Influence of Virginia Market-Type Cultivar and Fungicide Regime on Leaf Spot Disease and Peanut Yield in North Carolina

Abstract

Determining the effectiveness of fungicide programs based on cultivar resistance to pathogens, especially late leaf spot (caused by Nothopassalora personata (Berk. & M.A. Curtis) [U. Braun, C. Nakash., Videira & Crous]) is important in establishing recommendations to peanut (Arachis hypogaea L.) farmers. Research was conducted in North Carolina during 2021 and 2022 at three locations to compare the incidence of late leaf spot (e.g., visual estimates of percent of peanut leaflets with lesions), percentage of the peanut canopy defoliated caused by this disease, and yield of the peanut cultivars Bailey II, Emery, and Sullivan when exposed to five fungicide regimens including a non-treated control. Peanut yield was not affected by the interaction of cultivar × fungicide regimens. While differences in leaf spot incidence and canopy defoliation were noted for cultivars, these differences did not translate into differences in peanut yield. All fungicides regimens protected peanut yield from leaf spot disease regardless of the number of sprays during the cropping cycle (e.g., three, four, or five sprays). Peanut yield in the absence of fungicides was 4410 kg/ha compared with a range of 5000 to 5390 kg/ha when fungicides were applied. Peanut yield was greater when fungicides were applied four or five times compared with only three sprays or non-treated peanut. The regimen with five consecutive sprays of chlorothalonil alone for the first and final spray in the regimen and when this fungicide was applied with tebuconazole for the second, third, and fourth sprays was as effective as fungicide regimens including combinations of pydiflumetofen plus azoxystrobin plus benzovindiflupyr, mefentrifluconazole plus pyraclostrobin plus fluxapyroxad, bixafen plus flutriafol, and prothioconazole plus tebuconazole.

1. Introduction

Early and late leaf spot (caused by Passalora arachidicola (syn. Cercospora arachidicola [Hori]) and Nothopassalora personata (Berk. & M.A. Curtis) [U. Braun, C. Nakash., Videira & Crous], respectively) infect peanut, causing chlorotic lesions on the leaves of the peanut plant [1]. Symptoms can be present on petioles, stipules, stems, and pegs as well. Inoculum of the pathogen-causing leaf spot disease can overwinter in infected peanut residue on the soil [1]. As lesions progress in the peanut canopy, yield reductions as high as 50 to 70% can occur due to a reduction in photosynthesis in diseased leaves [1,2,3,4,5]. The incidence and severity of leaf spot are largely dependent on the temperature, relative humidity, the duration of favorable conditions, and the time of disease progression prior to harvest [1].

The progression of leaf spot disease in the Virginia–Carolina region of the US typically begins the first week of July, and, if weather conditions are favorable, risk can extend through September [2,6]. During this time, maintaining protection measures when conditions are favorable for disease progression is important to protect peanut yield. The management of leaf spot diseases in peanut includes rotation to non-susceptible host crops, cultural practices (e.g., tillage, planting pattern, plant population, irrigation, and resistant cultivars), and fungicides. Peanut is the only known host of the pathogens that cause leaf spot disease in peanut [2,7], making rotating away from peanut important for 3–4 years. Increasing the number of years between planting peanuts decreases initial leaf spot infection in the field [8]. Tillage practices can influence leaf spot infection by affecting peanut residue [9,10,11]. Irrigation can hasten pathogen activity and disease progression through elevated humidity in the peanut canopy [12].

Protecting yield with fewer fungicide inputs can be cost-effective for farmers and can be accomplished through resistant cultivars. Cultivars with resistance to leaf spot disease in peanut can decrease the risk of disease when fungicide applications are delayed due to inclement weather or logistical challenges. Partial resistance to leaf spot is expressed in some commercially available Virginia market-type cultivars [2,6]. Cultivars that show partial resistance to leaf spot contribute positively to management strategies by minimizing the yield loss of peanut, possibly reducing fungicide costs, and limiting equipment traffic in the field. The cultivar Bailey II [13] is the most popular Virginia market-type cultivar of peanut grown in the Virginia–Carolinas region, with approximately 70% of acres planted in 2024 [6]. The cultivar Emery [14] was planted on the second most acreage, followed by the cultivar Sullivan [15] in this region of the US [6]. These cultivars express various levels of resistance to leaf spot [2,6]. Mehl [16] reported effective leaf spot resistance for Sullivan when grown in southeastern Virginia.

In the Virginia–Carolinas region of the US, fungicide sprays to protect peanuts from leaf spot disease typically begin in July (~45 days after planting) at the R3 stage of peanut development [17]. The fungicide regimen often continues on a two-week basis for 5–6 sprays (~120 days after planting) during the cropping cycle [2,6]. Southern stem rot disease (caused by Athelia rolfsii Sacc.) also needs to be suppressed during this period of the cropping cycle. Fungicide cost for peanut in North Carolina can exceed USD 185/ha [14].

