Development of an Inoculation Method for Quantifying Fungicide Residues on Peanut Foliage

Abstract

Abstract A bioassay was developed to evaluate residues of three foliar applied fungicides on peanut leaflets and stems obtained from the upper, middle, and lower canopy. Experiments were conducted to determine the effects of wounding and nutrient source on the development of Sclerotium rolfsii lesions. Results indicated that wounding was not required for infection on either tissue type, and that adequate lesion development was obtained when ¼ strength potato dextrose agar (PDA) was used as a nutrient source. Significant differences in lesion development were observed among canopy layers for leaflets and stems. Tissues collected from the upper canopy were more rapidly colonized by S. rolfsii than tissues from the middle or lower canopy. One-quarter strength PDA was used as a nutrient source to determine an appropriate sample size, and to evaluate the response of S. rolfsii to varying concentrations of azoxystrobin, flutolanil and tebuconazole on non-wounded tissues. Based on differences in the sample mean, standard deviation, and coefficient of variation, a total of 8 to 10 samples are required to obtain a reliable estimate. The application of fungicides significantly reduced the size of S. rolfsii lesions compared to the non-treated controls. Consistent results were obtained from the leaflet and stem assays, and lesion size decreased linearly with increasing log10+1 transformed fungicide concentration. The EC50 values from the leaflet and stem assays were 17.2, 9.5, and 18.1 mg/L, and 18.1, 8.3, and 13.5 mg a.i. /L for azoxystrobin, tebuconazole, and flutolanil, respectively. These results demonstrate an effective method to determine differences in the activity of foliar applied fungicides.