Integrated Effects of Genetic Resistance and Prothioconazole + Tebuconazole Application Timing on Fusarium Head Blight in Wheat.

Integrated Fusarium head blight (FHB) management programs consisting of different combinations of cultivar resistance class and an application of the fungicide prothioconazole + tebuconazole at or after 50% early anthesis were evaluated for efficacy against FHB incidence (INC; percentage of diseased spikes), index (IND; percentage of diseased spikelets per spike), Fusarium damaged kernel (FDK), deoxynivalenol (DON) toxin contamination, grain yield, and test weight (TW) in inoculated field trials conducted in 11 U.S. states in 2014 and 2015. Mean log response ratios and corresponding percent control values for INC, IND, FDK, and DON, and mean differences in yield and TW relative to a nontreated, inoculated susceptible check (S_CK), were estimated through network meta-analyses as measures of efficacy. Results from the analyses were then used to estimate the economic benefit of each management program for a range of grain prices and fungicide applications costs. Management programs consisting of a moderately resistant (MR) cultivar treated with the fungicide were the most efficacious, reducing INC by 60 to 69%, IND by 71 to 76%, FDK by 66 to 72%, and DON by 60 to 64% relative to S_CK, compared with 56 to 62% for INC, 68 to 72% for IND, 66 to 68% for FDK, and 58 to 61% for DON for programs with a moderately susceptible (MS) cultivar. The least efficacious programs were those with a fungicide application to a susceptible (S) cultivar, with less than a 45% reduction of INC, IND, FDK, or DON. All programs were more efficacious under conditions favorable for FHB compared with less favorable conditions, with applications made at 50% early anthesis being of comparable efficacy to those made 2 to 7 days later. Programs with an MS cultivar resulted in the highest mean yield increases relative to S_CK (541 to 753 kg/ha), followed by programs with an S cultivar (386 to 498 kg/ha) and programs with an MR cultivar (250 to 337 kg/ha). Integrated management programs with an MS or MR cultivar treated with the fungicide at or after 50% early anthesis were the most likely to result in a 50 or 75% control of IND, FDK, or DON in a future trial. At a fixed fungicide application cost, these programs were $4 to $319/MT more economically beneficial than corresponding fungicide-only programs, depending on the cultivar and grain price. These findings demonstrate the benefits of combining genetic resistance with a prothioconazole + tebuconazole treatment to manage FHB, even if that treatment is applied a few days after 50% early anthesis.

Impact of Timing and Method of Virus Inoculation on the Severity of Wheat Streak Mosaic Disease.

Wheat streak mosaic virus (WSMV), transmitted by the wheat curl mite Aceria tosichella, frequently causes significant yield loss in winter wheat throughout the Great Plains of the United States. A field study was conducted in the 2013-14 and 2014-15 growing seasons to compare the impact of timing of WSMV inoculation (early fall, late fall, or early spring) and method of inoculation (mite or mechanical) on susceptibility of winter wheat cultivars Mace (resistant) and Overland (susceptible). Relative chlorophyll content, WSMV incidence, and yield components were determined. The greatest WSMV infection occurred for Overland, with the early fall inoculations resulting in the highest WSMV infection rate (up to 97%) and the greatest yield reductions relative to the control (up to 94%). In contrast, inoculation of Mace resulted in low WSMV incidence (1 to 28.3%). The findings from this study indicate that both method of inoculation and wheat cultivar influenced severity of wheat streak mosaic; however, timing of inoculation also had a dramatic influence on disease. In addition, mite inoculation provided much more consistent infection rates and is considered a more realistic method of inoculation to measure disease impact on wheat cultivars.

Effect of Temperature on Wheat Streak Mosaic Disease Development in Winter Wheat.

