Effective weed control is considered critical to maximize the yield of peanut ( Arachis hypogaea L.) ( Leon et al., 2019 ). In Ghana, where herbicides are seldom used by smallholder farmers, yield losses due to weed interference are estimated to reach 50 to 80% ( Dankyi, 2014; Dankyi et al., 2005 ; Dzomeku et al., 2009 ; Leon et al., 2019 ). Hand-weeding can be effective at controlling weeds; however, the time required to hand weed peanut in Ghana is a substantial component of the overall cost of peanut production ( Dankyi, 2014; Dankyi et al., 2005 ). Adequate labor is often not available especially during the first 3-6 weeks of the season when it is most needed to prevent yield loss in peanut ( El Naim et al., 2010 ; Everman et al., 2008 ). In countries where herbicides are available and affordable, farmers usually apply these products to the soil at planting and after weeds emerge later in the season to control weeds and protect yield ( Leon et al., 2019 ). When comparing herbicide applications versus hand-weeding, studies in northern Ghana demonstrated that herbicides are effective in controlling weeds while minimizing the labour needed for hand weeding ( Abudulai et al., 2017 ; Leon et al., 2019 ).
In addition to weeds, annual yield loss of peanut in Ghana from disease has been estimated to be as high as 50% ( Nutsugah et al., 2007a 2007b). Early and late leaf spot, peanut rosette virus ( Umbravirus) vectored by aphids ( Aphis craccivura), and rust ( Puccinia arachidis) are among the most important peanut diseases in Ghana and other countries in sub-Saharan Africa ( Abudulai et al., 2007 ; Gaikpa et al., 2015 ; Nigam et al., 2018 ; Nutsugah et al., 2007a ; Waliyar et al., 2000 ). Naab et al. (2005) demonstrated the value of incorporating fungicides into pest management strategies in peanut, but most smallholder farmers do not currently have access to fungicides or they do not have sufficient credit to purchase fungicides. Therefore, resistant cultivars are the most applicable strategy to minimize the negative impact of diseases on peanut ( Gaikpa et al., 2015 ). However, high yielding and disease resistant cultivars are often not available on a wide scale in Ghana due to a lack of quality seed ( Nigam et al., 2018 ; Owusu-Akyaw et al., 2019 ).
Recently, Abudulai et al. (2017) examined interactions of weed management practices that included herbicides and fungicides applied to protect peanut from leaf spot disease and improve production. In some instances, leaf spot disease was affected by weed management practice while disease management had no impact on weed management. For example, less canopy defoliation caused by leaf spot disease was observed in absence of fungicides when weeds were present compared with instances where weeds were controlled effectively by herbicides or hand-weeding. A limitation to these studies was the lack of comparison of the financial economic returns from the alternative treatments. Therefore, research was conducted to define interactions of weed management practices (e.g., combinations of herbicides and hand weeding) with fungicide treatments to control leaf spot in peanut. Possible interactions of pest management practices were also compared using measurements of weed and disease response, peanut pod yield, and estimated financial returns. The trials were conducted on-station and on-farm in the southern region of Ghana.
Materials and Methods
Field experiments were conducted during 2015 and 2016 near CSIR- Crops Research Institute Kwadaso Station (06°40'42.161"N 001°40'34.902"W), the Fumesua Station (06°43'24.588"N 001°31'58.164"W), and in a farmer’s field near Ejura (7°27'18.3"N 001°18'37.1"W). Soils were sandy loam alfisols at all locations. The improved leaf spot resistant cultivar Yenyawoso was used in one experiment in 2015 and two experiments in 2016, and the traditional leaf spot susceptible peanut cultivar Konkoma in two experiments in 2015 ( Gaikpa et al., 2015 ; Owusu-Akyaw et al., 2019 ). The cultivars were in different experiments and were not compared directly. The cultivar Yenyawoso has expressed tolerance to both early leaf spot and late leaf spot ( Gaikpa et al., 2015 ; Owusu-Akyaw et al., 2019 ). The number of days from planting to optimum kernel and pod maturity is approximately 90 days and is similar for both cultivars ( Owusu-Akyaw et al., 2019 ). All trials were planted in early June in plots with 8 rows spaced 50 cm apart and 8 m in length. Seed was planted in conventionally-prepared flat seedbeds at an in-row density of 5 seeds per m of row.
