Frogeye Leaf Spot Severity Research Articles

Evaluation of foliar fungicides for frogeye leaf spot control and yield protection in soybean in Arkansas

The efficacy of fifteen foliar fungicides to control frogeye leaf spot (caused by Cercospora sojina K. Hara) and protect soybean grain yield was investigated over three years (2014–2016) in two field experiments. Experiments were divided into two fungicide groups: premix fungicides, which included combinations of quinone outside inhibitors (QoI), demethylation inhibitors (DMI), succinate dehydrogenase inhibitors (SDHI), or methyl benzimidazole carbamates (MBC), and solo fungicides, all DMI fungicides. Fungicides were applied once at the full pod (R4) in 2014 and 2015 and at the beginning seed (R5) growth stage in 2016. A very susceptible frogeye leaf spot cultivar, Armor 48R40, was used. Frogeye leaf spot severity at fungicide application timing average 1.0, 0.5, and 8.0% in 2014, 2015, and 2016, respectively. Final disease severity averaged 21.7, 14.5, and 15.1% in 2014, 2015, and 2016, respectively on the non-treated control. Azoxystrobin provided no disease control, suggesting the presence of QoI-resistant C. sojina. Three fungicides: azoxystrobin + difenoconazole, flutriafol, and thiophanate-methyl + tebuconazole applied at R4 provided effective disease control compared to the non-treated control. Fungicides applied at R5 provided no disease control. No fungicide provided significant grain yield protection in either experiment. Overall, the average yield protection trend by fungicides applied at R4 and R5 was 6.8 and 3.0%, respectively compared to the non-treated control. These data suggest that although foliar fungicides can provide effective frogeye leaf spot control, yield protection may be limited when applied at the R4 growth stage, particularly when disease severity exceeds 0.5% on a very susceptible soybean cultivar.

Meta-Analytic Modeling of the Severity-Yield Relationships in Soybean Frogeye Leaf Spot Epidemics.

Frogeye leaf spot (FLS), caused by Cercospora sojina, is an important foliar disease affecting soybean in the United States. A meta-analytic approach including 39 fungicide trials conducted from 2012 to 2021 across eight states (Alabama, Arkansas, Illinois, Iowa, Kentucky, Louisiana, Mississippi, Tennessee) was used to assess the relationship between FLS severity and soybean yield. Correlation and regression analyses were performed separately to determine Fisher's transformation of correlation coefficients (Zr), intercept (β0) and slope (β1). Disease pressure (low severity, ≤34.5; high severity, >34.5%) and yield class (low, ≤3,352; high, >3,352 kg/ha) were included as categorical moderators. Pearson's [Formula: see text], obtained from back-transforming the [Formula: see text]r estimated by an overall random-effects model, showed a significant negative linear relationship between FLS severity and yield ([Formula: see text] = -0.60). The [Formula: see text]r was affected by disease pressure (P = 0.0003) but not by yield class (P = 0.8141). A random-coefficient model estimated a slope of -19 kg/ha for each percent severity for a mean attainable yield of 3,719.9 kg/ha. Based on the overall mean (95% CI) of the intercept and slope estimated by the random-coefficients model, the estimated overall relative damage coefficient was 0.51% (0.36 to 0.69), indicating that a percent increase in FLS severity reduced yield by 0.51%. The best model included yield class as a covariate, and population-average intercepts differed significantly between low (3,455.1 kg/ha) and high (3,842.7 kg/ha) yield classes. This highlights the potential impact of FLS on soybean yield if not managed and may help in disease management decisions.

Efficacy and Profitability of Fungicides for Managing Frogeye Leaf Spot on Soybean in the United States: A 10-Year Quantitative Summary.

