Meta-Analysis of the Effects of QoI and DMI Fungicide Combinations on Fusarium Head Blight and Deoxynivalenol in Wheat.

Field trials were conducted in 17 U.S. states to evaluate the effects of quinone outside inhibitor (QoI) and demethylation inhibitor (DMI) fungicide programs on Fusarium head blight index (IND) and deoxynivalenol (DON) toxin in wheat. Four DMI-only treatments applied at Feekes 10.5.1, five QoI-only treatments applied between Feekes 9 or Feekes 10.5, three QoI+DMI mixtures applied at Feekes 10.5, and three treatments consisting of a QoI at Feekes 9 followed by a DMI at Feekes 10.5.1 were evaluated. Network meta-analytical models were fitted to log-transformed mean IND and DON data and estimated contrasts of log means were used to obtain estimates of mean percent controls relative to the nontreated check as measures of efficacy. Results from the meta-analyses were also used to assess the risk of DON increase in future trials. DMI at Feekes 10.5.1 were the most effective programs against IND and DON and the least likely to increase DON in future trials. QoI-only programs increased mean DON over the nontreated checks and were the most likely to do so in future trials, particularly when applied at Feekes 10.5. The effects of QoI+DMI combinations depended on the active ingredients and whether the two were applied as a mixture at heading or sequentially. Following a Feekes 9 QoI application with a Feekes 10.5.1 application of a DMI reduced the negative effect of the QoI on DON but was not sufficient to achieve the efficacy of the Feekes 10.5.1 DMI-only treatments. Our results suggest that one must be prudent when using QoI treatments under moderate to high risk of FHB, particularly where the QoI is used without an effective DMI applied in combination or in sequence.

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.

A real-time qPCR assay to quantify Fusarium graminearum biomass in wheat kernels.

AIMS: To develop a real-time PCR assay to quantify Fusarium graminearum biomass in blighted wheat kernels. METHODS AND RESULTS: Primers designed to amplify a gene in the trichothecene biosynthetic cluster (TRI6) were evaluated for sensitivity and specificity. Primer pair Tri6_10F/Tri6_4R specifically and consistently amplified a 245-bp DNA fragment from F. graminearum. A workflow was developed and validated to extract DNA from infested grain. The assay detected as little as 10 µg of F. graminearum mycelia in 1 g of ground wheat grain with a high correlation between fungal biomass and cycle threshold values (R(2) = 0·9912; = 0·004). In field-inoculated grain, qPCR measurements of biomass correlated closely with deoxynivalenol levels (R = 0·82, P < 0·0001) and two visual techniques to assess grain quality (R = 0·88, P < 0·0001 and R = 0·81, P < 0·0001). CONCLUSIONS: The qPCR assay provided accurate and precise assessments of the amount of F. graminearum biomass in blighted wheat kernels. This method represents a technical advance over other approaches to quantify kernel colonization and real-time PCR detection methodologies for F. graminearum that do not correlate quantification of fungal genomic DNA to biomass. SIGNIFICANCE AND IMPACT OF THE STUDY: Quantifying F. graminearum biomass, especially low levels of growth associated with kernels that are visually asymptomatic, represents a new approach to screen for resistance to kernel infection, an understudied yet potentially important avenue to reduce the impact of head blight.

Emergence of a novel population of Puccinia striiformis f. sp. tritici in Eastern United States.

The geographic range of stripe rust of wheat, caused by Puccinia striiformis f. sp. tritici, has increased dramatically since 2000 in the United States. Yield losses to the disease have been most severe in the eastern United States, where measurable yield loss had been rare prior to 2000. The objective of this study was to examine the phenotypic and genotypic variation among isolates of P. striiformis f. sp. tritici collected from populations in the eastern United States before and since 2000. Virulence phenotype and amplified fragment length polymorphism (AFLP) markers were used to examine 42 isolates collected between 1960 and 2004. In addition, the genetic structure of 59 isolates collected in 2005 using a hierarchical sampling strategy was examined. The data indicated that the contemporary isolates (collected since 2000) were very distinct from older isolates (collected before 2000) based on virulence and AFLP markers, and that the old population prevalent before 2000 may have been replaced by the contemporary population. The old and new populations appear to be genetically distinct and may represent an exotic introduction rather than a mutation in isolates of the old population.

Aggressiveness of Puccinia striiformis f. sp. tritici Isolates in the South-Central United States.

Although stripe rust, caused by Puccinia striiformis f. sp. tritici, has been an occasional problem on wheat in the south-central United States from 1941 until 1999, the disease has been consistently severe in the region since 2000. Furthermore, since 2000, the geographic range of stripe rust in the eastern United States has expanded, and the old population of races has been replaced by a new population. The objective of this study was to determine whether new isolates of the pathogen were more aggressive and better adapted to warmer temperatures than old isolates. In all, 6 old isolates (collected before 2000) and 14 new isolates (collected since 2000) were evaluated at 12 and 18°C for latent period on wheat seedlings and urediniospore germination on Noble agar. At 12°C, old and new isolates had similar latent periods and spore germination percentages. However, at 18°C, new isolates averaged 2 days less for latent period and double the spore germination compared with old isolates. Therefore, the new isolates are better adapted and, thus, more aggressive at warmer temperatures than the old isolates. These differences may have contributed to the severity of recent epidemics in the region and to the expanded geographic range for stripe rust.

