Kiersten A. Wise

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 Piephos 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.

Meta-analysis of yield response of hybrid field corn to foliar fungicides in the U.S. corn belt

The use of foliar fungicides on field corn has increased greatly over the past 5 years in the United States in an attempt to increase yields, despite limited evidence that use of the fungicides is consistently profitable. To assess the value of using fungicides in grain corn production, random-effects meta-analyses were performed on results from foliar fungicide experiments conducted during 2002 to 2009 in 14 states across the United States to determine the mean yield response to the fungicides azoxystrobin, pyraclostrobin, propiconazole + trifloxystrobin, and propiconazole + azoxystrobin. For all fungicides, the yield difference between treated and nontreated plots was highly variable among studies. All four fungicides resulted in a significant mean yield increase relative to the nontreated plots (P < 0.05). Mean yield difference was highest for propiconazole + trifloxystrobin (390 kg/ha), followed by propiconazole + azoxystrobin (331 kg/ha) and pyraclostrobin (256 kg/ha), and lowest for azoxystrobin (230 kg/ha). Baseline yield (mean yield in the nontreated plots) had a significant effect on yield for propiconazole + azoxystrobin (P < 0.05), whereas baseline foliar disease severity (mean severity in the nontreated plots) significantly affected the yield response to pyraclostrobin, propiconazole + trifloxystrobin, and propiconazole + azoxystrobin but not to azoxystrobin. Mean yield difference was generally higher in the lowest yield and higher disease severity categories than in the highest yield and lower disease categories. The probability of failing to recover the fungicide application cost (p(loss)) also was estimated for a range of grain corn prices and application costs. At the 10-year average corn grain price of

Biology, Yield loss and Control of Sclerotinia Stem Rot of Soybean

0.12/kg (

Resistance to QoI Fungicides in Ascochyta rabiei from Chickpea in the Northern Great Plains

2.97/bushel) and application costs of

Corn yield loss estimates due to diseases in the United States and Ontario, Canada from 2012 to 2015.

40 to 95/ha, p(loss) for disease severity <5% was 0.55 to 0.98 for pyraclostrobin, 0.62 to 0.93 for propiconazole + trifloxystrobin, 0.58 to 0.89 for propiconazole + azoxystrobin, and 0.91 to 0.99 for azoxystrobin. When disease severity was >5%, the corresponding probabilities were 0.36 to 95, 0.25 to 0.69, 0.25 to 0.64, and 0.37 to 0.98 for the four fungicides. In conclusion, the high p(loss) values found in most scenarios suggest that the use of these foliar fungicides is unlikely to be profitable when foliar disease severity is low and yield expectation is high.

First Report of Goss's Bacterial Wilt and Leaf Blight on Corn Caused by Clavibacter michiganensis subsp. nebraskensis in Indiana

Sclerotinia stem rot (also known as white mold) of soybean is a significant yield-limiting problem in the North Central production region. This disease, caused by the fungus Sclerotinia sclerotiorum (Lib.) de Bary, varies in incidence and severity from year to year because of its sensitivity to weather conditions. Losses because of Sclerotinia stem rot can be substantial when environmental conditions and management practices favor high yield potential. Employing a disease management plan based on knowledge of field history and best disease management practices can help reduce losses from Sclerotinia stem rot. An effective disease management plan integrates several management tactics that include cultural practices, varietal resistance, as well as chemical and biological control. Understanding how different environmental variables and management practices influence infection by S . sclerotiorum and disease development are important to optimize disease management and reduce losses. This profile summarizes research-based knowledge of Sclerotinia stem rot, including the disease cycle, the scope of the losses that can occur because of this disease, how to identify both the pathogen S . sclerotiorum and the disease, and current management recommendations.

A Coordinated Effort to Manage Soybean Rust in North America: A Success Story in Soybean Disease Monitoring

Ascochyta blight, caused by Ascochyta rabiei (teleomorph: Didymella rabiei), is an important fungal disease of chickpea (Cicer arietinum). A monitoring program was established in 2005 to determine the sensitivity of A. rabiei isolates to the QoI (strobilurin) fungicides azoxystrobin and pyraclostrobin. A total of 403 isolates of A. rabiei from the Northern Great Plains and the Pacific Northwest were tested. Ninety-eight isolates collected between 2005 and 2007 were tested using an in vitro spore germination assay to determine the effective fungicide concentration at which 50% of conidial germination was inhibited (EC50) for each isolate-fungicide combination. A discriminatory dose of 1 μg/ml azoxystrobin was established and used to test 305 isolates from 2006 and 2007 for in vitro QoI fungicide sensitivity. Sixty-five percent of isolates collected from North Dakota in 2005, 2006, and 2007 and from Montana in 2007 were found to exhibit a mean 100-fold decrease in sensitivity to both azoxystrobin and pyraclostrobin when compared to sensitive isolates, and were considered to be resistant to azoxystrobin and pyraclostrobin. Under greenhouse conditions, QoI-resistant isolates of A. rabiei caused significantly higher amounts of disease than sensitive isolates on azoxystrobin- or pyraclostrobin-amended plants. These results suggest that disease control may be inadequate at locations where resistant isolates are present.