Frogeye leaf spot (FLS), caused by Cercospora sojina, is an economically important disease of soybean in the United States. Data from 66 uniform fungicide trials (UFTs) conducted from 2012 to 2021 across eight states (Alabama, Arkansas, Illinois, Iowa, Kentucky, Louisiana, Mississippi, and Tennessee) were gathered and analyzed to determine the efficacy and profitability of the following fungicides applied at the beginning pod developmental stage (R3): azoxystrobin + difenoconazole (AZOX + DIFE), difenoconazole + pydiflumetofen (DIFE + PYDI), pyraclostrobin (PYRA), pyraclostrobin + fluxapyroxad + propiconazole (PYRA + FLUX + PROP), tetraconazole (TTRA), thiophanate-methyl (TMET), thiophanate-methyl + tebuconazole (TMET + TEBU), and trifloxystrobin + prothioconazole (TFLX + PROT). A network meta-analytic model was fitted to the log of the means of FLS severity data and to the nontransformed mean yield for each treatment, including the nontreated. The percent reduction in disease severity (%) and the yield response (kg/ha) relative to the nontreated was the lowest for PYRA (11%; 136 kg/ha) and the greatest for DIFE + PYDI (57%; 441 kg/ha). A significant decline in efficacy over time was detected for PYRA (18 percentage points [p.p.]), TTRA (27 p.p.), AZOX + DIFE (18 p.p.), and TMET + TEBU (19 p.p.) by using year as a continuous covariate in the model. Finally, probabilities of breaking even were the greatest (>65%) for the most effective fungicide DIFE + PYDI and the lowest (<55%) for PYRA. Results of this meta-analysis may be useful to support decisions when planning fungicide programs.

Estimating Soybean Radiation Use Efficiency Using a UAV in Iowa

Radiation use efficiency (RUE) is difficult to estimate and unreasonable to perform on a small plot scale using traditional techniques. However, the increased availability of Unmanned Aerial Vehicles (UAVs) provides the ability to collect spatial and temporal data at high resolution and frequency, which has made a potential workaround. An experiment was completed in Iowa to (i) demonstrate RUE estimation of soybean [Glycine max (L.) Merr.] from reflectance data derived from consumer-grade UAV imagery and (ii) investigate the impact of foliar fungicides on RUE in Iowa. Some fungicides are promoted to have plant health benefits beyond disease protection, and changes in RUE may capture their effect. Frogeye leaf spot severity did not exceed 2%. RUE values ranged from 0.98 to 1.07 and 0.96 to 1.12 across the entire season and the period post-fungicide application, respectively, and fell within the range of previously published soybean RUE values. Plots treated with fluxapyroxad + pyraclostrobin had more canopy cover (p = 0.078) compared to the non-treated control 133 days after planting (DAP), but yields did not differ. A “greening effect” was detected at the end of the sample collection. RUE estimation using UAV imagery can be considered a viable option for the evaluation of management techniques on a small plot scale. Since it is directly related to yield, RUE could be an appropriate parameter to elucidate the impact of plant diseases and other stresses on yield.

Quantifying the Effects of Fungicides and Tillage on Cercospora sojina Severity and Yield of Soybean

A field study was conducted in 2014 through 2016 at the University of Tennessee Research and Education Center, Milan, TN, to determine fungicide effects on frogeye leaf spot (FLS) caused by Cercospora sojina and to determine disease severity and yield under no-till and tillage. Each plot was visually rated for FLS severity based on the percent leaf area covered from 0 to 100%. The result showed that tillage did not affect severity of the disease or yield. However, there were significant effects from fungicide, year-to-year effects, and fungicide × year interactions. Among the three years, disease severity in 2016 was greater than in 2014 and 2015. Among the six fungicides, Topsin, Topguard, Quadris TOP SBX, and Priaxor significantly (P ≤ 0.05) reduced maximum disease severity in 2016. All fungicide applications improved yield relative to the untreated check. Topsin and Quadris TOP SBX, however, had consistently the highest percent yield increases. These results suggest that not all fungicides that reduced FLS severity equally protected yield, indicating that yield and environmental conditions need to be considered when making appropriate fungicide recommendations in tilled and no-till systems.

