Peanut fungicides registered in the United States grouped by mode of action
The worlds leading peanut producing countries include India, China and the United States. In 2011, approximately 444,500 hectares of peanuts were harvested in the United States (NASS, 2011); with largest production region being the southeastern states of Alabama, Florida, Georgia, and South Carolina. Production is concentrated in this region due to the semi-tropical temperate climate conditions. Unfortunately, these environmental conditions are conducive for many pests, including weeds, insects and diseases. Other production regions in the United States include the southwestern region (New Mexico, Oklahoma, and Texas), as well as the Virginia-Carolina region (North Carolina and Virginia), each of which has their own disease issues.
1.1. Peanut leaf spot
Several fungal diseases are known to affect peanut leaves. Most notibly are early and late leaf spot, caused by
Optimal environmental conditions for infection and reproduction for the two pathogens are quite similar; 16-24 °C and 20-26 °C for
Several other diseases including pepper spot, caused by
1.2. Diseases of peanut stems, roots, pegs and pods
Numerous other fungal diseases are known to affect peanut stems, roots, pegs and pods. Diseases such as stem rot (Fig. 2), Rhizoctonia limb and pod rot (Fig. 3), Pythium pod rot (Fig. 4), Cylindrocladium black (Fig. 5) rot and Sclerotinia blight (Fig. 6) are among the most difficult to manage.
Stem rot, caused by the soilborne fungus
Upon germination of sclerotia,
Rhizoctonia solani (Kühn) anastomosis group 4 (AG-4) is capable of causing seed decay, pre- and post-emergence damping-off, as well as hypocotyl and root rot; however, it is most devastating on mature plants causing a rot of pegs, pods, and stems. Although variable from year to year, Rhizoctonia limb rot is considered a major disease of peanut in the southeast (Brenneman, 1997; Thompson, 1982) and Texas (personal observation). Substantial losses due to limb and pod rot can be experienced (Kemerait, 2003). Limb rot is more severe during cool wet periods and may be exacerbated by excessive nitrogen fertility (
In addition, Pythium pod rot can also be responsible for considerable losses and frequently occurs throughout Texas, Oklahoma and North Carolina. A severe pod rot in Nicaragua was also recently determined to be caused by
Cylindrocladium black root rot, caused by the fungus
Sclerotinia blight, caused primarily by
2. Chemical management of peanut diseases
Numerous chemical fungicides are available for control of the aforementioned diseases of peanut (Table 1). Applications of these products are made for the management of both foliar and soilborne diseases. These products have traditionally been the second largest variable expense in peanut production, behind seed cost. In the United States, management tactics vary among production regions; however, multiple applications of fungicides are typically required to minimize disease-associated losses within a given growing season (Melouk and Backman, 1995; Shokes and Culbreath, 1997). In the southeastern United States, applications of fungicides are typically made on calendar-based schedule; with initial applications beginning approximately 30 days after planting (DAP) and subsequent applications made on 14-day intervals. Due to the long growing season and high disease pressure in this region, a total of six to eight applications may be warranted. Whereas, two-to-three applications may be made in more arid production regions, such as west Texas. In the Virginia/Carolina region, several weather-based spray advisories have been developed and are currently being used to properly time applications (Phipps et al., 1997).