Temperature is one of the key factors that influence viral disease development in plants. In this study, temperature effect on Wheat streak mosaic virus (WSMV) replication and in planta movement was determined using a green fluorescent protein (GFP)-tagged virus in two winter wheat cultivars. Virus-inoculated plants were first incubated at 10, 15, 20, and 25°C for 21 days, followed by 27°C for 14 days; and, in a second experiment, virus-inoculated plants were initially incubated at 27°C for 3 days, followed by 10, 15, 20, and 25°C for 21 days. In the first experiment, WSMV-GFP in susceptible 'Tomahawk' wheat at 10°C was restricted at the point of inoculation whereas, at 15°C, the virus moved systemically, accompanied with mild symptoms, and, at 20 and 25°C, WSMV elicited severe WSMV symptoms. In resistant 'Mace' wheat (PI 651043), WSMV-GFP was restricted at the point of inoculation at 10 and 15°C but, at 20 and 25°C, the virus infected systemically with no visual symptoms. Some plants that were not systemically infected at low temperatures expressed WSMV-GFP in regrowth shoots when later held at 27°C. In the second experiment, Tomahawk plants (100%) expressed systemic WSMV-GFP after 21 days at all four temperature levels; however, systemic WSMV expression in Mace was delayed at the lower temperatures. These results indicate that temperature played an important role in WSMV replication, movement, and symptom development in resistant and susceptible wheat cultivars. This study also demonstrates that suboptimal temperatures impair WSMV movement but the virus rapidly begins to replicate and spread in planta under optimal temperatures.

Differential Transmission of Two Isolates of Wheat streak mosaic virus by Five Wheat Curl Mite Populations.

Wheat streak mosaic virus (WSMV), type member of the genus Tritimovirus in the family Potyviridae, is an economically important virus causing annual average yield losses of approximately 2 to 3% in winter wheat across the Great Plains. The wheat curl mite (WCM), Aceria tosichella, transmits WSMV along with two other viruses found throughout the Great Plains of the United States. Two common genotypes of WSMV (Sidney 81 and Type) in the United States share 97.6% nucleotide sequence identity but their transmission relationships with the WCM are unknown. The objective of this study was to determine transmission of these two isolates of WSMV by five WCM populations ('Nebraska', 'Montana', 'South Dakota', 'Type 1', and 'Type 2'). Nonviruliferous mites from each population were reared on wheat source plants mechanically inoculated with either Sidney 81 or Type WSMV isolates. For each source plant, individual mites were transferred to 10 separate test plants and virus transmission was determined by a double-antibody sandwich enzyme-linked immunosorbent assay. Source plants were replicated nine times for each treatment (90 individual mite transfers). Results indicate that three mite populations transmitted Sidney 81 at higher rates compared with Type. Two mite populations (Nebraska and Type 2) transmitted Sidney 81 and Type at higher rates compared with the other three populations. Results from this study demonstrate that interactions between virus isolates and mite populations influence the epidemiology of WSMV.

Incidence of Wheat streak mosaic virus, Triticum mosaic virus, and Wheat mosaic virus in Wheat Curl Mites Recovered from Maturing Winter Wheat Spikes.

Wheat curl mites (WCM; Aceria tosichella) transmit Wheat streak mosaic virus (WSMV), Triticum mosaic virus (TriMV), and Wheat mosaic virus (WMoV) to wheat (Triticum aestivum L.) in the Great Plains region of the United States. These viruses can be detected in single, double, or triple combinations in leaf samples. Information on incidence of viruses in WCM at the end of the growing season is scant. The availability of this information can enhance our knowledge of the epidemiology of WCM-transmitted viruses. This research was conducted to determine the frequency of occurrence of WSMV, TriMV, and WMoV in WCM populations on field-collected maturing wheat spikes and to determine differences in WCM densities in three geographical regions (southeast, west-central, and panhandle) in Nebraska. Maturing wheat spikes were collected from 83 fields across Nebraska in 2011 and 2012. The spikes were placed in proximity to wheat seedlings (three- to four-leaf stage) in WCM-proof cages in a growth chamber and on sticky tape. WCM that moved off the drying wheat spikes in cages infested the wheat seedlings. WCM that moved off wheat spikes placed on sticky tape were trapped on the tape and were counted under a dissecting microscope. At 28 days after infestation, the wheat plants were tested for the presence of WSMV, TriMV, or WMoV using enzyme-linked immunosorbent assay and multiplex polymerase chain reaction. WSMV was the most predominant virus detected in wheat seedlings infested with WCM from field-collected spikes. Double (TriMV+WSMV or WMoV+WSMV) or triple (TriMV+ WMoV +WSMV) virus detections were more frequent (47%) than single detections (5%) of TriMV or WSMV. Overall, 81% of the wheat seedlings infested with WCM tested positive for at least one virus. No significant association (P > 0.05) was found between regions for WCM trapped on tape. These results suggest that WCM present on mature wheat spikes harbor multiple wheat viruses and may explain high virus incidence when direct movement of WCM into emerging winter wheat occurs in the fall.