Occurrence of Frogeye Leaf Spot, Caused by Cercospora nicotianae, on Greenhouse Tobacco Transplants in Kentucky

Frogeye leaf spot (FLS), caused by Cercospora nicotianae, has traditionally been considered a minor disease occurring on older, low-growing leaves of tobacco (Nicotiana tabacum L.). Historically, FLS was even considered desirable by some burley tobacco grading experts as an indicator of crop maturity. In recent years, however, multiple cured crops grown in south-central Kentucky have been rejected by buyers due to excessive FLS. In May 2017, symptoms of FLS were identified for the first time in burley tobacco transplants grown in greenhouse floatbeds in Metcalfe, Allen, and Barren Counties, the same region of Kentucky where FLS severity has resulted in rejected crops. Lesions were characterized by a tan-white center surrounded by a narrow brown ring with a chlorotic halo. Black sporulation was macroscopically visible in lesion centers under high humidity. Incidence of FLS was 0.5% - 5% of transplants, with a maximum of 10% severity on the total leaf area of an individual transplant. Diseased transplants w...

Value of Damage Control with Foliar Fungicide in Soybean Production in Tennessee

Core Ideas Annual application of foliar fungicide in continuous soybean increased yields. Breakeven soybean price for applying a foliar fungicide ranged from US$0.13 to $0.27kg−1. Results suggest a high likelihood that foliar fungicide application is profitable. With the expansion in soybean (Glycine max L.) land area in the Southeast, producers are wondering how to profitably manage the disease frogeye leaf spot (FLS) (Cercospora sojina); however, little is known about the profitability of using foliar fungicide on soybean infected with FLS. We determined the effect of total water applied, growing degree days, and foliar fungicide treatment on FLS severity and yield of soybean in maturity group (MG) III, IV, and V. A two‐stage severity/treatment outcome model was estimated and results were used to calculate the breakeven price of soybean for applying a foliar fungicide by MG. Data came from an 11‐yr soybean fungicide (quinone) experiment in Tennessee. Applying a foliar fungicide to soybean in a high disease pressure location reduced FLS severity and increased yields for each MG. Annual application of foliar fungicide in continuous soybean was found to increase yields by 475 kg ha−1 for MG III, 321 kg ha−1 for MG IV, and 408 kg ha−1 for MG V. The breakeven price of soybean, for applying a foliar fungicide, ranged from US$0.13 to $0.27 kg−1 for all MGs, depending on the application cost. A profit‐maximizing producer would apply a foliar fungicide as part of the annual production practices to manage FLS since the soybean price has historically been above the breakeven price. Results from this study will inform Tennessee and southeastern producers on the profitability of spraying soybean with a foliar fungicide to manage FLS.

Quantitative Trait Loci Underlying Partial Resistance to Cercospora sojina Race 2 Detected in Soybean Seedlings in Greenhouse Assays

Cercospora sojina (Hara), an air-borne pathogen, infects soybean [Glycine max (L.) Merr.] leaves causing frog-eye leaf spot (FLS). Three major genes (Rcs1-3) underlie resistance to the major races of FLS but two were not yet mapped. In addition quantitative trait loci provide partial resistance to many strains. FLS race 2 was an isolate first collected in the 1950’s when damaging FLS first arose. ‘Essex’ was partially resistant while ‘Forrest’ was partially susceptible to mixed races of FLS. The objective here was to identify quantitative trait loci underlying resistance to FLS race 2 in the greenhouse using recombinant inbred lines (RILs) derived from the cross of Essex by Forrest. C. sojina race 2 (ATCC 44531) was used to induce leaf symptoms on one hundred F5:14 RILs derived from the cross of Essex by Forrest. The leaf symptoms were measured at 21 days after manual infestation by wounding (dai) and again at 42 dai to show resistance to reinfestation of new leaves from the primary lesions without wounding. Bags over leaves were not used to better simulate field conditions. However, there was no significant correlation between FLS severity at 21 and 42 dai (r =0.08 and P= 0.005). At 21 dai there was a strongly significant QTL near Satt319 on LG C2 (chromosome 7; LOD 3.8; R2 52%) where the Essex allele reduced leaf symptoms by 0.7 units. At 42 dai there was a strongly significant QTL near Satt632 on LG A2 (chromosome 8; R2 was 15%; LOD was 3.6) where the Essex allele reduced leaf symptoms by 0.4 units. Neither locus mapped to the location of Rcs3. By ANOVA thirteen additional minor loci were detected on LGs A1, B1, F, G, H, I, J, K, L, M and O. At two loci (LG B1 and O) the Forrest allele appeared to reduce FLS at both 21 and 42 dai. Eight loci may have reduced FLS at 21 dai (0.006 &lt; P &lt; 0.049; 4% &lt; R2 &gt; 9%) of which 5 had beneficial alleles from Forrest. Seven loci may have reduced FLS at 42 dai (0.001 &lt; P &lt; 0.04; 4% &lt; R2 &lt; 15%) of which 4 had beneficial alleles from Forrest. Therefore, quantitative resistance to race 2 of FLS was inferred to have major loci contributions from Essex and minor loci contributions from both Forrest and Essex. Resistance was dependent on plant age. Breeding and selection for FLS will be complex and may be more efficient with the markers, germplasm and models of inheritance reported here.