2.1. Management of peanut leaf spot with fungicides
Copper and sulfur dusts, were among the first fungicides used in peanut production for management of foliar diseases (Smith and Littrell, 1980). Most inorganic copper and sulfur compounds are relatively insoluble, thus, preventative applications create a protectant barrier on leaf surfaces. Small quantities are absorbed by fungal spores, and accumulations result in their lethal effect. Dust formulations are no longer being utilized due to high usage rates, poor plant coverage, and the potential contamination of non-target locations (Backman et al., 1975; Backman, 1978). Some of the early liquid fungicides, such as benomyl and chlorothalonil were or are used to manage
Chlorothalonil, a broad-spectrum fungicide, is among the most effective fungicides registered for leaf spot control and has been the standard fungicide for leaf spot management since the 1970s (Smith and Litrell, 1980; Culbreath et al., 1992). Unfortunately, chlorothalonil is not active against
Brenneman and Culbreath (1994) studied various application schedules of chlorothalonil and tebuconazole for leaf spot and stem rot.. They evaluated different application schedules and found that a block of four applications of tebuconazole beginning at the third spray, reduced the severity of both foliar and soilborne diseases, and increased pod yields and kernel quality when compared to the full-season chlorothalonil program. Similar trends were observed when less than four tebuconazole applications were made (Brenneman and Culbreath, 1994). Recommendations in eastern production regions call for chlorothalonil to be added to tebuconazole due to the development of tebuconazole insensitive populations of
|A1 (4)||Phenylamide||mefenoxam or metalaxyl||Ridomil Gold EC, Ridomil Gold GR, Ridomil Gold SL||locally systemic|
|Topsin M||locally systemic|
|Respiration||C2 (7)||Carboxamide||penthiopyrad||Fontelis||locally systemic|
|flutolanil||Artisan (+ propiconazole),|
|C3 (11)||Strobilurin -|
|trifloxystrobin||Absolute (+ tebuconazole),|
Stratego (+ propiconazole)
|F1 (2)||Dicarboximide||iprodione||Rovral||locally systemic|
inhibitor - DMI
|propiconazole||Tilt, Propiconazole, Propimax, Artisan (+ flutolanil),|
Stratego (+ trifloxystrobin)
Provost (+ tebuconazole)
|tebuconazole||Folicur, Muscle, Orius, Tebuzole, Trisum,|
Absolute (+ trifloxystrobin)
|M1 (M1)||Inorganic||copper salts||Kocide, Copper-Count-N||protectant|
|M5 (M5)||Chloronitrile||chlorothalonil||Bravo, Equus, Echo||protectant|
2.2. Management of diseases caused by soilborne pathogens with fungicides
Peanut producers have more options now than ever when it comes to fungicides. While many of the products currently on the market have activity against diseases caused by both foliar and soilborne pathogens, flutolanil was registered in 1995 and is only active against
This same strategy was applied to newer fungicides, such as the ergosterol biosynthesis inhibitors as they were evaluated in peanut. Granular formulations of diniconazole and tebuconazole were examined, but results were inconsistent (Csinos, 1987). Suppression of diseases caused by soilborne pathogens was observed when liquid formulations of these compounds were applied to foliage in leaf spot studies (Backman and Crawford, 1985; Csinos et al., 1987; Brenneman and Culbreath, 1994; Besler et al., 2003). By mixing dyes with the foliar-applied fungicides and applying irrigation, Csinos (1988) documented how these materials were delivered to the soil. He demonstrated that the architecture of the peanut plant served to funnel rain or irrigation water along the stems and increase deposition of fungicides at the plant crown and pegs. This redistribution is important since these structures serve as primary infection courts for several pathogens (Melouk and Backman, 1995).
Various factors are known to affect fungicide deposition and efficacy. Differences in the leaf cuticle can influence the retention of fungicides (Neely, 1970; Neely, 1971), and changes in the composition of the cuticle have been attributed to different environmental factors (Skoss, 1955). Pesticide deposition is also greatly affected by canopy density. Researchers have found that higher levels of chlorothalonil are deposited on the upper plant canopy, compared to the lower canopy (Brenneman et al., 1990; Hamm and Clough, 1999). Zhu et al. (2004) demonstrated that spray deposits in the upper and lower peanut canopy differed significantly, and deposits in the lower canopy decreased as plants aged. The deposition and retention of chlorothalonil may differ within the peanut canopy layer and volume of water used for application (Brenneman et al., 1990). O’leary et al. (1997) found that both formulation and application method of flutolanil resulted in significant increases in chemical residues on subterranean plant parts and the lower canopy, respectively, characteristics that impacted managment of stem rot.