Resistance to fungicides to the pathogen causing late leaf spot has increased over the past two decades [2,6]. Leaf spot resistance to demethylation inhibitors (DMIs) and quinone outside inhibitor (QoI) fungicides is present in the Virginia–Carolinas region of the US [2,6,18,19]. Management strategies to reduce the likelihood that further resistance to fungicides include the following: (1) the use of resistant cultivars, (2) rotating peanut with other crops, (3) rotating fungicide with different sites of action, and (4) the co-application of fungicides with different sites of action [20]. Cultivar resistance can also contribute to fungicide resistance management because it offers an alternative method of suppression not associated with sites of action present in fungicides [6,21]. Multisite fungicides are considered low risk for resistance to develop in pathogens [22,23].

Chlorothalonil is a popular and effective fungicide used in peanuts. It is relatively low cost, and the development of resistance in leaf spot is unlikely due to the multisite mode of action [22]. However, this fungicide does not provide curative activity against disease, and growers can be at risk if applications are delayed and a 14-day schedule is not maintained [2,6,24]. This fungicide is often applied as the first and final spray in fungicide programs because it is inexpensive and aids in fungicide resistance management. However, overuse of this fungicide can increase the likelihood of Sclerotinia blight (caused by Sclerotinia minor Jagger) [25] and increased infestation by two-spotted spider mite (Tetranychus urticae Koch) [26]. Co-applying pydiflumetofen with the commercial package mixture of azoxystrobin plus benzovindiflupyr is a popular fungicide mixture for the protection of peanut from leaf spot and stem rot disease [2,6]. This combination can provide protection from disease in peanuts up to 28 days [27]. While not used extensively in North Carolina and surrounding states, the commercial package mixtures of mefentrifluconazole plus pyraclostrobin plus fluxapyroxad and bixafen plus flutriafol offer leaf spot protection with multiple sites of action [2,6,23].

Developing the most effective leaf spot management practice for peanuts that includes cultivar resistance and fungicides is important for farmers and their advisors in the Virginia–Carolinas production region of the US. To address this goal, research with peanuts was conducted in North Carolina to determine leaf spot incidence, canopy defoliation, and pod yield when fungicides and cultivars currently grown in the region are grown.

2. Materials and Methods

Research was conducted in North Carolina in 2021 and 2022 near Lewiston-Woodville (36.07° N, 77.11° W), Rocky Mount (35.90° N, 77.67° W), and Whiteville (34.41° N, 78.79° W). The Virginia market-type cultivars Bailey II, Emery, and Sullivan were planted in mid-May in rows spaced 91 cm apart in conventionally prepared, raised seedbeds. Plot size for each combination of treatment factors (e.g., cultivars and fungicide regimens) was 4 rows wide by 9 m in length. The seeding rate was designed to establish a final in-row population of 15 plants/m. Rainfall was supplemented with overhead sprinkler irrigation to ensure optimum growth and development of peanut.

Each cultivar received four different fungicide programs as well as a control treatment without any fungicides applied. Each fungicide program started at the R3 stage of peanut development [17], roughly 45 days after planting. The combination of fungicide sequence and duration of protection for each spray is referred to as fungicide regimen (

Table 1. Fungicide programs used to protect peanut yield from leaf spot disease ^a.

Days After Planting
Fungicide Regimen	45	60	75	90	105
Non-treated	-	-	-	-	-
Four-spray multiple fungicide	Chlorothalonil	Pydiflumetofen, azoxystrobin, benzovindiflupyr	-	Prothioconazole, tebuconazole	Chlorothalonil
Five-spray multiple fungicide	Chlorothalonil	Prothioconazole, tebuconazole	Mefentrifluconazole, pyraclostrobin, fluxapyroxad	Bixafen, flutriafol	Chlorothalonil
Three-spray chlorothalonil–tebuconazole	Chlorothalonil	-	Chlorothalonil, tebuconazole	-	Chlorothalonil
Five-spray chlorothalonil–tebuconazole	Chlorothalonil	Chlorothalonil, tebuconazole	Chlorothalonil, tebuconazole	Chlorothalonil, tebuconazole	Chlorothalonil

^a Chlorothalonil, pydiflumetofen, azoxystrobin plus benzovindiflupyr, mefentrifluconazole plus pyraclostrobin plus fluxapyroxad, bixafen plus flutriafol, prothioconazole plus tebuconazole, and tebuconazole are applied at 1.26 kg ai/ha, 0.05 kg ai/ha, 0.20 kg ai/ha plus 0.10 kg ai/ha, 0.13 kg ai/ha plus 0.17 kg ai/ha plus 0.084 kg ai/ha, 0.074 kg ai/ha plus 0.13 kg ai/ha, and 0.23 kg ai/ha, respectively.