A set of standard area diagrams to assess severity of frogeye leaf spot on soybean

A set of standard area diagrams (SADs) was developed and validated to aid visual assessment of severity of frogeye leaf spot (FLS) caused by Cercospora sojina. The SAD has eight color images of diseased leaflets with severity values that ranged from 0.1 to 39.9 %. The SAD was validated by a group of 20 raters [10 experienced (ER) and 10 inexperienced (IR)], who assessed the same set of 50 images twice, the first without SADs and the second using SADs as an aid. The SADs significantly improved accuracy [coefficients of bias (C b ) were 0.64 and 0.99 for IR and 0.98 and 0.99 for ER, without and with SADs, respectively], precision [correlation coefficients (r) were 0.89 and 0.95 for IR and 0.94 and 0.97 for ER, without and with SADs, respectively] and overall agreement [Lin’s concordance correlation coefficients (ρ c ) were 0.57 and 0.94 for IR and 0.92 and 0.97 for ER without and with SADs, respectively]. The estimates of severity of FLS were more reliable when using SADs. Both the inter-rater reliability (coefficient of determination, R 2) and intra-class coefficient (ρ) were significantly increased by using SADs. Therefore, it is believed that the SADs proposed in the present study will be a useful tool to aid accurate, precise and reliable estimates of severity of FLS in experiments (e.g., fungicide screening, assessment of partial resistance of soybean genotypes to FLS) and to aid in decision-making purposes.

Soybean Resistance to Cercospora sojina Infection Is Reduced by Silicon.

Frogeye leaf spot, caused by Cercospora sojina, is one of the most important leaf diseases of soybean worldwide. Silicon (Si) is known to increase the resistance of several plant species to pathogens. The cultivars Bossier and Conquista, which are susceptible and resistant, respectively, to frogeye leaf spot, supplied and nonsupplied with Si were examined for the activities of defense enzymes and the concentrations of total soluble phenolics (TSP) and lignin-thioglycolic acid (LTGA) derivatives at 8, 14, and 16 days after inoculation (dai) with C. sojina. The importance of cell wall degrading enzymes (CWDE) to the infection process of C. sojina and the effect of Si on their activities were also determined. Soybean plants were grown in hydroponic culture containing either 0 or 2 mM Si (-Si and +Si, respectively) and noninoculated or C. sojina inoculated. Severity of frogeye leaf spot was higher in cultivar Bossier plants than cultivar Conquista and also in the +Si plants compared with their -Si counterparts. Except for the concentrations of TSP and LTGA derivatives, activities of defense enzymes and the CWDE did not change for +Si noninoculated plants regardless of the cultivar. The activities of lipoxygenases, phenylalanine ammonia-lyases, chitinases, and polyphenoloxidases as well as the activities of CWDE decreased for the +Si inoculated plants. The results from this study demonstrated that defense enzyme activities decreased in soybean plants supplied with Si, which compromised resistance to C. sojina infection.

Tillage, Fungicide, and Cultivar Effects on Frogeye Leaf Spot Severity and Yield in Soybean.