2.3. Improving fungicide deposition and efficacy via application method
Thorough coverage of foliage or the ability of fungicides to reach target organisms is essential in maximizing disease control. Environmental conditions such as relative humidity, wind speed, temperature and rainfall can greatly affect fungicide deposition. Changes in nozzle type, carrier volumes or pressure may also improve deposition. Application method is known to affect the deposition of fungicide by influencing penetration within the the plant canopy (Brenneman et al., 1990). Fungicides can be applied to peanut through various ground sprayers, fixed wing aerial applicators, or injected through irrigation systems (chemigation). Brenneman and Sumner (1990) reported that cholorthalonil applied via chemigation provided a similar level of leaf spot control as ground applications under low to moderate levels of disease; however, control was not sufficient with severe epidemics. Chemigation with propiconazole (Brenneman et al., 1994) or tebuconazole (Brenneman and Sumner, 1989) in place of foliar applications of chlorothalonil resulted in increased leaf spot incidence. Chemigation wets the entire leaf surface and residues may be displaced from the tissues due to the cuticle (Neely, 1970; Neely, 1971; Skoss, 1955). Johnson et al. (1986) found that only 10% of chlorothalonil applied was retained on the foliage after chemigation. Backman (1982) speculated that the displacement of PCNB and carboxin due to chemigation led to improved efficacy of stem rot in Alabama. A subsequent report evalauting tebuconazole found that Rhizoctonia limb rot was less severe where the fungicide was applied via chemigation (Brenneman and Sumner, 1989). Chemigation is permitted on several fungicide labels including azoxystrobin, metalaxyl and mefenoxam which are used predominantly for pod rot in in Texas where the majority of peanut acres are irrigated (Woodward and Black, 2007). In greenhouse studies simulating chemigation with mefenoxam, Wheeler et al. (2007) found that the chemical should be applied in an appropriate volume of water that places the fungicide at a depth where pods are developing. Higher irrigation rates led to increased concentrations at depths of 10 and 20 cm; however, excesive irrigation can leach the fungicide from the zone completely and compromise efficacy.
Fungicide penetration and deposition may also be affected by canopy density and architecture. Older peanut plants tend to have a more dense canopy, thus reducing deopsits to the lower canopy (Zhu et al., 2003). Much research has been conducted to evaluate methods of improving fungicide penetration into the lower canopy for control of soilborne diseases. The application of benomyl in conjuntion with the pruning of peanut vines increased stem rot control (Backman et al., 1975). Likewise, the application of iprodione following pruning has improved control of Sclerotinia blight (Bailey and Brune, 1997; Butzler et al., 1998). Implements designed to open the canopy have been used to concentrate fungicides near the crown area. Grichar (1995) found that use of an A-sweep boom attachment improved the efficacy of several fungicides towards stem rot. Targeting applications of fluazinam using a canopy opener allowed for reduced rates to be used in the control of Sclerotinia blight in Oklahoma (Damicone and Jackson, 2001).
More recently, Augusto et al. (2010a) found that fungicide applications made at night (when peanut leaves are folded) rather than the day (when peanut leaves are unfolded) were more effective for the control of stem rot and increased yields. While stem rot control was enhanced, incidence of early leaf spot was not affected by application timing with systemic fungicides, but protectants such as chlorothalonil were less effective for leaf spot when sprayed at night. Additional studies found that early morning applications (applied betwen 3:00 and 5:00 A.M.) of pyraclostrobin and prothioconazole plus tebuconazole decreased stem rot compared to day-time or evening (between 9:00 and 10:00 PM) applications (Augusto et al., 2010b). In that study, applications of systemic fungicides applied prior to sunrise increased yields compared to day applications. This resulted from increased spray coverage, density and droplet size in the lower canopy, as well as improved redistribution downward with movement in dew that was present in the morning applications.
