Field experiments conducted in 2015 at the University of Georgia, Coastal Plain Experimental Station (UGA-CPES) were utilized for the leaf spot assessment comparisons. There were four trials altogether. One trial was conducted at the UGA-CPES Gibbs Farm, one trial at the UGA-CPES Rigdon Farm, and two trials at the UGA-CPES Lang Farm. All fields had a history of severe epidemics of early and late leaf spot. However, in recent years, late leaf spot was more prevalent. Trials utilized natural inoculum of the leaf spot pathogen in the field.

A randomized complete block design was used in all trials. All plots were irrigated as needed to maintain favorable conditions for leaf spot epidemics and for peanut growth

A field trial was conducted at the University of Georgia Gibbs Farm. The trial included a total of ten treatments consisting of nine advanced breeding lines (Jordan et al. 2017) and cultivar Georgia-12Y (Branch 2013). There were three replications. Planting date was 10 April 2015. Plots were 6 m long by 1.8 m wide.

A field trial with a total of eight treatments was conducted at the University of Georgia Rigdon Farm. Treatments consisted of two cultivars, Georgia-06G (Branch 2007) and Georgia-12Y, factorially arranged with two cultivation treatments, non-cultivated, and cultivated with a tine weeder, and two seed bed configurations, flat seed bed and planting in a furrow (Jordan 2017). There were three replications. Planting date was 6 May 2015. Plots were 9.2 m long by 1.8 m wide.

A planting date experiment conducted at the University of Georgia Lang Farm was utilized. 24 treatments consisted of two cultivars, Georgia-06G and Georgia-12Y in factorial arrangement with treated and nontreated seed, and six planting dates, April 24, April 27, May 4, May 11, May 19, and May 26. Seed treatment consisted of azoxystrobin 0.128 g/kg of seed, fludioxanil 0.008 g/kg of seed, and mefenoxam 0.0016 g/kg of seed applied as Dynasty PD (2.5 g/kg of seed) (Syngenta Crop Protection, Greensboro, NC). Plots were 9.2 m long by 1.8 m wide. There were four replications.

A field experiment was conducted in which 13 treatments consisted of nine advanced breeding lines, planted without seed treatment, and cultivars Georgia-06G and Georgia-12Y. Each of Georgia-06G and Georgia-12Y were planted with and without seed treatment (Jordan et al. 2017). Seed treatment was same as that described previously. Planting date was May 20. Plots were 9.2 m long by 1.8 m wide. There were three replications. In all trials, seeding density was 14.8 seed/m of row.

Leaf spot severity was assessed for each plot using three different methods.

For each evaluation date, leaf spot severity was assessed visually using the Florida 1-10 leaf spot severity scale where 1 = no disease, 0% defoliation, and 10 = 100% defoliation, plants dead, killed by leaf spot (Chiteka et al., 1988). The assessment included parting the branches of the upper canopy to allow estimation of severity in the lower canopy. Percent defoliation was calculated from Florida 1-10 severity ratings using the formula developed by Li et al (2012): where FLSc is the Florida scale value.

On each evaluation date, canopy reflectance was measured using active sensor reflectance meter (Crop Circle model ACS-210, Holland Scientific, Lincoln, NE) and a GreenSeeker Handheld (Trimble, Westminster, CO). The Crop Circle meter was the same device used by Navia Gine (2012). Both sensors simultaneously measure canopy reflectance in the visible and near infrared portions of the light spectrum. The Crop Circle sensor measures visible portions (VIS) centered at 590 nm wavelength and near infrared (NIR) centered at 880 nm wavelength. The GreenSeeker sensor measures VIS and NIR centered around 660 nm and 780 nm wavelengths, respectively. For each plot, a vegetation index, presented as the normalized difference vegetation index (NDVI), was calculated using the formula: where ref NIR and ref VIS are reflectance measurements for the near infrared and visible portions of the light spectrum, respectively. NDVI readings from both instruments can range from 0 to 1.0.