Oomycete Species Associated with Soybean Seedlings in North America—Part I: Identification and Pathogenicity Characterization

Annual decreases in corn yield caused by diseases were estimated by surveying members of the Corn Disease Working Group in 22 corn-producing states in the United States and in Ontario, Canada, from 2012 through 2015. Estimated loss from each disease varied greatly by state and year. In general, foliar diseases such as northern corn leaf blight, gray leaf spot, and Goss’s wilt commonly caused the largest estimated yield loss in the northern United States and Ontario during nondrought years. Fusarium stalk rot and plant-parasitic nematodes caused the most estimated loss in the southernmost United States. The estimated mean economic loss due to yield loss by corn diseases in the United States and Ontario from 2012 to 2015 was

Fungicide and Cultivar Effects on Sudden Death Syndrome and Yield of Soybean

76.51 USD per acre. The cost of disease-mitigating strategies is another potential source of profit loss. Results from this survey will provide scientists, breeders, government, and educators with data to help inform and prioritize research, policy, and educational efforts in corn pathology and disease management. M U E L L E R E T A L . , P L A N T H E A L T H P R O G R E S S 1 7 (2 0 1 6 )

Oomycete Species Associated with Soybean Seedlings in North America—Part II: Diversity and Ecology in Relation to Environmental and Edaphic Factors

In August of 2008, leaves of hybrid corn (Zea mays L.) and popcorn from Pulaski and Jasper counties in northwest Indiana were submitted to the Purdue Plant and Pest Diagnostic Lab with symptoms characteristic of a bacterial disease. Symptomatic leaves had large, tan-to-gray necrotic lesions with dark freckling present within the lesions. Shiny bacterial exudate was present on the surface of many of the lesions. Microscopic observation revealed no fungal structures within the lesions, and bacterial streaming was observed from the cut edge of symptomatic tissue under ×100 magnification with phase contrast. A commercially available ELISA test (Agdia Inc., Elkhart, IN) determined that samples were negative for Pantoea stewartii, the causal agent of Stewarts bacterial leaf blight and wilt. A bacterial suspension was prepared from symptomatic tissue and streaked onto Kings B medium and subcultured on semiselective CNS medium (1,2). Axenic, peach-colored colonies present on the CNS medium tested gram positive with a KOH test. Analysis of fatty acid methyl esters (MIDI Inc, Newark, DE) indicated that the strain was very similar (0.611) to Clavibacter michiganensis. Amplification of the 500-bp 16S rRNA region of the bacterial gene and subsequent BLAST alignments of the resulting sequence indicated a 99% match for C. michiganensis subsp. nebraskensis (GenBank Accession Nos. AM410697 and U09763; D16S2 gene bacterial library, version 2.10; MIDI Inc,). Kochs postulates were used to confirm pathogenicity of the isolated bacteria on corn inbred B73. Eighteen plants were mechanically inoculated at growth stage V1 to V2 with a bacterial suspension of approximately 1 × 108 CFU/ml prepared from cultures grown on CNS for 10 days at 28°C (2). Inoculum was rubbed onto leaves dusted with Carborundum and 0.1 ml of the bacterial suspension was injected into stems with a hypodermic needle. Nine control plants were inoculated with sterile water. Plants were kept at greenhouse conditions (24°C) with supplemental 400W high-pressure sodium light. Within 5 to 8 days, leaves and stems of all 18 inoculated plants developed water-soaked, necrotic lesions. No symptoms were observed in control plants. Bacteria were reisolated from symptomatic plants on CNS medium as described above, and gram-positive colonies were obtained. Reisolated strains were identical to C. michiganensis subsp. nebraskensis by D16S2 DNA sequence analysis, confirming the causal agent of the disease. Disease incidence in affected fields ranged from 20 to 60% and significant yield loss was reported. This confirmation is of regulatory importance because of potential export restrictions of Indiana-grown seed corn and popcorn to select countries. To our knowledge, this is the first report of Gosss bacterial wilt and leaf blight on corn in Indiana. References: (1) D. C. Gross and A. K. Vidaver. Phytopathology 69:82, 1979. (2) L. M. Shepherd. M.S. thesis. Iowa State University. Ames, 1999.