Impact of Wheat streak mosaic virus and Triticum mosaic virus Coinfection of Wheat on Transmission Rates by Wheat Curl Mites.

Wheat streak mosaic virus (WSMV) and Triticum mosaic virus (TriMV) are transmitted by the wheat curl mite (WCM, Aceria tosichella), and coinfections of wheat by these viruses are common in the field. Previous work has shown that mite genotypes vary in their ability to transmit TriMV. However, the degree to which coinfection of wheat modifies WCM vector competence has not been studied. The objective was to determine whether mite genotypes differed in virus transmission ability when feeding on wheat coinfected by WSMV and TriMV. First, WCM genotype type 2 was used to determine virus transmission rates from mock-, WSMV-, TriMV-, and coinfected wheat plants. Transmission rates were determined by using single-mite transfers from replicated source plants. Coinfection reduced WSMV transmission by type 2 WCM from 50 to 35.6%; however, coinfection increased TriMV transmission from 43.3 to 56.8%. Mite survival on single-mite transfer test plants indicates that the reduction in WSMV transmission may result from poor mite survival when TriMV is present. In a second study, two separate colonies of WCM genotype type 1 were tested to assess the impact of coinfection on transmission. Type 1 mites did not transmit TriMV from coinfected plants but the two colonies varied in transmission rates for WSMV (20.9 to 36.5%). Even though these changes in mite transmission rates are moderate, they help explain the high relative incidence of TriMV-positive plants that are coinfected with WSMV in field observations. These findings begin to demonstrate the complicated interactions found in this mite-virus complex.

Quantification of Yield Loss Caused by Triticum mosaic virus and Wheat streak mosaic virus in Winter Wheat Under Field Conditions.

Triticum mosaic virus (TriMV) and Wheat streak mosaic virus (WSMV) infect winter wheat (Triticum aestivum) in the Great Plains region of the United States. The two viruses are transmitted by wheat curl mites (Aceria tosichella), which also transmit High Plains virus. In a field study conducted in 2011 and 2012, winter wheat cultivars Millennium (WSMV-susceptible) and Mace (WSMV-resistant) were mechanically inoculated with TriMV, WSMV, TriMV+WSMV, or sterile water at the two-leaf growth stage. Chlorophyll meter (soil plant analysis development [SPAD]) readings, area under the SPAD progress curve (AUSPC), grain yield (=yield), yield components (spikes/m2, kernels/spike, 1,000-kernel weight), and aerial dry matter were determined. In Millennium, all measured variables were significantly reduced by single or double virus inoculation, with the greatest reductions occurring in the double-inoculated treatment. Among the yield components, the greatest reductions occurred in spikes/m2. In Mace, only AUSPC was significantly reduced by the TriMV+WSMV treatment in 2012. There was a significant (P ≤ 0.05), negative linear relationship between SPAD readings and day of year in all inoculation treatments in Millennium and in the TriMV+WSMV treatment in Mace. There were significant (P ≤ 0.05), positive linear relationships between yield and SPAD readings and between yield and aerial dry matter in Millennium but not in Mace. The results from this study indicate that under field conditions, (i) Mace, a WSMV-resistant cultivar, is also resistant to TriMV, and (ii) double inoculation of winter wheat by TriMV and WSMV exacerbates symptom expression and yield loss in a susceptible cultivar.

