Weed and Disease Control and Peanut Response Following Post—Emergence Herbicide and Fungicide Combinations

3.1.1. Weed efficacy studies

Weed efficacy studies were divided into two groups: 1) a grass herbicide study and 2) a broadleaf weed study. The grass herbicide study included clethodim at 0.14 kg ai/ha or sethoxydim at 0.21 kg ai/ha while the broadleaf weed study included aciflurofen at 0.42 kg ai/ha, imazapic at 0.07 kg ai/ha, imazethapyr at 0.07 kg ai/ha, lactofen at 0.22 kg ai/ha, or 2,4-DB at 0.42 kg ai/ha. Fungicides evaluated included pyraclostrobin at 0.27 kg ai/ha, tebuconazole at 0.23 kg ai/ha, and the premix of prothioconazole at 0.084 kg ai/ha plus tebuconazole at 0.168 kg ai/ha.

Herbicides and fungicides were applied alone and in combination to determine efficacy against various weeds. A crop oil concentrate (Agri-Dex, a blend of 83% paraffin-based petroleum oil and 17% surfactant) at 2.3 L/ha was added to each treatment except in 2007 at Yoakum where a non-ionic surfactant (X-77, 90% nonionic surfactant) at 0.25% v/v was added. Herbicide and fungicides at Yoakum were applied with a CO₂-pressurized backpack sprayer equipped with TeeJet 11002 DG flat fan spray tips (Spraying Systems Company, P.O. Box 7900, North Avenue, Wheaton, IL 60188) that delivered a spray volume of 190 L/ha at 180 kPa while on the Texas High Plains locations, fungicides and herbicides were applied with a CO₂ pressurized backpack sprayer using TeeJet 110015 TT flat fan nozzles calibrated to deliver a spray volume of 94 L/ha at 207 kPa. At Yoakum, the peanut variety Tamrun OL02 [68] was planted in each year at a seeding rate of 112 kg/ha. At the Texas High Plains locations, Flavor Runner 458 [69] was planted at the rate of 100 kg/ha.

Texas millet and southern crabgrass were present at Yoakum in 2007 and 2009 while broadleaf signalgrass was present in 2008. Texas millet was present at Lamesa in 2007. Palmer amaranth was present at Yoakum in 2007, 2008, and 2009, Lamesa in 2007, and Halfway in 2008 and 2009. Smellmelon (Cucumis meloL. var. Dudaim Naud.) was present at Yoakum in 2007, 2008, and 2009 while horse purslane (Trianthema portulacastrumL.) was present at Yoakum only in 2009. When present, all field plots were naturally infested with dense populations of Texas millet and broadleaf signalgrass at 4 to 6 plants/m², southern crabgrass at 6 to 8 plants/m², horse purslane at 6 to 8 plants/m², smellmelon at 6 to 8 plants/m², or Palmer amaranth at 4 to 6 plants/m². Typically, treatments were applied when annual grasses were 10 to 26 cm tall, Palmer amaranth was 15 to 30 cm tall, horse purslane was 10 to 20 cm tall, and smellmelon was 15 to 30 cm in length. No attempt was made to harvest peanut in the efficacy studies due to the difficulty in digging weedy plots [10,13,17].

3.1.2. Weed-free studies

Studies also were conducted under weed-free conditions at the Lamesa and Halfway in 2008 and 2009. Plots were maintained weed-free with ethalfluralin (Sonalan HFP®, Dow AgroSciences, 9330 Zionsville Road, Indianapolis, IN 46268) at 0.84 kg/ha applied preplant incorporated. At Lamesa, Flavor Runner 458 was planted in 2008 while Tamrun OL02 was planted in 2009; at Halfway, the Spanish market type, OLin [70] was planted both years of the study. Seeding rate for the runner market cultivars (Flavor Runner 458, Tamrun OL02) was 90 kg/ha while OLin was planted at 100 kg/ha. Peanut phytotoxicity ratings were recorded throughout the growing season and peanut yield was obtained by digging each plot separately, air-drying in the field for 4 to 7 days, and harvesting pods from each plot with a combine. Weights were recorded after soil and trash were removed from plot samples were adjusted to 10% moisture.