Frogeye leaf spot (FLS) of soybean, caused by Cercospora sojina, has been a problem in the southern United States for many years but has become an increasing problem in the northern United States more recently, causing significant yield losses. This increase in disease severity in the northern United States has been attributed to increased utilization of no-till planting and changes in climate. A field study was conducted at the University of Tennessee, Research and Education Center in Milan, TN from 2007 to 2010 to determine severity in tilled and no-till plots treated with or without fungicide at R3 and R5 growth stages. Three FLS-susceptible cultivars, one each in Maturity Groups III, IV, and V, were treated with pyraclostrobin (Headline) fungicide. Analysis of variance using the area under the disease progress curve (AUDPC) indicated no significant difference (P ≤ 0.05) in disease severity between tilled and no-till plots without fungicide. Fungicide did not significantly reduce disease under no-till, but did under tilled plots. This is the first study showing that no-till plots did not reduce or enhance the severity of FLS when no fungicide was applied. Fungicide application significantly reduced (P ≤ 0.05) disease severity and AUDPC and increased yield in tilled plots. The yield gains in tilled, fungicide-treated plots ranged from 1 to 17%. When fungicide was applied, disease severity was not reduced as significantly in no-till as in treated tilled plots, suggesting that fungicide programs under a no-till system may require further study to minimize the risk of FLS severity.

The biocontrol agent Bacillus sp. CHEP5 primes the defense response against Cercospora sojina.

Glycine max (soybean) production can be dramatically affected by frogeye leaf spot (FLS) caused by Cercospora sojina Hara. The inoculation of biocontrol agents may be an alternative strategy for C. sojina control. The native biocontrol bacterium Bacillus sp. CHEP5 reduced the severity of FLS in soybean by inducing systemic resistance. We suggest that the defense response was primed since the expression of the defense related gene GmAOS was enhanced in induced plants treated with both methyl jasmonate and C. sojina. Furthermore, as GmAOS is related to jasmonic acid biosynthesis, we assume that this phytohormone is involved in induced systemic resistance signaling defense pathway in soybean against C. sojina.

Quantitative Trait Loci Underlying Partial Resistance to Cercospora sojina Race 2 Detected in Soybean Seedlings in Greenhouse Assays

Cercospora sojina (Hara), an air-borne pathogen, infects soybean [Glycine max (L.) Merr.] leaves causing frog-eye leaf spot (FLS). Three major genes (Rcs1-3) underlie resistance to the major races of FLS but two were not yet mapped. In addition quantitative trait loci provide partial resistance to many strains. FLS race 2 was an isolate first collected in the 1950’s when damaging FLS first arose. ‘Essex’ was partially resistant while ‘Forrest’ was partially susceptible to mixed races of FLS. The objective here was to identify quantitative trait loci underlying resistance to FLS race 2 in the greenhouse using recombinant inbred lines (RILs) derived from the cross of Essex by Forrest. C. sojina race 2 (ATCC 44531) was used to induce leaf symptoms on one hundred F5:14 RILs derived from the cross of Essex by Forrest. The leaf symptoms were measured at 21 days after manual infestation by wounding (dai) and again at 42 dai to show resistance to reinfestation of new leaves from the primary lesions without wounding. Bags over leaves were not used to better simulate field conditions. However, there was no significant correlation between FLS severity at 21 and 42 dai (r =0.08 and P= 0.005). At 21 dai there was a strongly significant QTL near Satt319 on LG C2 (chromosome 7; LOD 3.8; R2 52%) where the Essex allele reduced leaf symptoms by 0.7 units. At 42 dai there was a strongly significant QTL near Satt632 on LG A2 (chromosome 8; R2 was 15%; LOD was 3.6) where the Essex allele reduced leaf symptoms by 0.4 units. Neither locus mapped to the location of Rcs3. By ANOVA thirteen additional minor loci were detected on LGs A1, B1, F, G, H, I, J, K, L, M and O. At two loci (LG B1 and O) the Forrest allele appeared to reduce FLS at both 21 and 42 dai. Eight loci may have reduced FLS at 21 dai (0.006 &lt; P &lt; 0.049; 4% &lt; R2 &gt; 9%) of which 5 had beneficial alleles from Forrest. Seven loci may have reduced FLS at 42 dai (0.001 &lt; P &lt; 0.04; 4% &lt; R2 &lt; 15%) of which 4 had beneficial alleles from Forrest. Therefore, quantitative resistance to race 2 of FLS was inferred to have major loci contributions from Essex and minor loci contributions from both Forrest and Essex. Resistance was dependent on plant age. Breeding and selection for FLS will be complex and may be more efficient with the markers, germplasm and models of inheritance reported here.