). Specific fungicide regimens are presented in Table 1. Chlorothalonil (Bravo Weather Stik, Syngenta Crop Protection, Greensboro, NC, USA), pydiflumetofen (Miravis, Syngenta Crop Protection, Greensboro, NC, USA), azoxystrobin plus benzovindiflupyr (Elatus, Syngenta Crop Protection, Greensboro, NC, USA), mefentrifluconazole plus pyraclostrobin plus fluxapyroxad (Revytek, BASF Corporation, Research Triangle Park, NC, USA), bixafen plus flutriafol (Lucento, FMC Corporation, Philadelphia, PA, USA), prothioconazole plus tebuconazole (Provost Silver, Bayer CropScience, Research Triangle Park, NC, USA), and tebuconazole (Tebuzol 3.6F, UPL NA, Inc., King of Prussia, PA, USA) were applied at 1.26 kg a.i./ha, 0.05 kg a.i./ha, 0.20 kg a.i./ha plus 0.10 kg a.i./ha, 0.13 kg a.i./ha plus 0.17 kg a.i./ha plus 0.084 kg a.i./ha, 0.074 kg ai/ha plus 0.13 kg a.i./ha, and 0.23 kg a.i./ha, respectively.

Fungicides were applied using a CO₂-pressurized backpack sprayer calibrated to deliver 140 L/ha aqueous solution at 210 kPa using regular flat fan nozzles (Teejet FF 11002, Spraying Systems Co., Wheaton, IL, USA) at a ground speed of 5 km/h. Production and pest management practices other than cultivar and fungicide program were held constant across all experiments [6].

Peanut pods were dug, and vines inverted based on pod mesocarp color, a reflection of pod and kernel maturity, to optimize pod yield using a sample collected from a peanut plot without lesions and no canopy defoliation [28]. Pod maturity of all three cultivars was similar, allowing digging and vine inversion to occur on the same day for the entire experiment.

The test was designed as a split plot at each site–year combination with peanut cultivar considered the whole plot and fungicide regimens considered the sub-plot. Each treatment was replicated 4 times.

Visual estimates of percentage of leaves expressing lesions caused by leaf spot disease (referred to as leaf spot incidence) and percentage of leaflets fallen from the plant (referred to as canopy defoliation) caused by late leaf spot were recorded within 2 days and approximately 14 days before digging peanut and inverting vines 2021 and approximately 2, 7, 14, 21, and 28 days prior to digging and vine inversion in 2022. The percentage of disease incidence was rated on a 0 to 100% scale where 0 = no lesions caused by leaf spot in the canopy and 100 = all leaflets in the canopy expressing lesions. Peanut canopy defoliation was recorded on a scale of 0 to 100% where 0 = no loss of leaflets and 100 = no leaflets remaining in the canopy.

Leaf spot rating events were increased in 2022 to accommodate an area under disease progress curve (AUDPC) calculation. AUDPC associated with disease incidence and canopy defoliation was calculated in R package (Version 1.3-7) with the following expression utilizing the sum of the trapeze under the curve [29].

$$\mathrm{AUDPC}={\sum }_{i=1}^{n-1}{\displaystyle \frac{y_i+y_{i+1}}{2}}\times (t_{i+1} - t_i)$$

• y_i = Incidence or defoliation rating at the ith observation.
• t_i = Time in days after planting at the ith observation.
• n = Total number of observations.

Percentages of leaf spot incidence, canopy defoliation, AUDPC (2022 only), and pod yield were subjected to ANOVA with the GLIMMIX procedure in SAS (SAS v9.4, Cary, NC, USA) with respect to the split-plot design for cultivar and fungicide regimen [30,31]. Site-year combinations and replication were considered random effects. Cultivars and fungicide regimens were considered fixed effects. Means were separated using Tukey’s Honestly Significant Difference test at α = 0.05 when pooled over site–year combinations. Standard error of the mean was provided for all means. Pearson correlation coefficients were constructed using the CORR procedure in SAS for percent leaf spot incidence, percent canopy defoliation, AUDPC for leaf spot incidence and peanut canopy defoliation, and peanut yield at α < 0.05.

3. Results

3.1. Leaf Spot Incidence and Canopy Defoliation at 14 and 2 Days Prior to Digging

When pooled over locations and years (2021 and 2022), the main effect of cultivar was significant for leaf spot incidence 14 days before digging (p =< 0.0001). Leaf spot incidence 2 days before digging (p = 0.3367) and peanut yield (p = 0.9985) were not affected by the main effect of cultivar. Leaf spot incidence was affected by the main effect of fungicide regimen at 14 and 2 days prior to digging (p =< 0.0001). The interaction of cultivar × fungicide regimen was significant for peanut canopy defoliation at 14 and 2 days before digging (p =< 0.0001). The interaction of cultivar × fungicide regimen was not significant for leaf spot incidence at 14 or 2 days prior to digging (p = 0.0886 and p = 0.5649, respectively) or peanut yield (p = 0.0658).

Leaf spot incidence was greater for the cultivar Emery than Bailey II and Sullivan 14 days prior to digging; leaf spot incidence was the same for Bailey II and Sullivan (

Table 2. Influence of peanut cultivar on leaf spot incidence 2 and 14 days prior to peanut digging and vine inversion in 2021 and 2022 ^a.