Weed control and peanut phytotoxicity, expressed as chlorosis and necrosis of leaf tissue, was visually estimated on a scale of 0 to 100 (0 indicating no weed kill or leaf chlorosis or necrosis and 100 indicating complete weed or peanut kill), relative to the non-treated control. Weed control was recorded approximately four weeks after POST herbicide applications while peanut phytotoxicity was recorded 5 to 14 days after herbicide application.

3.1.3. Data analysis

Weed control and peanut injury data were transformed to the arcsine square root prior to analysis of variance, but are expressed in their original form for clarity because the transformation did not alter interpretation. Visual estimates of weed control and peanut injury, and yield were subjected to analysis of variance to test effects of POST herbicide and fungicide. Means were compared with the appropriate Fisher’s Protected LSD test at the 5% probability level. The non-treated was not included in weed efficacy or peanut injury analysis but was included in peanut yield analysis.

3.2.1. Disease efficacy studies

Studies in south Texas were conducted from 2008 to 2010 on early leaf spot and southern blight. These studies included the fungicides pyraclostrobin at 0.27 kg ai/ha, tebuconazole at 0.23 kg ai/ha, and the premix of prothioconazole at 0.084 kg ai/ha plus tebuconazole at 0.168 kg ai/ha and the herbicides aciflurofen at 0.42 kg ai/ha, clethodim at 0.14 kg ai/ha, imazapic at 0.07 kg ai/ha, imazethapyr at 0.07 kg ai/ha, lactofen at 0.22 kg ai/ha, sethoxydim at 0.21 kg ai/ha, or 2,4-DB at 0.42 kg ai/ha. Fungicides and herbicides were applied alone and in combination to determine efficacy against foliar and soilborne diseases. No adjuvant was included in these studies in 2008 or 2009; however, in 2010 a crop oil concentrate (Agri-Dex, a blend of 83% paraffin-based petroleum oil and 17% surfactant) at 2.3 L/ha was added to each treatment.

Fungicides and herbicides alone and in combination were applied with a CO2-pressurized backpack sprayer equipped with three D2-23 hollow-cone spray nozzles per row in 140 L of water/ha at a pressure of 504 kPa. The experimental design was a randomized complete block with a factorial arrangement of seven herbicides by three fungicides. All studies included a non-treated control. Each plot consisted of four rows spaced 97 cm apart and 6.3 m long. The variety Tamrun OL02 [68] was planted in 2008 and 2009 while Florida 07 [71] was planted in 2010 at the rate of 112 kg/ha. Planting dates were June 16, 2008, July 1, 2009, and May 24, 2010.

Studies conducted in central Texas focused on early leaf spot and Sclerotinia blight caused by Sclerotina minorJagger. These studies included the herbicides clethodim at 0.14 kg ai/ha and sethoxydim at 0.21 kg ai/ha and the fungicides boscalid at 0.49 kg ai/ha and fluazinam at 0.88 kg ai/ha. Agridex at 1.0% v/v was included in all treatments. Each plot consisted of two rows spaced 91 cm apart and 7.9 m long. Fungicides and herbicides were applied alone and in combination with a CO2-pressurized backpack sprayer equipped with two 8002VS flat fan spray nozzles per row in 140 L of water/ha at a pressure of 134 kPa. The runner-type variety Flavor Runner 458 [69] was planted each year of the study at 95 kg/ha.

Typical peanut injury resulted in rapid damage to plant tissue after application and manifested as small necrotic lesions. The visible injury on leaflets with 2,4-DB was common and consisted of typical 2,4-DB damage which consisted of elongated leaflets with a slightly faded appearance [10]. This symptomology was not visible on new growth and remained visible on lower leaves throughout the growing season. Peanut phytotoxicity ratings were recorded 7 days after treatment at Yoakum. Peanut injury was estimated visually on a scale of 0 to 100 (0 indicating no leaf chlorosis or necrosis and 100 indicating complete peanut kill), relative to the non-treated control. Severity of leaf spot was rated in the center two rows using the Florida leaf spot scoring system where 1 = no leaf spot, and 10 = plants completely defoliated and dead because of leaf spot [49,59]. Values of 1 through 4 on the scale reflect increasing incidence of leaflets with spots, and occurrence of spots in lower versus upper canopy of the plots; whereas values 4 through 10 reflect increasing levels of defoliation [51]. The leaf spot rating was recorded immediately prior to peanut digging.