2.4. Redistribution of fungicides via irrigation
Historically, suppresion of soilborne pathogens was achieved through applications of granular fungicides banded over the center of the row (Csinos, 1987). These formulations were thought to sift through the canopy ultimately arriving at the soil; however, control using these materials was costly and inconsistent. The registration of the flutolanil has provided producers with a more effective means of managing soilborne diseases (Hagan et al., 2004). Furthermore, the registration of tebuconazole and azoxystrobin, has greatly improved both stem rot and leaf spot management over the past decade (Brenneman and Culbreath, 1994; Brenneman and Murphy, 1991; Grichar et al., 2000). In contrast to granular fungicides, broadcast-spray applications of these compounds are made to peanut foliage. Fungicide deposition within the canopy contributes to efficacy for leaf spot, but the management of stem rot is more difficult since the target of spray deposition for stem rot control is at the base of the plant or even below ground (Punja, 1985). The mechanism by which foliar-applied fungicides affect stem rot is not fully understood. It is believed that initial deposits of fungicides within the upper canopy are washed on to stems and pegs at the base of the plant via dew, rainfall, or irrigation (Taylor, 1996). This hypothesis was tested by Csinos and Kvien (1988), by using methyl-blue dye to demonstrate fungicide redistribution with irrigation. As a result of these studies and observations of sporadic reductions in efficacy of foliar-applied fungicides in non-irrigated fields, producers in Georgia are advised to administer irrigation following fungicide applications in order to maximize stem rot control (Kemerait et al., 2006). It is recognized that administering irrigation too quickly may compromise leaf spot control, but the timings needed to optimize control of diseases caused by foliar and soilborne pathogens are not well documented.
There is currently limited information available regarding the redistribution of fungicides from rainfall or irrigation. Most of what has been reported pertains to the influence of rainfall and the rainfastness of protectant compounds in vegetables or fruit crops (Smith and MacHardy, 1984; Neely, 1971; Kudsk et al., 1991). Information regarding mechanisms of suppressing soilborne pathogens with foliar applied fungicides is even more limited. Csinos and Kvien (1988) suggested that initial fungicide deposits applied to peanut foliage are washed to the base of the plant, thus improving contact with soilborne pathogens. Presumably, fungicides were redistributed from the foliage to crowns and pegs.
Flutolanil (Araki, 1980) and tebuconazole (Taylor, 1996) are rapidly absorbed by the leaf, whereas, azoxystrobin remains on the leaf surface for a longer period (Bartlett et al., 1995). The persistence of azoxystrobin on the leaf surface may help explain the differences in the pod colonization for the non-irrigated controls. Earlier irrigation timings led to maximum stem rot control, while longer drying times were required to maximize leaf spot control. In the study conducted by Woodward (2006), a period of 18 hours drying time was required between the application of select fungicides and admistering an irrigation event. More recently, Augusto and Brenneman (2011) evaluated the interactive effects of fungicide timing and subsequent irrigation. Leaf spot control was not effected by irrigation, which was applied approximately 24 hours after fungicide applications. Overall, the application of irrigation was less effective at reducing stem rot incidence compared to nighttime applications of fungicides; however, effects of neither fungicide timing or subsequent irrigation were the same for all fungicides evaluated. This could be attributed to differences in retention, absorbtion or systemicity of the fungicides. Systemic fungicides used to manage leaf spot and stem rot move acropetally within the plant; however, applications of prothioconazole, or prothioconazole plus tebuconazole have been shown to reduce disease in the lower non-treated areas of the plant (Augusto and Brenneman, 2012). A better understanding of fungicide systemicity is needed to maximize foliar and soilborne disease control in peanut. Furthermore, the increased residual activity of newer peanut fungicides has led to changes of commercial fungicide regimes under reduced disease pressure.
2.5. Use of extended interval fungicide programs and forecasting models
While fungicides are typically applied on a 14-day schedule to manage fungal diseases, the use of extended spray intervals could certainly be beneficial to producers by reducing production costs if they could maintain similar yields. In a study conducted by Brenneman and Culbreath (1994), fungicides applied on a 14-day schedule and 21-day schedule provided similar levels of leaf spot and stem rot suppression. Disease suppression decreased in plots treated on a 28-day interval; however, leaf spot and stem rot suppression was lower than what was observed in the non-treated control. A similar trend was observed for yield, where 3-year averages for the non-treated control, 14-day and 21-day intervals were 2914, 5153, and 4704 kg per hectare, respectively. Additional studies have shown that fungicides applied on 21- or 28-day intervals are capable of providing sufficient control of diseases and provide yields comparable to those achieved by the standard 14-day applications interval (Brenneman et al., 2001; Culbreath, 1993; Culbreath et al., 1992; Monfort, 2002; Phatak et al., 2002). Results of one study in particular showed that plots receiving as few as four chlorothalonil applications applied on a 28-day interval had yields as high as plots treated with seven applications made on a 14-day interval (Culbreath et al., 1992). Chandra et al. (1998), found that one properly timed application provided adequate control of leaf spot; however, timings differed within years. More recently Culbreath et al. 2006 also demonstrated excellent leaf spot control with pyraclostrobin applied at more extended intervals, and even when the initial sprays were greatly delayed. Delayed initial applications with this fungicide are now widely used by growers in the southeastern United States with good results.