Leaf Spot Incidence
Days Before Digging
Cultivar	14	2
	__________________________________________ % ____________________________________________
Bailey II	37 (3) b	52 (3) A
Emery	45 (3) a	55 (3) A
Sullivan	39 (3) b	52 (3) A

^a Means within a column followed by the same letter are not significantly different at α = 0.05 based on Tukey’s Honestly Significant Difference test. Data are pooled over site–year combinations and fungicide regimens. Standard error of the mean in parentheses.

). However, by 2 days prior to digging, no difference in leaf spot incidence was noted among cultivars.

Leaf spot incidence was greater at both evaluation dates when fungicides were not applied (

Table 3. Influence of fungicide regimen on leaf spot incidence and peanut canopy defoliation caused by late leaf spot disease 14 and 2 days prior to digging pods and inverting vines in 2021 and 2022 ^a.

Leaf Spot Incidence
Days Before Digging
Fungicide Regimen b,c	14	2
	_____________________________________ % ________________________________________
Non-treated control	94 (1) a	98 (1) A
Four-spray multiple fungicide	28 (2) c	41 (4) C
Five-spray multiple fungicide	18 (2) cd	34 (4) C
Three-spray chlorothalonil–tebuconazole	57 (3) b	77 (3) B
Five-spray chlorothalonil–tebuconazole	12 (2) d	14 (2) D

^a Means within a response variable followed by the same letter are not significantly different at α = 0.05 based on Tukey’s Honestly Significant Difference test. Data are pooled over site–year combinations. Standard error of the mean in parenthesis. ^b Fungicide regimens: (1) Non-treated control; (2) four-spray multiple fungicide (chlorothalonil, then pydiflumetofen plus azoxystrobin plus benzovindiflupyr, then prothioconazole plus tebuconazole, then chlorothalonil); (3) five-spray multiple fungicide (chlorothalonil, then prothioconazole plus tebuconazole, then mefentrifluconazole plus pyraclostrobin plus fluxapyroxad, then bixafen plus flutriafol, then chlorothalonil); (4) three-spray chlorothalonil–tebuconazole (chlorothalonil, then chlorothalonil plus tebuconazole, then chlorothalonil); (5) five-spray chlorothalonil–tebuconazole (chlorothalonil, then chlorothalonil plus tebuconazole, then chlorothalonil plus tebuconazole, then chlorothalonil plus tebuconazole, then chlorothalonil). ^c Chlorothalonil, pydiflumetofen, azoxystrobin plus benzovindiflupyr, mefentrifluconazole plus pyraclostrobin plus fluxapyroxad, bixafen plus flutriafol, prothioconazole plus tebuconazole, and tebuconazole are applied at 1.26 kg ai/ha, 0.05 kg ai/ha, 0.20 kg ai/ha plus 0.10 kg ai/ha, 0.13 kg ai/ha plus 0.17 kg ai/ha plus 0.084 kg ai/ha, 0.074 kg ai/ha plus 0.13 kg ai/ha, and 0.23 kg ai/ha, respectively.

). Incidence of leaf spot was lower with the four-spray multiple fungicide, five-spray multiple fungicide, and five-spray chlorothalonil–tebuconazole fungicide regimens when compared with the three-spray regimen that included chlorothalonil and tebuconazole. At 14 days prior to digging, leaf spot incidence was similar for the four-spray multiple regimen and five-spray multiple fungicide regimen. Leaf spot incidence was similar for the five-spay multiple fungicide regimen and the five-spray chlorothalonil–tebuconazole regimen. Leaf spot incidence was lower for the five-spray chlorothalonil–tebuconazole regimen than the four-spray multiple fungicide regimen. However, when evaluated closer to digging, no difference in leaf spot incidence was observed for the four-spray multiple fungicide and the five-spray multiple fungicide regimens. The five-spray chlorothalonil–tebuconazole regimen resulted in lower leaf spot incidence compared to all fungicide regimens at 2 days before digging.

At 14 days prior to digging, the use of a fungicide regimen lowered peanut canopy defoliation, regardless of the cultivar (

Table 4. Influence of the interaction of peanut cultivar and fungicide regimen on peanut canopy defoliation caused by late leaf spot disease 14 and 2 days prior to digging pods and inverting vines in 2021 and 2022 ^a.