Soybean Yield Response to Pyraclostrobin and Drainage Water Management

Although pyraclostrobin {carbamic acid, [2,[[[1‐(4‐chlorophenyl)‐1H‐pyrazol‐3‐yl]oxy]methyl]phenyl]methoxy‐,methyl ester} has been used to protect soybean [Glycine max (L.) Merr.] from foliar diseases, its interaction with drainage water management (DWM) systems was unknown. Field research during two wet years (95 to 97 mm greater than the past decade) evaluated the effects of pyraclostrobin application timing (R3, R5, R3+R5, and R3+R5+lambda‐cyhalothrin) and DWM system (nondrained and drainage only [DO] or drainage plus subirrigation [DSI] at 6.1 and 12.2 m drain tile spacings) on soybean yield, grain quality, and severity of Septoria brown spot (SBS) (Septoria glycines) and frogeye leaf spot (FLS) (Cercospora sojina). Grain yields increased 18 to 22% with DO or DSI at 6.1 and 12.2 m spacings compared to a nonfungicide treated, nondrained control. In the absence of drainage, pyraclostrobin with or without lambda‐cyhalothrin increased yields 20 to 27% compared to the nondrained, nonfungicide treated control. The combination of DWM and pyraclostrobin increased grain yields up to 36%. Pyraclostrobin plus lambda‐cyhalothrin at R3+R5 increased yield 8 to 12% except with DO at 12.2 m compared to similar nonfungicide‐treated DWM systems. A DWM and pyraclostrobin interaction was detected for grain oil and protein concentration, but differences were minimal. Pyraclostrobin with or without lambda‐cyhalothrin reduced severity of SBS and FLS 2 to 8% depending on the year, but DWM did not affect severity of these diseases. The greatest synergistic yield increase on a claypan soil occurred when foliar disease management and DWM systems were used together in years with higher than normal rainfall.

Soybean Response to Preplant and Foliar‐Applied Potassium Chloride with Strobilurin Fungicides

Potassium chloride (KCl) preplant or foliar applied with fungicides to soybean [Glycine max (L.) Merr.] may allow farmers to increase yields when soil K availability is reduced and/or when Septoria brown spot (SBS) (Septoria glycines), frogeye leaf spot (FLS) (Cercospora sojina), or sudden death syndrome (SDS) (Fusarium solani (Mart.) Sacc. f. sp. glycines) are present. Interactions between fertilizer and fungicide management programs have not been examined in the central United States. Our objective was to evaluate the effect of preplant‐ and foliar‐applied KCl alone or combined with pyraclostrobin, azoxystrobin, or azoxystrobin plus lambda‐cyhalothrin on soybean response and severity of SBS, FLS, and SDS. Experiments were conducted in northeastern (Novelty) and southeastern (Qulin) Missouri in 2006 and 2007 on soils with low to medium K. Leaf K concentrations increased 1.4 to 6.1 g kg−1 following preplant KCl compared to nontreated or foliar KCl. Leaf Cl concentrations increased significantly with preplant KCl at Qulin and foliar KCl at R4 at Qulin and Novelty. At Novelty, preplant KCl reduced the severity of SBS and FLS up to 6%, and increased yield 340 kg ha−1, while foliar KCl increased yield 110 kg ha−1 An R4 application of strobilurin fungicides increased yields 230 to 360 kg ha−1 at Novelty. At Qulin, soybean yield increased 390 kg ha−1 with preplant KCl, while there were variable effects of fungicides on the severity of SBS, FLS, or SDS, and no yield increase due to fungicides or foliar KCl. Foliar applications of KCl were no substitute for preplant KCl.

Incidence and severity of frogeye leaf spot and associated yield losses in soybeans in agroecological zone II of Zambia.