By better defining the environmental conditions that favor disease development, peanut producers can improve disease control by timely application of fungicides. Forecasting models use environmental data such as temperature, rainfall and relative humidity, to predict when conditions are favorable for pathogen and disease development (Campbell and Madden, 1990). Over the past 40 years, various forecasting models have been developed and successfully implemented for peanut diseases. Jenson and Boyle (1966) and Phipps and Powell (1984) are credited with developing some of the first forecasting models to manage peanut leaf spot. More recently, an early leaf spot spray advisory, developed in Virginia, was effective in reducing number of sprays required for satisfactory disease control and has been highly accepted by growers (Cu and Phipps, 1993; Phipps, 1993). Spray advisories for late leaf spot have been implemented in other peanut producing states, such as Georgia, Alabama, North Carolina and Oklahoma (Nutter and Brenneman, 1989; Davis et al., 1993; Bailey et al., 1994; Damicone 1994).
In Georgia, AU-Pnut is the predominant leaf spot advisory used in research; however, it is not widely used by producers. This model was developed in the late 1980s, and is based solely on precipitation (the number of precipitation events and the five-day forecasted probability of precipitation) (Davis et al., 1993). Studies to evaluate the AU-Pnut advisory for timing applications of fungicides aimed at soilborne fungi have shown suppression of stem rot, but the results have been inconsistent (Brenneman and Culbreath, 1994; Rideout, 2003).
Several spray advisories based on the environmental conditions that incite Sclerotinia blight have been developed in Virginia and North Carolina (Phipps, 1995, Langston, 1998, Langston et al., 2002). Such advisories have been shown to improve disease control when compared to calendar applications. These advisories are based on air and soil temperatures, precipitation, relative humidity, vine growth, and canopy closure. Adaptations of these models have been evaluated for the control of stem rot. Rideout (2003) demonstrated that fungicide application timing has a significant effect on stem rot control and yield in Georgia. Furthermore, he concluded that the application of fungicides according to advisories based on soil temperature, precipitation and host growth provided similar or better disease control than the typical calendar-based programs.
Peanut is susceptible to various foliar and soilborne pathogens. Currently there is a wide range of fungicides labeled for management of peanut diseases (Table 1). Standard fungicides, such as chlorothalonil or tebuconazole, commonly comprise fungicide regimes designed to control leaf spot and stem rot, respectively. Other diseases, such as pod rot and Sclerotinia blight are managed with fungicides such as azoxystrobin and fluazinam, respectively. Several other fungicides with different modes of action are avaliable for use in peanut. While some fungicides, such as pyraclostrobin have post-infection activity, efficacy is typically greatest when applications are made in a preventative manner. Utilization of integrated disease managment strategies that incorporate factors such as field history, cultural practices and partially resistant cultivars may be used to reduce disease pressure and increase profitability. Resistance to several classes of fungicides used in peanut have been identified in populations of leaf spot pathogens. Most recently, resistance to triazole fungicides, such as tebuconazole, have been reported in eastern production regions of the United States. Furthermore, the potential exists for resistance to develop in other fungicide classes, primarily the strobilurin; therefore, it is imperative that producers rotate chemistries to ensure the sustainability and longterm use of these fungicides. Future research evaluating aspects of peanut fungicides, such as initial application timing, systemic and residual activity and interactive effects of tank-mixtures are warranted. For diseases caused by soilborne pathogens, a better understanding of spatial and temporial aspects of the pathogen could allow for more precise applications of fungicides.
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