Peanut Canopy Defoliation
	14 Days Before Digging			2 Days Before Digging
Fungicide Regimen b,c	Bailey II	Emery	Sullivan	Bailey II	Emery	Sullivan
	___________________ % ____________________			__________________ % ___________________
Non-treated control	34 (2) c	62 (2) a	49 (2) b	58 (2) A	78 (2) A	66 (2) B
Four-spray multiple fungicide	4 (1) g	5 (1) fg	5 (1) fg	6 (1) EF	9 (1) EF	7 (1) EF
Five-spray multiple fungicide	4 (1) g	7 (1) efg	5 (1) fg	7 (1) EF	7 (1) EF	5 (1) F
Three-spray chlorothalonil–tebuconazole	13 (1) ef	21 (2) d	14 (2) de	16 (1) DE	26 (1) C	19 (2) CD
Five-spray chlorothalonil–tebuconazole	2 (1) g	4 (1) g	2 (1) g	5 (1) F	4 (1) F	4 (1) F

^a Means within a rating event followed by the same letter are not significantly different at α = 0.05 based on Tukey’s Honestly Significant Difference test. Data are pooled over site–year combinations. Standard error of the mean in parentheses. ^b Fungicide regimens: (1) Non-treated control; (2) four-spray multiple fungicide (chlorothalonil, then pydiflumetofen plus azoxystrobin plus benzovindiflupyr, then prothioconazole plus tebuconazole, then chlorothalonil); (3) five-spray multiple fungicide (chlorothalonil, then prothioconazole plus tebuconazole, then mefentrifluconazole plus pyraclostrobin plus fluxapyroxad, then bixafen plus flutriafol, then chlorothalonil); (4) three-spray chlorothalonil–tebuconazole (chlorothalonil, then chlorothalonil plus tebuconazole, then chlorothalonil); (5) five-spray chlorothalonil–tebuconazole (chlorothalonil, then chlorothalonil plus tebuconazole, then chlorothalonil plus tebuconazole, then chlorothalonil plus tebuconazole, then chlorothalonil). ^c Chlorothalonil, pydiflumetofen, azoxystrobin plus benzovindiflupyr, mefentrifluconazole plus pyraclostrobin plus fluxapyroxad, bixafen plus flutriafol, prothioconazole plus tebuconazole, and tebuconazole are applied at 1.26 kg ai/ha, 0.05 kg ai/ha, 0.20 kg ai/ha plus 0.10 kg ai/ha, 0.13 kg ai/ha plus 0.17 kg ai/ha plus 0.084 kg ai/ha, 0.074 kg ai/ha plus 0.13 kg ai/ha, and 0.23 kg ai/ha, respectively.

). When fungicides were not applied, the greatest amount of peanut canopy defoliation was observed in peanut cultivar Emery followed by Sullivan and then Bailey II with the least peanut canopy defoliation. When a fungicide regimen was used, the greatest peanut canopy defoliation was noted in Emery or Sullivan in the three-spray chlorothalonil–tebuconazole regimen. When Sullivan was applied with the three-spray chlorothalonil–tebuconazole regimen, it was similar to Bailey II with the three-spray chlorothalonil–tebuconazole regimen and Emery with the five-spray multiple fungicide regimen. When Bailey II was applied with the three-spray chlorothalonil–tebuconazole regimen, it had similar results to when Emery or Sullivan were sprayed with the four- or five-spray multiple fungicide regimens.

At 2 days prior to digging, when pooled over site–year combination, peanut canopy defoliation was greatest in the absence of fungicides; Emery resulted in greater peanut canopy defoliation compared to Bailey II and Sullivan, which were similar (Table 4). Similar to 14 days prior to digging, peanut canopy defoliation was lowest in the four-spray multiple fungicide regimen and both of the five-spray fungicide regimens, regardless of the peanut cultivar at 2 days prior to digging. Within the three-spray chlorothalonil–tebuconazole regimen, Bailey II resulted in less defoliation compared to Emery; no difference in defoliation was noted when comparing Sullivan to Emery or Sullivan to Bailey II. Emery and Sullivan with the three-spray chlorothalonil–tebuconazole regimen resulted in greater peanut canopy defoliation when a fungicide regimen was applied. The three-spray chlorothalonil–tebuconazole regimen on Bailey II resulted in similar defoliation to the four-spray regimen regardless of cultivar and the five-spray multiple fungicide regimen with Bailey II and Emery.

3.2. AUDPC for Leaf Spot Incidence in 2022

The interaction of cultivar × fungicide regimen was significant for the AUDPC for leaf spot incidence (p = 0.0042). When fungicides were not applied, greater leaf spot incidence was noted with all peanut cultivars compared to the use of fungicides irrespective of the regimen (

Table 5. Influence of the interaction of peanut cultivar and fungicide regimen on area under the disease progress curve for leaf spot incidence and peanut canopy defoliation in 2022 ^a.