Occurrence and Distribution of Triticum mosaic virus in the Central Great Plains.

Wheat curl mite (WCM)-transmitted viruses-namely, Wheat streak mosaic virus (WSMV), Triticum mosaic virus (TriMV), and the High Plains virus (HPV)-are three of the wheat-infecting viruses in the central Great Plains of the United States. TriMV is newly discovered and its prevalence and incidence are largely unknown. Field surveys were carried out in Colorado, Kansas, Nebraska, and South Dakota in spring and fall 2010 and 2011 to determine TriMV prevalence and incidence and the frequency of TriMV co-infection with WSMV or HPV in winter wheat. WSMV was the most prevalent and was detected in 83% of 185 season-counties (= s-counties), 73% of 420 season-fields (= s-fields), and 35% of 12,973 samples. TriMV was detected in 32, 6, and 6% of s-counties, s-fields, and samples, respectively. HPV was detected in 34, 15, and 4% of s-counties, s-fields, and samples, respectively. TriMV was detected in all four states. In all, 91% of TriMV-positive samples were co-infected with WSMV, whereas WSMV and HPV were mainly detected as single infections. The results from this study indicate that TriMV occurs in winter wheat predominantly as a double infection with WSMV, which will complicate breeding for resistance to WCM-transmitted viruses.

Efficacy and Stability of Integrating Fungicide and Cultivar Resistance to Manage Fusarium Head Blight and Deoxynivalenol in Wheat.

Integration of host resistance and prothioconazole + tebuconazole fungicide application at anthesis to manage Fusarium head blight (FHB) and deoxynivalenol (DON) in wheat was evaluated using data from over 40 trials in 12 U.S. states. Means of FHB index (index) and DON from up to six resistance class-fungicide management combinations per trial (susceptible treated [S_TR] and untreated [S_UT]; moderately susceptible treated [MS_TR] and untreated [MS_UT]; moderately resistant treated [MR_TR] and untreated [MR_UT]) were used in multivariate meta-analyses, and mean log response ratios across trials were estimated and transformed to estimate mean percent control ( ) due to the management combinations relative to S_UT. All combinations led to a significant reduction in index and DON (P < 0.001). MR_TR was the most effective combination, with a of 76% for index and 71% for DON, followed by MS_TR (71 and 58%, respectively), MR_UT (54 and 51%, respectively), S_TR (53 and 39%, respectively), and MS_UT (43 and 30%, respectively). Calculations based on the principle of treatment independence showed that the combination of fungicide application and resistance was additive in terms of percent control for index and DON. Management combinations were ranked based on percent control relative to S_UT within each trial, and nonparametric analyses were performed to determine management combination stability across environments (trials) using the Kendall coefficient of concordance (W). There was a significant concordance of management combinations for both index and DON (P < 0.001), indicating a nonrandom ranking across environments and relatively low variability in the within-environment ranking of management combinations. MR_TR had the highest mean rank (best control relative to S_UT) and was one of the most stable management combinations across environments, with low rank stability variance (0.99 for index and 0.67 for DON). MS_UT had the lowest mean rank (poorest control) but was also one of the most stable management combinations. Based on Piepho's nonparametric rank-based variance homogeneity U test, there was an interaction of management combination and environment for index (P = 0.011) but not for DON (P = 0.147), indicating that the rank ordering for index depended somewhat on environment. In conclusion, although the magnitude of percent control will likely vary among environments, integrating a single tebuconazole + prothioconazole application at anthesis with cultivar resistance will be a more effective and stable management practice for both index and DON than either approach used alone.

Effects of Single and Double Infections of Winter Wheat by Triticum mosaic virus and Wheat streak mosaic virus on Yield Determinants.