Loci of southern stem rot or Sclerotinia blight (where applicable) were counted immediately after peanut plants were inverted. A locus represented 31 cm or less of linear row with one or more plants infected with S. rolfsiior S. minor[72]. Plots were harvested in south Texas in 2008 and 2010, but not in 2009 due to extremely wet conditions which persisted during late October and November and prevented digging of individual plots (Table 1). Plots were harvested in 2008 and 2009 in central Texas.

All test areas were maintained weed-free with a preemergence tank-mix application of pendimethalin at 1.06 kg ai/ha plus S-metolachlor at 1.42 kg ai/ha. Overhead sprinkler irrigation was applied on a 1- to 2-week schedule throughout the growing season as needed.

3.2.2. Data analysis

Peanut yields were obtained by digging each plot separately, air-drying in the field for 4 to 7 days, and harvesting pods from each plot with a combine. Weights were recorded after soil and trash were removed from plot samples were adjusted to 10% moisture. Leaf spot ratings and incidence of soilborne disease development were used for comparison of tank-mix combinations. Data were analyzed using PROC GLM with SAS (SAS Institute, Inc., Cary, NC) and a model statement appropriate for a factorial design. Treatments means were separated by Fisher’s protected least significant difference test at P≤0.05.

4.1.1. Annual grass control

No differences in broadleaf signalgrass, Texas millet, or southern crabgrass control were noted between clethodim or sethoxydim when applied alone or in combination with any of the fungicides (Table 2). Grichar [73] reported that clethodim and sethoxydim controlled 3 to 10 cm tall Texas millet and southern crabgrass at least 85%. Clethodim applied to 15 to 25 cm tall Texas millet or southern crabgrass provided no better than 89% Texas millet control while southern crabgrass control varied from 51 to 95% [73]. Sethoxydim applied to the same height Texas millet or southern crabgrass controlled Texas millet 79 to 87% and southern crabgrass control was no better than 76% [73].

Lancaster et al. [8,9] reported large crabgrass control was reduced with clethodim when applied with pyraclostrobin, chlorothalonil, and azoxystrobin; however, fluazinam, propiconazole plus trifloxystrobin, and tebuconazole did not reduce large crabgrass control by clethodim. Similarly, Jordan et al. [66] reported that azoxystrobin and chlorothalonil, but not tebuconazole, reduced annual grass control by clethodim. Also Lancaster et al. [8,9] reported that large crabgrass control was reduced when sethoxydim was applied with azoxystrobin or pyraclostrobin, but not fluazinam, propiconazole plus trifloxystrobin, or tebuconazole.

4.1.2. Palmer amaranth control

At Yoakum in 2007 and 2008 and Halfway in 2008 there was an herbicide by fungicide interaction; therefore, those data are presented as a 2-way interaction of broadleaf herbicide by fungicide (Table 3). However, only herbicide effects were significant at Lamesa in 2007 and Halfway and Yoakum in 2009 (Table 4).

In 2007 at Yoakum, lactofen, aciflurofen, and imazapic alone controlled Palmer amaranth at least 91% while 2,4-DB and imazethapyr alone provided 83% and 68% control, respectively (Table 3). Lactofen plus tebuconazole and aciflurofen plus either the premix of prothioconazole plus tebuconazole or tebuconazole reduced Palmer amaranth control over each respective herbicide applied alone. In 2008 at Yoakum, all herbicides alone controlled Palmer amaranth at least 92%. Reduced control from each respective herbicide alone was noted with acifluorfen plus either pyraclostrobin or tebuconazole and imazethapyr or 2,4-DB plus pyraclostrobin. At the Halfway location, lactofen and aciflurofen alone provided poor control (≤ 25%) of Palmer amaranth while imazethapyr, imazapic, and 2,4-DB controlled Palmer amaranth at least 77% (Table 3). Only the combination of 2,4-DB plus the premix of prothioconazole plus tebuconazole reduced control when compared to 2,4-DB alone.