Weed management consisted of: 1) no weed control; 2) hand-weeding at 3 and 6 weeks after planting (WAP); 3) a preemergence (PRE) application immediately after planting; 4) a postemergence (POST) application 4 WAP; 5) a PRE herbicide supplemented by hand-weeding 6 WAP; 6) a POST herbicide supplemented by hand-weeding at 6 WAP; 7) PRE and POST herbicides; and 8) PRE and POST herbicides supplemented by hand-weeding at 6 WAP ( Table 1). The PRE herbicide metolachlor (Maestro 960 EC, Shandong Weifang Rainbow Chemical Co., LTD, Shandong, China) was applied at 1.15 kg ai/ha within 2 days after planting. The POST herbicide imazethapyr (Vezir 240 SL, ADAMA Makhteshim, Kiryat, Israel) was applied at 70 g ai/ha 4 WAP at the R1 or R2 growth stage ( Boote, 1982). Nonionic surfactant at 0.25% (v/v) was included with imazethapyr.
Fungicide treatments consisted of: 1) no fungicide or 2) sequential fungicide applications at 4 and 6 WAP. In 2015, the fungicide program consisted of sequential applications of tebuconazole (Raintebzol 430SC, Shandong Weifang Rainbow Chemical Co., LTD, Shandong, China) at 100 g ai/ha. The fungicide program in 2016 consisted of sequential applications of azoxystrobin plus difenoconazole (Fivestar 325 SC, Shandong Weifang Rainbow Chemical Co., LTD, Shandong, China) applied at 0.09 plus 0.04 kg ai/ha, respectively, at the R2 growth stage ( Boote, 1982). The formulated product in 2016 was used because it contained two fungicides and would serve as a more effective resistance management option ( Shew, 2020). These fungicides are considered efficacious against the pathogens that cause leaf spot disease ( Johnson et al., 2018 ; Shew, 2020). When combined with a fungicide treatment, the POST herbicide was applied 3 days prior to the fungicide. All herbicides and fungicides were applied at a carrier volume of 145 L/ha aqueous solution using a hand-held, backpack sprayer equipped with a single flat fan nozzle (Solo Backpack Sprayers, Newport News, VA).
The experimental design was a randomized complete block with 4 replications. Each combination of weed management practice and fungicide program was randomly assigned to plots in each replication. Visual estimates of leaf spot severity were determined from 10 randomly selected plants 12 WAP using an ordinal scale of 1 to 5 where 1 = plants with no visible lesions caused by leaf spot disease, 2 = 1 to 20% of peanut leaflets expressing lesions, 3 = 21 to 50% of leaflets with lesions, 4 = 51 to 70% of leaflets with lesions, and 5 = 71% to 100% of leaflets with lesions. The mid-point value for each category of the ordinal scale was used for statistical analysis ( Chiang et al., 2014 ). Predominant weeds at Ejura, Fumesua, and Kwadodo are provided in Table 2. Dry weight of weed biomass and pod yield were determined from the 4 center rows (50-cm spacing) of each plot with a length of 4 m 10 WAP. Pod yield was adjusted to 8% moisture.
Results and Discussion
A significant and positive correlation of yield and financial return was observed for both cultivars ( Table 3). This was expected as yield is a major contributor to financial return even though cost of pest management can affect financial return. Yield and financial return were negatively correlated with leaf spot severity for Konkoma but not for Yenyawoso. Leaf spot disease can reduce yield if effective disease management is not imposed when severity is high. Total weed biomass was negatively correlated with yield for both Konkoma and Yenyawoso. Financial return was negatively correlated with total weed biomass for Konkoma but not Yenyawoso. Decreasing weed interference with peanut often increases yield and financial return ( Leon et al., 2019 ).
