhyt® (2.3, 3.5, 5.3 and 7.0 L/Ha), three rates of Rampart® (2.3, 4.7 and 7.0 L/Ha), or two rates of K-phite® 7LP (2.3 and 7.0 L/Ha) were tested. Treatment with the higher rates of phosphonate product most often significantly or numerically reduced severity of scab compared to the lower rates of the products applied on foliage and fruit. Indeed, regardless of epidemic intensity, higher rates of phosphonate provided more efficacious control. Maximum Residue Limit (MRL) for phosphonate in pecan has recently been set at 500 ppm. 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. The maximum residue recorded was 928.0 ppm and was from a tree receiving 9 sprays of phosphonate. In further 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. 1) Our results demonstrate that higher rates contribute to more efficacious control of this yield-limiting disease, and 2) even with the recommended usage for phosphonate type products, the risk of exceeding the EU MRL is small.