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.

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

In August of 2011, the North Dakota State University Plant Diagnostic Lab received a hybrid corn (Zea mays) leaf sample from Burleigh County in south-central North Dakota (ND). The leaf had long, irregular, water-soaked lesions consistent with Goss's leaf blight of corn. Using a light microscope at 10× magnification, bacterial streaming was observed from the excised edge of leaf tissue. A bacterial suspension was created, streaked onto a semi-selective CNS medium (1), and incubated at 22°C. Dark yellow-orange colonies appeared on the medium after 5 days. Single colonies were subcultured onto additional CNS media. To verify the identity of the bacterial isolate, PCR amplification of the 16S ribosomal DNA from this isolate along with a known Clavibacter michiganensis spp. nebraskensis (Cmn) isolate collected in Indiana (4) was performed using the eubacterial universal primers 27f and 1525r (3). The 1,431-bp 16S rDNA region was obtained for each isolate and they were compared with each other and with those deposited in NCBI GenBank. Sequence alignment identified only one nucleotide difference between the ND isolate and the Indiana isolate. BLASTn search against the NCBI database showed the first 100 hits were described as C. michiganensis or unidentified Clavibacter sp. The ND isolate had a two-nucleotide difference with Cmn isolate NCPPB2581 (HE614873), and a three nucleotide difference was found with the C. michiganensis spp. michiganensis isolate NCPPB 382 (AM711867). To satisfy Koch's postulates, eight corn plants (Golden Cross Bantam) were grown in the greenhouse at 22 to 24°C. Four corn plants were inoculated at growth stage V4-V5 using a suspension of approximately 1 × 109 CFU/ml from cultures grown on CNS for 6 days. Wounds were created on the fifth leaf approximately 7 cm from the leaf tip using a tongue-seizing forceps outfitted with a rubber stopper composed of pins (2). Simultaneously, 1 ml of the bacterial suspension was delivered into the wounds through a hole on top of the rubber stopper. Four control plants were inoculated with sterile water in a similar fashion. No symptoms were observed on the control plants. After 6 days, long water-soaked symptoms were observed on leaves inoculated with the bacterial suspension. Using leaves with water-soaked lesions, the pathogen was re-isolated onto CNS media and subjected to PCR amplification, and the resulting amplicons were sequenced as before. The sequence of the amplicon from the re-isolation matched that of the original ND isolate. To our knowledge, this is the first account of Goss's leaf blight and wilt identified in ND. As the corn acreage and no-till production systems in the state have increased, the economic implications of this disease may become more significant. Recognition of symptoms and proper identification of this bacterial disease in the field should help reduce unnecessary foliar fungicide sprays. References: (1) D. C. Gross and A. K. Vidaver. Phytopathology 69:82, 1979. (2) W. A. Hagborg. Can. J. Bot. 48:1135, 1970. (3) X. Li and S. H. DeBoer. Can. J. Microbiol. 41:925, 1995. (4) G. Ruhl et al. Plant Dis. 93:841, 2009.

First Report of Virulence Phenotypes of Puccinia helianthi, Causal Agent of Sunflower Rust in Illinois.

Sunflower rust caused by Puccinia helianthi (Schw.) is an autoecious and macrocyclic rust that occurs on wild perennial, wild annual, and cultivated sunflower species (Helianthus annuus L.). Severe epidemics of sunflower rust can significantly reduce yield and quality of cultivated sunflower (4). Uredinia of P. helianthi were observed in individual sunflower fields in four Illinois counties in 2008, namely; Champaign, LaSalle, McLean, and Livingston. Leaves with uredinia from each field were collected and shipped to the USDA-ARS Sunflower Pathology Laboratory in Fargo, ND. Urediniospores were harvested by rinsing leaves with Soltrol 170 isoparaffin solvent (Chevron Phillips Chemicals, Woodlands, TX). Urediniospores were increased by inoculating the spore suspension onto 21-day-old seedlings of susceptible oilseed hybrid Myc-7350 with a vacuum-pump powered atomizer. After the Soltrol 170 evaporated, plants were placed in a dew chamber for 24 h and then moved to a greenhouse at approximately 20 to 25°C with a 14-h photoperiod. When sporulating uredinia were visible, a bulk collection of urediniospores was made from samples of each location. Additionally, one single-pustule derived isolate from each location was obtained by harvesting urediniospores from a single pustule and increasing as above. To determine the virulence phenotypes, each single-pustule derived isolate and bulk collection were inoculated as above onto at least two replicated sets of the nine international differential lines (each set containing three plants per line) and evaluated after 14 days (1,2). The single-pustule isolates from LaSalle, Livingston, and McLean counties were determined to be virulence phenotype 300. The single-pustule isolate from Champaign produced no infection on any differential lines, including the susceptible, and was therefore considered not viable. The virulence phenotypes of the bulk samples were coded as 330 (Champaign), 324 (McLean), and 336 (Livingston and LaSalle). Virulence of all aforementioned virulence phenotypes was found to be common in a recent survey of 400 Midwestern P. helianthi samples collected in 2007 and 2008 (1). Although sunflower production is limited in Illinois, expansion could occur. This is particularly true in southern Illinois, where double cropping sunflower after winter wheat is a possibility. Urediniospore germination and infection is favored by free moisture and temperatures ranging from 10 to 25°C, while optimum temperature for spore development ranges from 20 to 35°C (3). These environmental conditions are often more common in Illinois than in the major sunflower-producing states in the Great Plains, where sunflower rust is a concern annually. Thus, determination of P. helianthi virulence phenotypes in Illinois may be important for future management of sunflower rust. References: (1) T. J. Gulya and S. G. Markell. National Sunflower Association. Online Publication/Gulya_RustStatus_09, 2009. (2) T. Gulya and S. Masirevic. FAO Eur. Res. Network on Sunflower. Bucarest, Romania. 31, 1995. (3) T. Gulya et al. Sunflower Diseases. Page 263 in: Sunflower Technology and Production. A. A. Schneiter, ed. American Society of Agronomy, Madison, WI, 1997. (4) S. Markell et al. Plant Dis. 93:668, 2009.