At Lamesa, all herbicides controlled Palmer amaranth less than 60% while at Yoakum there was no difference in Palmer amaranth control following all herbicide treatments (Table 4). At the Halfway location, lactofen, imazapic, and imazethapyr controlled Palmer amaranth at least 98% while 2,4-DB and aciflurofen controlled this weed 75% and 54%, respectively.

Grichar [74] reported that imazapic at 0.04 to 0.07 kg/ha controlled Palmer amaranth at least 95% when applied to weeds that were less than 15 cm tall while imazethapyr provided at least 90% control in 2 of 3 years. In other research, Jordan et al. [66] reported that smooth pigweed (A. hybridusL.) control by imazethapyr was reduced by tank mixing with fungicides.

4.1.3. Horse purslane control

There was an herbicide by fungicide interaction for horse purslane in 2009. Lactofen and 2,4-DB alone and in combination with fungicides provided almost complete control of horse purslane (Table 3). Aciflurofen alone controlled 97% horse purslane while antagonism was noted with acifluorfen plus the premix of prothioconazole plus tebuconazole combinations. All imazethapyr plus fungicide combinations reduced horse purslane control compared to imazethapyr alone. Imazapic alone or in combination failed to control horse purslane.

Horse purslane can be a stronger competitor with peanut early in the growing season than common purslane due to a more upright growth than that of common purslane [75]. Grichar [75] reported that aciflurofen and lactofen alone or combinations of these herbicides with 2,4-DB controlled horse purslane at least 70% when evaluated 21 days after treatment (DAT), but no greater than 75% control was observed when rated up to 115 DAT. In later work, Grichar [76] reported that in one year, lactofen applied to horse purslane less than 15 cm tall controlled this weed 93% while in another year, lactofen applied to horse purslane less than 15 cm tall or 20 to 30 cm tall provided at least 93% control while acifluorfen applied to horse purslane less than 15 cm tall controlled this weed 77%.

4.1.4. Smellmelon control

Only herbicides were significant for smellmelon control (Table 4). No difference in smellmelon control was noted with any herbicides in 2007, while in 2008 imazethapyr produced the worst control. In 2009, lactofen controlled less smellmelon than imazapic. Grichar [76] reported that imazapic provided the most consistent control of smellmelon while acifluorfen, imazethapyr, imazapic, and lactofen controlled at least 80% smellmelon in some years but in other years control was less than 70%. Imazapic at 0.04 to 0.07 kg/ha controlled smellmelon greater than 90% in corn (Zea maysL.) regardless whether applied PRE, early POST, or late POST [77]. Grichar [78] reported that imazapic provided consistent control (> 85%) of citronmelon (Citrullus lanatusvar. citroides) in peanut. Typically, season-long smellmelon control with 2,4-DB is poor. This can be attributed to lack of any residual activity of 2,4-DB and continued germination of seed and smellmelon growth [76].

4.1.5. Peanut phytotoxicity with tank mix combinations

4.1.6. Peanut yield as influenced by tank mix combinations

Under weed-free conditions, when using either the grass or broadleaf herbicides with fungicides, no negative response with respect to peanut yield was noted when compared with the non-treated control for either runner or Spanish market types (data not shown). Most studies conducted on herbicide-fungicide interactions on peanut have focused on either weed efficacy or disease control and few have reported on effect on peanut yield. No studies could be found that reported any peanut yield reductions with clethodim or sethoxydim under weed-free conditions. Although lactofen at 0.22 kg/ha caused peanut leaf bronzing and spotting [74], lactofen produced a similar yield when compared to the untreated, weed-free control [79]. Richburg et al. [80] reported no yield differences with runner, Spanish, or Virginia peanut cultivars with imazethapyr at 0.07 kg/ha in Georgia or Texas. No reduction in peanut grade or yield following imazapic treatments have been observed in several studies [76,81,82]. Grichar et al. [83] reported that single and multiple applications of 2,4-DB at 0.45 kg/ha did not affect runner-type yield.