Results of ANOVA revealed an interaction of weed management practices × fungicide treatment was not significant in most instances for each cultivar (susceptible vs. resistant to leaf spot). However, the main effect of herbicide program was significant for biomass of broadleaf weeds, grass weeds, nutsedge, pod yield, and financial return regardless of cultivar. The interaction of weed management practices and fungicide program was significant for leaf spot severity for both cultivars. The main effect of fungicide program was significant for pod yield and financial return for the leaf spot susceptible cultivar Konkoma. However, when the leaf spot resistant cultivar Yenyawoso was grown, the main effect of fungicide was not significant for grain yield and financial returns. Differences among weed management practices for weed biomass, yield, and financial return were expected due to previous research with these approaches to weed management ( Abudulai et al., 2017 ; Leon et al., 2007 ; Naab et al., 2005 ). Differences in leaf spot severity were somewhat surprising even though in most cases the differences were subtle. None-the-less, some practices may move soil on plants, and weeds may affect movement of spores onto the peanut canopy. It is also postulated that weeds could interfere with deposition of fungicide in the peanut canopy. Lack of response of Yenyawoso to fungicide was expected because of leaf spot tolerance reported in this cultivar ( Owusu-Akyaw et al., 2019 ). The positive response of the cultivar Konkoma to fungicide was expected because this cultivar is susceptible to leaf spot disease ( Owusu-Akyaw et al., 2019 ).
When comparing weed management practices, total weed biomass was similar for hand-weeding only (57.4 kg/m2), PRE herbicide only (73.6 kg/m2), and the non-treated control (75.7 kg/m2) for Konkoma ( Table 4). In general, total weed biomass decreased as the number of weed management practices increased for this cultivar. For Yenyawoso, all weed management practices resulted in less total weed biomass than the non-treated control. The least effective weed management approach, with respect to total weed biomass, was noted when herbicides were applied PRE only (65.1 kg/m2) or POST only (62.7 kg/m2). A single weed management practice is often insufficient to control weeds completely ( Hare et al., 2019). For example, employing two practices resulted in similar biomass reductions with values of 28.7 to 38.1 kg/m2. Hand-weeding only or in combination with PRE and/or POST herbicides resulted in weed biomass values of 16.2 to 33.6 kg/m2).
In absence of fungicides, differences in leaf spot severity ranged from 68 to 76% across weed management practices for Konkoma and 32 to 52% for Yenyawoso ( Table 5). Although differences in weather conditions across the experiments were noted and the experimental design does not allow direct comparisons, the lower leaf spot incidence for Yenyawoso was expected because this cultivar expresses a greater degree of tolerance to leaf spot disease compared with the susceptible cultivar Konkoma ( Owusu-Akyaw et al., 2019 ).
The interaction of weed management practice × fungicide treatment was significant for leaf spot severity for both Konkoma and Yenyawoso. Applying fungicide reduced leaf spot severity for the cultivar Konkoma except when weed management consisted of a PRE herbicide only or when no weed management ( Table 5). Leaf spot severity ranged from 14% to 27% for the other weed management practices when fungicide was applied. The relatively poor weed control provided by PRE only weed management or no weed control may have resulted in limited deposition of fungicide into the peanut canopy compared with deposition when greater weed management was included. Greater deposition of fungicide would result in more effective pathogen control and less disease. Also, greater weed biomass may have limited air flow in the peanut canopy resulting in a more favorable environment for pathogens and subsequent disease progression. However, the mechanism of greater leaf spot severity for PRE only management and the treatment without weed management was not elucidated in this research. These results do suggest that additional research defining the mechanism of weed and disease management interactions would be informative. In contrast to results with Konkoma, no difference in leaf spot severity was observed for fungicide-treated and non-treated peanut for the PRE only treatment or when a weed control practice was not employed for Yenyawoso ( Table 5). Applying fungicide resulted in lower severity of leaf spot when hand-weeding only was used or when two strategies were used for weed management. When PRE and POST herbicides were applied along with hand-weeding there was no difference in leaf spot severity when comparing fungicide treatments.
Pod yield and financial return were not affected by the interaction of weed management practice and fungicide program regardless of cultivar. However, both fungicide program and weed management practice affected these measurements. When data were pooled over weed management practices, grain yield and financial return were greater when fungicides were applied to Konkoma ( Table 6). However, this was not the case for the cultivar Yenyawoso where yield and financial returns were similar for Yenyawoso regardless of fungicide treatment.
This publication was made possible in part through support provided by the Office of Agriculture, Research and Policy, Bureau of Food Security, U.S. Agency for International Development, under the terms of Award No. AID-ECG-A-00-07-0001 to The University of Georgia as management entity for U.S. Feed the Future Innovation Lab on Peanut Productivity and Mycotoxin Control (2012-2017). The opinions expressed herein are those of the authors and do not necessarily reflect the views of the U.S. Agency for International Development. Appreciation is expressed to technical staff and farmers for assistance with this research.
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