Triticum mosaic virus (TriMV) is a recently discovered virus infecting wheat (Triticum aestivum) in the Great Plains region of the United States. It is transmitted by wheat curl mites (Aceria tosichella) which also transmit Wheat streak mosaic virus (WSMV) and Wheat mosaic virus. In a greenhouse study, winter wheat 'Millennium' (WSMV susceptible) and 'Mace' (WSMV resistant) were mechanically inoculated with TriMV, WSMV, TriMV+WSMV, or sterile water at the two-leaf growth stage. At 28 days after inoculation, final chlorophyll meter (soil plant analysis development [SPAD]) readings, area under the SPAD progress curve (AUSPC), the number of tillers per plant, shoot and root weight, and total nitrogen and carbon content were determined. In Millennium, all measured variables were significantly reduced by single or double virus infections, with the greatest reductions occurring in the double-infection treatment. In Mace, only final SPAD readings, AUSPC, and total nitrogen were significantly reduced by single or double virus infections. There was a significant (P ≤ 0.05), positive linear relationship between SPAD readings and shoot weight in Millennium but not in Mace. The relationship between total nitrogen and shoot weight was positive, linear, and significant in both cultivars. The results from this study indicate that Mace, a WSMV-resistant cultivar, is also resistant to TriMV, and double infection of winter wheat by TriMV and WSMV exacerbates symptom expression and loss of biomass in susceptible cultivars.

First Report of Downy Mildew Caused by Plasmopara obducens on Impatiens in California.

During February 2004, diseased double impatiens (Impatiens walleriana) plants were received from a commercial grower in southern California. The upper surfaces of symptomatic leaves were pale yellow with no distinct lesions. Diseased leaves later wilted, and severely affected leaves abscised from the stem. At the nursery, only double impatiens plants in the Fiesta series were infected, and some cultivars were more heavily infected than others. Disease incidence in cv. Sparkler Hot pink was nearly 100%. The interior of infected leaves was colonized by coenocytic mycelium. A conspicuous white growth was observed only on the underside of leaves. Sporangiophores were hyaline, thin walled, emergent from stomata, and had slightly swollen bases. Sporangiophore branching was distinctly monopodial. Smaller sporangiophore branches were arranged at right angles to the supporting branches, and tips of branches measured 8 to 14 µm long. Sporangia were ovoid and hyaline with a single pore on the distal ends. Distal ends of sporangia were predominantly flat but occasionally had a slight papilla. Short pedicels were present on the attached ends. Sporangia measured 19.4 to 22.2 (-25.0) µm × 13.9 to 16.7 (-19.4) µm. Oospores were not observed in leaf tissue. On the basis of symptoms and morphology of the organism, the pathogen was identified as Plasmopara obducens J. Schröt. Pathogenicity tests were done on double type cvs. Fiesta, Tioga Red, and Tioga Cherry Red and on single type cvs. Cajun Watermelon and Accent Lilac. Plants were spray inoculated with sporangiospore suspensions (1 × 104 sporangiospores per milliliter), incubated for 24 h in a dew chamber (18 to 20°C), and then maintained in a greenhouse (22 to 24°C). Symptoms and signs of downy mildew developed after 12 days only on inoculated cv. Fiesta plants, and the pathogen morphology matched that of the originally observed pathogen. Nontreated control plants did not develop downy mildew. To our knowledge, this is the first report of downy mildew on impatiens in California. P. obducens is one of two causal agents of downy mildew of impatiens (2,4). The other pathogen, Bremiella sphaerosperma, has dichotomous sporangiophore branching and causes lesions with well-defined margins (2,4). In the United States, the disease has been recorded in the eastern and northeastern states and in Indiana, Minnesota, Mississippi, Montana, and Wisconsin (3). In Canada, the disease has been recorded in Manitoba and Quebec (1). References: (1) I. L. Conners. An Annotated Index of Plant Diseases in Canada and Fungi Recorded on Plants in Alaska, Canada, and Greenland. Research Branch, Canada Department of Agriculture, Publication 1251, 1967. (2) O. Constantinescu. Mycologia 83:473, 1991. (3) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, 1989. (4) G. W. Wilson. Bull. Torrey Bot. Club 34:387, 1907.