For each evaluation within each trial, visual leaf spot ratings and reflectance measurements were made on the same day in rapid succession. All ratings and reflectance measurements were done by the same evaluator. Visual leaf spot assessments and sensor readings were made at Gibbs Farm on 140 and 145 DAP, at the Rigdon Farm trial on 119 DAP.

Leaf spot ratings were made at 133 and 140 DAP for each planting date in the Lang Farm planting date trial and 105, 112, 119, 126, 133, and 140 DAP in the Lang Farm genotype trial.

For each evaluation, the respective sensor was carried manually and positioned directly over the center of the row in the nadir view at a distance of approximately 1.0 m above the plant canopy. Scans are made of the entire length of both rows of each plot by walking at a speed of approximately 0.9 m/sec without interruption. Both devices emit their own light. The Crop Circle instrument generates visible amber light (590 nm) and near infrared light (880 nm). For the Crop Circle instrument, sensor readings were recorded 10 times per second. The final output of the sensor was a pseudo-reflectance value, an invertible linear transformation, for both NIR and VIS bands and the calculated NDVI. The mean of sensor readings for NDVI was calculated for each plot, which was used for analysis. The GreenSeeker emits bursts of visible red (660 nm) and near infrared light (770 nm) and measures the amount of light that reflects back at the sensor. The unit continues to sample the scanned area as long as the trigger remains engaged. The unit automatically averages the NDVI rating, and the average is displayed as soon as the trigger is released. The NDVI reading was recorded manually for each plot.

Regression analysis was used to examine the linear and nonlinear relationships between NDVI and final percent defoliation for each location. Proc GLM (SAS v 9.4) was used to examine linear and quadratic relationships among percent defoliation and NDVI measured by the two reflectance meters. Regression was used to examine the relationships within individual factors such as cultivar, treatment, and evaluation date. When responses were similar for various factors, regression was conducted using data pooled across the factors or evaluation times. If a significant (P ≤ 0.05) quadratic response was observed that improved fit of the data compared to a linear model based on R² values and plots of residuals, the quadratic model is presented. Otherwise, a linear model is presented if a significant (P ≤ 0.05) linear response was observed.

Across sample dates, visual estimates of percent defoliation ranged from 0 to 48%. NDVI measured with the GreenSeeker (NDVI_GS) instrument decreased linearly with increasing percent defoliation (Figure 1A). There was no significant regression for the relationship between NDVI measured with the Crop Circle instrument (NDVI_CC) and percent defoliation (P > 0.36) (Figure 1A) or the relationship between NDVI measured between the two instruments (P > 0.58) (Figure 1B).

Visual estimates of percent defoliation ranged from 3.8% to 76.5%. Across cultivars and treatments, there was no significant regression (P > 0.57) for NDVI_GS on percent defoliation (Figure 2A). NDVI_CC decreased linearly of with increasing percent defoliation, but data were highly variable (Figure 2A). There was no significant regression (P > 0.69) for the relationship between NDVI_CC and NDVI_GS (Figure 2B).

Visual estimates of percent defoliation ranged from 0 to 100%. Across planting dates and evaluation dates, NDVI_GS decreased linearly with increasing visual estimates of defoliation on Georgia-06G (Figure 3A) and decreased according to a quadratic function of percent defoliation on Georgia-12Y (Figure 3B). NDVI_CC decreased linearly with increasing percent defolation on both Georgia-06G (Figure 3A) and Georgia-12Y (Figure 3B). Across all factors, NDVI_CC increased with increasing NDVI_GS according to a quadratic function (Figure 3C).

Percent defoliation ranged from 0 to 100%. Across all cultivars, treatments, and evaluation dates, NDVI_GS decreased with increasing defoliation according to two linear segments in a piecewise regression with rate of decline decreasing above 47.9 % defoliation (Figure 4A). NDVI_CC decreased with increasing defoliation as described by a two linear sector piecewise regression with a break point at approximately 91.7% defoliation (Figure 4B). NDVI_CC declined more rapidly after that point as defoliation increased. NDVI_CC increased according to a quadratic function of NDVI_GS measured with the GreenSeeker instrument (Figure 4C).