	Leaf Spot Incidence			Peanut Canopy Defoliation
Fungicide Regimen b,c	Bailey II	Emery	Sullivan	Bailey II	Emery	Sullivan
	___________ disease-days ___________			____________ disease-days _______________
Non-treated control	2118 (12) b	2510 (7) a	2286 (15) ab	611 (15) C	1121 (16) A	878 (24) B
Four-spray multiple fungicide	508 (14) fg	684 (19) ef	556 (10) fg	89 (4) EF	92 (3) EF	111 (4) EF
Five-spray multiple fungicide	420 (8) fg	490 (11) fg	455 (5) fg	90 (4) EF	111 (5) EF	108 (2) EF
Three-spray chlorothalonil–tebuconazole	867 (10) de	1164 (8) cd	1222 (14) c	190 (5) DEF	318 (8) D	249 (8) DE
Five-spray chlorothalonil–tebuconazole	384 (17) g	373 (11) g	430 (10) fg	70 (4) F	92 (6) EF	50 (3) F

^a Means within a response variable followed by the same letter are not significantly different at α = 0.05 based on Tukey’s Honestly Significant Difference test. Data are pooled over site–year combinations. Standard error of the mean in parentheses. ^b Fungicide regimens: (1) Non-treated control; (2) four-spray multiple fungicide (chlorothalonil, then pydiflumetofen plus azoxystrobin plus benzovindiflupyr, then prothioconazole plus tebuconazole, then chlorothalonil); (3) five-spray multiple fungicide (chlorothalonil, then prothioconazole plus tebuconazole, then mefentrifluconazole plus pyraclostrobin plus fluxapyroxad, then bixafen plus flutriafol, then chlorothalonil); (4) three-spray chlorothalonil–tebuconazole (chlorothalonil, then chlorothalonil plus tebuconazole, then chlorothalonil); (5) five-spray chlorothalonil–tebuconazole (chlorothalonil, then chlorothalonil plus tebuconazole, then chlorothalonil plus tebuconazole, then chlorothalonil plus tebuconazole, then chlorothalonil). ^c Chlorothalonil, pydiflumetofen, azoxystrobin plus benzovindiflupyr, mefentrifluconazole plus pyraclostrobin plus fluxapyroxad, bixafen plus flutriafol, prothioconazole plus tebuconazole, and tebuconazole are applied at 1.26 kg ai/ha, 0.05 kg ai/ha, 0.20 kg ai/ha plus 0.10 kg ai/ha, 0.13 kg ai/ha plus 0.17 kg ai/ha plus 0.084 kg ai/ha, 0.074 kg ai/ha plus 0.13 kg ai/ha, and 0.23 kg ai/ha, respectively.

). When fungicides were not applied, Bailey II expressed lower leaf spot incidence than Emery; no difference was noted between Sullivan and the other two cultivars. When treated with fungicides, the greatest leaf spot incidence was observed on Emery and Sullivan in the three-spray chlorothalonil–tebuconazole regimen; within this regimen, leaf spot incidence was lower in Bailey II compared to Sullivan but similar to Emery. Sullivan and Bailey II applied with the four-spray multiple fungicide regimen, and all peanut cultivars applied with either of the five-spray fungicide regimens resulted in the lowest disease incidence. Emery in the four-spray multiple fungicide regimen had greater leaf spot incidence compared to Bailey II and Emery in the five-spray chlorothalonil–tebuconazole regimen.

3.3. AUDPC for Canopy Defoliation in 2022

Similar to the AUDPC for leaf spot incidence, the interaction of cultivar × fungicide regimen was significant for canopy defoliation (p =< 0.0001). Regardless of the peanut cultivar, peanut canopy defoliation was lower when fungicides were applied compared to non-treated peanut (Table 5). In the absence of fungicides, Bailey II had the lowest peanut canopy defoliation, followed by Sullivan, and then Emery. Within the three-spray chlorothalonil–tebuconazole regimen, peanut canopy defoliation was similar across all peanut cultivars. When the three-spray chlorothalonil–tebuconazole regimen was used with Bailey II, peanut canopy defoliation was comparable to the four-spray multiple fungicide and the five-spray regimens. Higher peanut canopy defoliation was noted when Emery was used with the three-spray chlorothalonil–tebuconazole regimen compared to the four-spray multiple fungicide and the five-spray regimens. Across all peanut cultivars, the five-spray chlorothalonil–tebuconazole regimen resulted in lower peanut canopy defoliation compared to Emery in the three-spray chlorothalonil–tebuconazole regimen. Sullivan applied with the three-spray chlorothalonil–tebuconazole regimens had similar results to Emery with the five-spray chlorothalonil–tebuconazole regimen.

3.4. Peanut Yield

Peanut yield was affected by the main effect of fungicide regimen (p =< 0.0001) but not by the main effect of cultivar (p = 0.9985) or the interaction of cultivar × fungicide regimen (p = 0.0658). Peanut yield for the cultivars Bailey II, Emery, and Sullivan was 4990 kg/ha (SE = 81 kg/ha), 4980 kg/ha (SE = 90 kg/ha), and 4990 kg/ha (SE = 87), respectively. Fungicides protected yield compared with non-treated peanut regardless of the fungicide regimen (

Table 6. Influence of fungicide regimen on peanut yield in 2021 and 2022 ^a.