Strawberry Plant Extracts Stimulate Secondary Conidiation by Colletotrichum acutatum on Symptomless Leaves.

ABSTRACT Conidial suspensions of Colletotrichum acutatum were prepared in 1:27, 1:45, and 1:81 (wt/vol) dilutions of an extract of strawberry (cv. Tristar) flowers or leaves in water. Strawberry leaves and plastic coverslips were sprayed with the conidial suspensions, incubated at 25 degrees C and continuous wetness for 48 h, and the number of conidia and appressoria were counted. In another experiment, leaves and coverslips were sprayed with a conidial suspension in water, incubated for 72 h to establish C. acutatum populations, and placed in a growth chamber under dry conditions for up to 6 weeks. At each sampling time, leaves and coverslips were sprayed with flower extracts, leaf extracts, or water, incubated for 48 h at 25 degrees C and continuous wetness, and the number of conidia and appressoria were counted. Flower extracts significantly (P

Influence of Temperature and Wetness Duration on Conidia and Appressoria of Colletotrichum acutatum on Symptomless Strawberry Leaves.

ABSTRACT Strawberry leaves (cv. Tristar) inoculated with Colletotrichum acuta-tum conidia were incubated at 10, 15, 20, 25, 30, and 35 degrees C under continuous wetness, and at 25 degrees C under six intermittent wetness regimes. The number of conidia and appressoria was quantified on excised leaf disks. In order to assess pathogen survival, inoculated leaves were frozen and incubated to induce acervular development. Germination, secondary3 conidiation, and appressorial development were significantly (P /= 0.95) related to appressorial populations prior to this treatment and was greatest following periods of continuous wetness. Production of secondary conidia and appressoria of C. acutatum on symptomless strawberry leaves under a range of environmental conditions suggests that these processes also occur under field conditions and contribute to inoculum availability during the growing season.

Germination and Sporulation of Colletotrichum acutatum on Symptomless Strawberry Leaves.

ABSTRACT The germination and sporulation of Colletotrichum acutatum were characterized over time on strawberry leaves (cv. Tristar) and plastic coverslips incubated at 26 degrees C under continuous wetness. Conidia germinated within 3 h after inoculation and formed melanized appressoria with pores by 9 h after inoculation. Host penetration was not observed up to 7 days after inoculation. Production of secondary conidia on conidial and hyphal phialides began within 6 h after inoculation. Secondary conidiation was responsible for up to a threefold increase in the total number of conidia within 7 days after inoculation. Primary conidia and hyphae began to collapse 48 h after inoculation, whereas melanized appressoria remained intact. These findings suggest that appressoria and secondary conidia of C. acutatum produced on symptomless strawberry foliage may be significant sources of inoculum for fruit infections.

Soybean Cultivar Responses to Sclerotinia sclerotiorum in Field and Controlled Environment Studies.