5.1.1. Early leaf spot control in South Texas

There was an herbicide by fungicide interaction for early leaf spot control in 2008 and 2009. In 2010, the main plots of herbicide and fungicide were significant for early leaf spot control; therefore, that data were averaged over herbicides and fungicides only. Foliar disease development was moderate in 2008 due to extreme drought and hot conditions that persisted throughout the 2008 and the early portion of the 2009 growing seasons. Typically, early leaf spot epidemics are favored by temperatures of approximately 16 to 250 C and long periods of high relative humidity are required for infections to occur [84]. All herbicides alone, with the exception of sethoxydim and lactofen, were not different from the non-treated control with respect to early leaf spot development in 2008 (Table 8). All fungicides alone or in combination with any of the herbicides produced leaf spot levels that were less than the non-treated control. When individual fungicides were compared with the respective fungicide plus herbicide treatments some differences were noted. Pyraclostrobin alone resulted in less early leaf spot than pyraclostrobin plus either imazapic, lactofen, or sethoxydim. No differences were noted between tebuconazole alone or in combination with any herbicide. Prothioconazole plus tebuconazole alone resulted in less early leaf spot than prothioconazole plus tebuconazole in combination with acifluorfen (Table 8).

Although early-season rainfall was below normal in 2009, September rainfall was above normal leading to conditions for late-season development of high levels of foliar diseases. No differences were noted between the non-treated control and any herbicide with respect to early leaf spot control (Table 8). All fungicides alone or in combination with herbicides resulted in less early leaf spot than the non-treated control. When fungicides were compared alone or in combination, pyraclostrobin alone resulted in less early leaf spot than the combination of pyraclostrobin plus imazapic while tebuconazole alone resulted in less leaf spot than tebuconazole plus either imazapic or aciflurofen. No differences were noted between prothioconazole plus tebuconazole alone or in combination with any herbicides.

Weather conditions in 2010 were conducive for development of early leaf spot (Table 7). When herbicides were compared, aciflurofen, imazethapyr, and lactofen resulted in greater early leaf spot than where no herbicide was used (Table 9). All fungicides resulted in less early leaf spot than where no fungicide was used.

5.1.2. Early leaf spot control in central Texas

Early leaf spot data was collected only in 2008 and neither fungicide nor herbicide effects were significant. Due to dry conditions, early leaf spot pressure was moderate and there were no differences with any factors (Table 10). Management of early and late leaf spot of peanut is essential for peanut production in most areas of the world [59]. In the southeastern United States, control of these diseases is heavily reliant upon multiple fungicide applications [59,84] while far fewer applications are necessary in the southwestern United States [53,56,85].

5.1.3. Southern blight control

Control of southern blight was not significant for any factor in 2008; however, in 2010 there was a fungicide by herbicide interaction. Since peanut were not dug in 2009, no southern blight ratings were taken. In 2008, no differences were noted with respect to development of southern blight (Table 8). In 2010, under low to moderate pressure, sethoxydim alone produced the highest levels of southern blight with over 85% disease incidence (Table 9). No differences were noted between fungicides alone or the combinations of a fungicide with a herbicide.

5.1.4.Sclerotinia blight control

Sclerotinia blight control was significant for both fungicides in both years; whereas herbicides did not impact disease control. Sclerotinia blight pressure was moderate to heavy in each year (Table 10). In 2008, fluazinam provided the best control of Sclerotinia blight compared with the non-treated control while both boscalid and fluazinam reduced Sclerotinia blight compared to the non-treated control in 2009. Fluazinam has provided good to excellent disease control depending on the rate applied [86-88]. Smith et al. [89] reported in field studies that the application of boscalid or fluazinam that preceded the largest incremental increase in disease incidence provided the best control of disease or increased yield. They advised that disease advisories or intensive scouting should be used to determine when epidemics initiate so that a fungicide can be applied prior to infection.