Fungicide Regimen b,c	Peanut Yield
	kg/ha
Non-treated	4410 (144) c
Four-spray multiple fungicide	5100 (126) ab
Five-spray multiple fungicide	5020 (117) b
Three-spray chlorothalonil–tebuconazole	5000 (114) b
Five-spray chlorothalonil–tebuconazole	5390 (112) a

^a Means within a column followed by the same letter are not significantly different at α = 0.05 based on Tukey’s Honestly Significant Difference test. Data are pooled over site–year combinations and cultivars. Standard error of the mean in parentheses. ^b Fungicide regimens: (1) Non-treated control; (2) four-spray multiple fungicide (chlorothalonil, then pydiflumetofen plus azoxystrobin plus benzovindiflupyr, then prothioconazole plus tebuconazole, then chlorothalonil); (3) five-spray multiple fungicide (chlorothalonil, then prothioconazole plus tebuconazole, then mefentrifluconazole plus pyraclostrobin plus fluxapyroxad, then plus bixafen and flutriafol then chlorothalonil); (4) three-spray chlorothalonil–tebuconazole (chlorothalonil, then chlorothalonil plus tebuconazole, then chlorothalonil); (5) five-spray chlorothalonil–tebuconazole (chlorothalonil, then chlorothalonil plus tebuconazole, then chlorothalonil plus tebuconazole, then chlorothalonil plus tebuconazole, then chlorothalonil). ^c Chlorothalonil, pydiflumetofen, azoxystrobin plus benzovindiflupyr, mefentrifluconazole plus pyraclostrobin plus fluxapyroxad, bixafen plus flutriafol, prothioconazole plus tebuconazole, and tebuconazole are applied at 1.26 kg ai/ha, 0.05 kg ai/ha, 0.20 kg ai/ha plus 0.10 kg ai/ha, 0.13 kg ai/ha plus 0.17 kg ai/ha plus 0.084 kg ai/ha, 0.074 kg ai/ha plus 0.13 kg ai/ha, and 0.23 kg ai/ha, respectively.

). When pooled over cultivars and site-year combinations, peanut yield was similar following the four-spray multiple fungicide, five-spray multiple fungicide, and the three-spray chlorothalonil–tebuconazole regimen. Additionally, no difference in peanut yield was observed when comparing the four-spray multiple fungicide regimen with the regimen including five sprays of chlorothalonil alone or with tebuconazole. The five-spray regimen with chlorothalonil alone or with tebuconazole applied bi-weekly resulted in greater yields than the five-spray multiple fungicide regimen and the regimen with only three sprays of chlorothalonil during the cropping cycle.

3.5. Pearson Correlations for Disease Incidence, AUDPC, Canopy Defoliation, and Peanut Yield

Pearson correlation coefficients for leaf spot incidence and peanut yield were significant for recordings at 14 and 2 days prior to peanut digging and vine inversion (p =< 0.0001) (

Table 7. Pearson correlation coefficients constructed for leaf spot incidence (2021 and 2022), percent canopy defoliation (2021 and 2022), and AUDPC for leaf spot incidence and peanut canopy defoliation (2022) with peanut yield ^a.

Correlation	P > F	R
Percent disease incidence 14 days prior to digging vs. peanut yield	<0.0001	−0.32
Percent disease incidence 2 days prior to digging vs. peanut yield	<0.0001	−0.36
Percent peanut canopy defoliation 14 days prior to digging vs. peanut yield	<0.0001	−0.35
Percent peanut canopy defoliation 2 days prior to digging vs. peanut yield	<0.0001	−0.25
Percent disease incidence for AUDPC vs. peanut yield	<0.0001	−0.29
Percent peanut canopy defoliation for AUDPC vs. peanut yield	<0.0001	−0.32

^a Data are pooled over site–year combinations.

). Peanut canopy defoliation and peanut yield correlation coefficients were significant for ratings at 14 and 2 days prior to digging (p =< 0.0001). Pearson correlation coefficients for the AUDPC for disease incidence and peanut canopy defoliation with peanut yield were significant (p =< 0.0001). No single observation or calculation (e.g., incidence of leaf spot, timing of the observation relative to digging pods and inverting vines, or the AUDPC) explained peanut yield loss more effectively than the other observations or calculations. The range of R values was from −0.25 to −0.36 (Table 7).

4. Discussion

Results from these experiments were not unexpected given that peanut was planted annually at these locations, and the rotation sequence between peanut in the crop sequence does not exceed three years [6]. Leaf spot disease can be higher when fungicides are either not applied or when the interval between fungicide sprays exceeds the recommended interval [6]. The fungicides used in the regimens with four or five sprays have been shown to be effective in suppressing leaf spot disease when applied in a timely manner [6]. Monfort et al. [10] reported that extended intervals of chlorothalonil, tebuconazole, or azoxystrobin sprays resulted in greater leaf spot progression compared to the standard bi-weekly applications. A similar response was observed in our study for the three-spray regimen of chlorothalonil, followed by chlorothalonil plus tebuconazole, followed by chlorothalonil when 28 days separated each fungicide spray.