The responses of 12 soybean cultivars to Sclerotinia sclerotiorum were evaluated under field and controlled environment conditions. The 12 cultivars were planted in fields naturally or artificially infested with S. sclerotiorum and evaluated for disease incidence. In controlled environment studies, the cultivars were compared with respect to lesion size on detached leaves and stems inoculated with mycelial disks, incidence of stem rot following mycelial inoculation of foliage, lesion lengths on stems discolored by oxalic acid, and levels of soluble pigment(s) in stems. Significant differences (P ≤ 0.05) in disease incidence, lesion sizes, and levels of soluble pigment(s) were detected among the 12 cultivars in all evaluations, but ranking of cultivars varied among methods and experiments within methods. Corsoy 79 and S19-90 were consistently most resistant in all methods; whereas Kenwood, A2242, Bell, and Williams 82 were least resistant. Pearson correlation coefficients (R) for disease incidence between location years in field experiments (FE) ranged from 0.86 to 0.95. R values between FE and controlled environment experiments (CEE) ranged from 0.01 to 0.62 for detached leaf assays (DLA), -0.20 to 0.47 for lesion lengths on stems inoculated with mycelial disks (LLM), 0.38 to 0.45 for incidence of stem rot from mycelial inoculation of foliage (MIF), 0.08 to 0.66 for lesion lengths on stems discolored by oxalic acid (LLO), and -0.55 to -0.37 for levels of soluble stem pigment(s) (SSP). Absolute values of R between FE and CEE were 0.40 or greater in 40, 17, 33, 29, and 83% of all correlation analyses for DLA, LLM, MIF, LLO, and SSP, respectively. Spearman's coefficients of rank correlation (r s) between FE and CEE based on average performance were 0.55, -0.20, 0.40, 0.42, and -0.44 for DLA, LLM, MIF, LLO, and SSP, respectively. Disease development was slow when foliage was inoculated with a mycelial suspension. Based on experiment (E) by cultivar (C) interaction and rs values between replicate experiments within each method, determination of soluble stem pigment levels (0.86 ≤ rs ≤ 0.97; P < 0.001 for 100% of six analyses; P = 0.98 for E × C interaction) and measurement of lesion lengths on stems discolored by oxalic acid (0.27 ≤ rs ≤ 0.85; rs ≥ 0.58 and P ≤ 0.05 for 68% of 28 analyses; P = 0.07 for E × C interaction) were the most repeatable methods. The results from this study suggest that determination of levels of soluble pigments in stems, measurement of lesion lengths on stems discolored by oxalic acid, and the detached leaf assay may be better than mycelial inoculation of stems or foliage in evaluating soybean cultivars for field resistance to S. sclerotiorum.

Efficacy of Fungicide Treatments for Control of Common Rust and Northern Leaf Spot in Hybrid Corn Seed Production.

Field experiments were conducted during the 1992 and 1993 growing seasons in 13 commercial fields in Iowa to assess the efficacy of foliar applications of fungicides to seed corn for control of common rust, Puccinia sorghi, and northern leaf spot, Bipolaris zeicola. Chlorothalonil, mancozeb, and/or propiconazole sprays were initiated at growth stage (GS) 2.5, GS 4.0, or approximately 2 weeks before detasseling, and applications were repeated up to five times at various growth stages. The majority of fungicide treatments resulted in an increase in salable seed units (1 unit = 80,000 seeds) compared to untreated control treatments. The greatest increase in salable seed occurred in the large and medium seed size units. Although disease severity on the final assessment date in both years and standardized area under the disease progress curve in 1993 were significantly higher (P ≤ 0.05) in untreated control plots than in fungicide-treated plots in all experiments, the best disease control was provided by 5 applications of chlorothalonil or 2 to 4 applications of chlorothalonil, mancozeb, or propiconazole initiated on the first spray date.

Model for Economic Analysis of Fungicide Usage in Hybrid Corn Seed Production.

A model was used to determine the economic benefits of using fungicides to control foliar diseases of seed corn. The components of the model were (i) increased net return to the seed company from fungicide usage, (ii) increased income, based on a wholesale price of $30 per unit of 80,000 seeds and a premium of $2 per unit of medium-sized seed, and (iii) increased expenses, calculated as the sum of fungicide and fungicide application costs, cost of processing increased seed, and increased payment to the grower by the seed company. Increased payment to the grower was calculated as the product of increased seed yield (before separation into fractions), May futures price for no. 2 commercial corn on 1 March, and an average inbred production factor. Increased net return was calculated by subtracting increased expenses from increased income. Yield data were obtained from fungicide trial experiments in commercial hybrid corn seed production fields in Iowa from 1990 to 1993. Of 169 fungicide treatments applied, 64 and 79% were profitable to the seed company and to the grower, respectively. The results from this study indicate that fungicide usage in hybrid corn seed production can be economically beneficial.