The incidence and severity of frogeye leaf spot of soybean (Glycine max (L.) Merr.) was studied in agroecological region II of Zambia during the 1997/98 crop growing season. A survey was conducted on farmers' fields on SCSI Kaleya, Magoye and Hernon-147 cultivars. Disease incidence and severity was assessed by monitoring disease increments at two weeks interval (beginning of January to April) from nine fields, three from each province. Soybean cultivars were evaluated for yield losses resulting from frogeye leaf spot. Field plots of each cultivar were either sprayed twice with benomyl (benlate) or not sprayed at all. The results showed that the incidence of frogeye leaf spot was highest in Southern province (5.1), followed by Lusaka province (4.9) while Central province had the lowest disease incidence (1.8). Values for area under disease progress curve (AUDPC) were significantly greater (P < 0.05) for Lusaka and Southern provinces than for Central province. Yields in benomyl protected plots ranged from 1444 kg ha-1 to 2320 kg ha-1 and were significantly different among the cultivars. Average yields of non protected plants were reduced by 30.5% for Kaleya, 35.6% for Hernon-147 and 37.2% for SCS1. Incidence and severity increased with time and varied depending on weather parameters and susceptibility of cultivars to the disease. Yield losses due to frogeye leaf spot occurred through a reduction in seed size. Differences in weather conditions and amount of inocula are believed to contribute to the observed variation in incidence and severity of the disease at different locations.

The effect of timing of fungicide applications on control of frogeye leaf spot and grain yield of soybeans

Soybean cultivar Samsoy 1, and the breeding lines TGx 849-313D and TGx 996-26E, grown in a field with a heavy epidemic of frogeye leaf spot caused byCercospora sojina, were treated with double foliar applications of the fungicide benomyl. The treatments were made using four application schedules at six different growth stages, starting from V3 (fully developed leaves, beginning with trifoliate nodes) to R5 (beginning seed_, to determine the effect of the fungucide timing on frogeye leaf spot severity, soybean grain yield and grain quality. Generally, applications at R1 (beginning bloom) and R3 (beginning pod) significantly (P<-0.05) reduced disease severity in the 2 susceptible genotypes, Samsoy 1 and TGx 849-313D. Plot yields of these genotypes were also significantly greater than the untreated controls when the fungicide applications were made at R1 and R3. There was no significant difference in diseave severity or grain yield, between the untreated control and the different times of application, on the resistant genotype TGx 996-26E. Improved seed germination and lower levels of seed infection byC. sojina occurred for all fungicide timings in the susceptible genotypes. The results suggest that fungicide spraying initiated at R1 and followed up at R3 is most effective in frogeye leaf spot control and can also result in higher grain yields, than applications made earlier or later in the season. Control of frogeye leaf spot, however, is best achieved by growing resistant cultivars.

Effect of planting date on severity of frogeye leaf spot and grain yield of soybeans

The influence of soybean ( Glycine max) planting date on seasonal epidemics of frogeye leaf spot caused by Cercospora sojina and on grain yield, was evaluated during the 1991 and 1992 cropping seasons. Two susceptible (Samsoy 1 and TGx 849-313D) and one resistant (TGx 996-26E) soybean cultivars were used in field trials at Zonkwa, in the southern Kaduna state of Nigeria. Four field plantings were made at 14-day intervals from late May to mid0July. For each planting date, half the plots received foliar applications of benomyl at R 1 and R 3 growth stages, and the other half was untreated. Frogeye disease severity ratings were taken for all plots at R 4 to R 5 growth stages, and grain yield for each plot was measured at harvest. There was a significant ( p < 0.05) increase in disease severity between the first and third plantings on the two susceptible cultivars in 1991, and on TGx 849-313D only in 1992. Corresponding yield reductions for the two susceptible cultivars averaged 13 and 31% for the treated and untreated plots, respectively, with each 2-week delay beyond the first planting date in 1991, and 5 and 6%, respectively, in 1992. There was no significant difference in disease among all treatments on the resistant cultivar, TGx 996-26E. Under Nigerian conditions, an increase in frogeye leaf spot severity can be expected with delayed planting of soybean after 1 June, and this can result in a corresponding loss of crop yield.