Woodward et al. [5] reported increased leaf spot disease in a reduced fungicide regimen compared to disease in a fungicide regimen that provided protection during the time when peanut was at risk for leaf spot disease. Woodard et al. [5] also reported a difference in cultivar resistance to leaf spot disease. However, Woodard et al. [5] evaluated runner market-type cultivars in a different region of the US. None of the cultivars that they tested were included in our study. Kaur et al. [32] reported that leaf spot-resistant peanut cultivars yielded greater in the presence of leaf spot disease compared with cultivars expressing greater sensitivity to disease. They also reported that peanut yield was often greater for cultivars susceptible to leaf spot disease when the incidence of this disease was low and not yield-limiting. Yield response to fungicides was often greater for susceptible cultivars than for leaf spot-resistant cultivars. Similar to Woodard et al. [5], Kaur et al. [32] evaluated cultivars not used in our study in a different geography.

The cultivar Bailey II is less susceptible to leaf spot disease than Emery [6]. Bailey II expressed less disease progress than Sullivan in some but not all instances. However, these differences in leaf spot incidence and canopy defoliation did not translate into differences in peanut yield when comparing cultivars. The fungicide regimen that included chlorothalonil applied five times, either alone or with tebuconazole, was equal or more effective in protecting peanut yield than other fungicide regimens. However, Porter [25] demonstrated that multiple applications of chlorothalonil can increase Sclerotinia blight disease. With respect to leaf spot disease and the effectiveness of five sprays of chlorothalonil without or with tebuconazole, applications of fungicides in this study were made on a consistent and defined schedule because of the nature of small-plot research. It is possible that growers experiencing logistical challenges or weather conditions that delay applications of chlorothalonil, or chlorothalonil plus tebuconazole would have negative impacts on protection from leaf spot disease. Chlorothalonil is a protectant fungicide with no curative action, and tebuconazole is ineffective in suppressing leaf spot disease at these locations in North Carolina due to evolved resistance [6].

Two of the fungicide regimens included fungicides that more recently entered the market for farmers to use in their peanut production systems compared with chlorothalonil and tebuconazole. The four-spray and five-spray regimens offer advantages over the chlorothalonil–tebuconazole regimen with five sprays. First, while chlorothalonil is a multisite fungicide not prone to developing resistance to leaf spot disease, there is curative activity offered by prothioconazole plus tebuconazole used in both of the alternative regimens (e.g., four or five fungicide sprays). Including fungicides in the regimen with curative activity would be valuable for farmers when the schedule of fungicide applications is altered due to logistical constraints caused by equipment issues or weather patterns. Thirdly, the value of regimens with a more diverse array of fungicides could provide more effective protection from other economically important diseases in peanuts. The combination of pydiflumetofen plus azoxystrobin plus benzovindiflupyr suppresses Sclerotinia blight as effectively as the historical commercial standard fluazinam [33]. Fluazinam does not control leaf spot disease in peanut [33], while the combination of pydiflumetofen plus azoxystrobin plus benzovindiflupyr protects peanut from leaf spot disease, southern stem rot, and Sclerotinia blight [2,6,33]. The regimen with this fungicide combination also has a greater length of protection from leaf spot disease than the five-spray chlorothalonil–tebuconazole regimen (e.g., 28 days versus 14 days between spray 2 and spray 3 in the regimen) [34]. The five-spray program with a greater diversity of fungicide sites of action than the chlorothalonil–tebuconazole regimen includes a degree of curative activity and the potential for fungicide resistance management. Finally, while the five-spray regimen of chlorothalonil alone or with tebuconazole was effective in protecting peanuts from leaf spot disease and southern stem rot [2], the excessive use of chlorothalonil can exacerbate issues with Sclerotinia blight in fields where this disease is present [6,25]. Lux et al. [35] reported greater Sclerotinia blight incidence and lower peanut yield when a five-spray chlorothalonil regimen without or with tebuconazole was used compared with more diverse fungicide regimens with fewer chlorothalonil sprays similar to those used in the current study.

5. Conclusions

Results from this research provide information on the risk associated with various fungicide regimes for Virginia market-type cultivars grown in the Virginia–Carolinas production region of the US. Even though differences in disease incidence of leaf spot disease, canopy defoliation caused by leaf spot disease, and the AUDPC were noted when comparing cultivars, peanut yield did not differ based on cultivar. This suggests that practitioners can plant these cultivars and expect a similar yield response across environments and fungicide inputs. Practitioners can also expect a similar response to the fungicide regimens used in the current study when these cultivars are grown. Additional research is needed to determine the response of these cultivars to other fungicide regimens and disease other than leaf spot.

The relatively minor differences in leaf spot resistance observed in our study by these cultivars and subsequent lack of peanut yield response most likely reflect improvements in cultivar releases from the North Carolina State University genetics and breeding program [6,36]. The cultivars Bailey II, Emery, and Sullivan contain significant disease-resistant traits compared with previous cultivar releases [6,36]. Differences in these three cultivars reflect demands from consumers for food products including market-grade characteristics while expressing varying degrees of resistance to leaf spot disease [6,36].