Evaluation of Components of Fusarium Head Blight Resistance in Soft Red Winter Wheat Germ Plasm Using a Detached Leaf Assay.

A large environmental influence on phenotypic estimates of disease resistance and the complex polygenic nature of Fusarium head blight (FHB) resistance in wheat (Triticum aestivum) are impediments to developing resistant cultivars. The objective of this research was to investigate the utility of a detached leaf assay, inoculated using inoculum from isolates of Microdochium nivale var. majus, to identify components of FHB resistance among 30 entries of U.S. soft red winter wheat in the 2002 Uniform Southern FHB Nursery (USFHBN). Whole plant FHB resistance of the USFHBN entries was evaluated in replicated, mist-irrigated field trials at 10 locations in eight states during the 2001-2002 season. Incubation period (days from inoculation to the first appearance of a dull gray-green water-soaked lesion) was the only detached leaf variable significantly correlated across all FHB resistance parameters accounting for 45% of the variation in FHB incidence, 27% of FHB severity, 30% of Fusarium damaged kernels, and 26% of the variation in grain deoxynivalenol (DON) concentration. The results for incubation period contrasted with previous studies of moderately resistant European cultivars, in that longer incubation period was correlated with greater FHB susceptibility, but agreed with previous findings for the Chinese cultivar Sumai 3 and CIMMYT germ plasm containing diverse sources of FHB resistance. The results support the view that the detached leaf assay method has potential for use to distinguish between specific sources of FHB resistance when combined with data on FHB reaction and pedigree information. For example, entry 28, a di-haploid line from the cross between the moderately resistant U.S. cultivar Roane and the resistant Chinese line W14, exhibited detached leaf parameters that suggested a combination of both sources of FHB resistance. The USFHBN represents the combination of adapted and exotic germ plasm, but four moderately resistant U.S. commercial cultivars (Roane, McCormick, NC-Neuse, and Pat) had long incubation and latent periods and short lesion lengths in the detached leaf assay as observed in moderately FHB resistant European cultivars. The dichotomy in the relationship between incubation period and FHB resistance indicates that this may need to be considered to effectively combine exotic and existing/adapted sources of FHB resistance.

Effect of Previous Crop, Seedborne Inoculum, and Fungicides on Development of Stagonospora Blotch.

Stagonospora blotch of wheat has been difficult to control in the eastern United States. The objectives of this research were to evaluate the effects of field inoculum, seedborne inoculum, and seed treatment and foliar fungicides on Stagonospora blotch development and to develop more effective management strategies. In 1995 and 1996, similar experiments were established in "infested" and "clean" fields using two seed lots of Coker 9543 ("low" and "high" levels of seed infection) and six seed or foliar fungicide treatments. Planting in clean fields, planting seed with a low level of seedborne inoculum, treating seed with difenoconazole or triadimenol + thiram, and applying propiconazole or tebuconazole to the foliage all contributed toward reducing leaf infections by Stagonospora nodorum, severity of leaf and glume blotch, and incidence of S. nodorum in the harvested seed. Propiconazole alone was the least effective treatment. Planting in an infested field tended to negate the beneficial effects of low level of seed infection and fungicide seed treatments. Crop rotations and tillage that allow wheat debris to decompose before the next wheat crop along with difenoconazole or triadimenol seed treatment to reduce seedborne inoculum should be sufficient to avoid serious losses. In fields where wheat is grown every year, tillage and seed treatment would still be helpful, but a foliar fungicide at GS 8 may be necessary for adequate control. Applying an effective fungicide to seed appears to be a more efficient means of reducing seedborne inoculum than does producing seed with low levels of inoculum.

Efficacy of Bacterial Seed Treatments for Controlling Pythium Root Rot of Winter Wheat.

Pythium root rot, caused by various Pythium spp., is a widespread disease of wheat. The objective of this study was to identify bacterial strains from wheat roots in Arkansas that suppressed Pythium root rot and to compare their efficacy with that of bacterial strains from other areas. Bacterial strains (applied as seed treatments) that suppressed Pythium root rot in growth chamber assays were evaluated further for in vitro antibiosis against three Pythium spp. and for efficacy under field conditions. Pseudomonas fluorescens strain 2-79R, Burkholderia cepacia strain 1-23, and Pseudomonas sp. strain 1-30 were the most effective for suppressing Pythium root rot under field conditions and significantly (P = 0.10) increased yield in one experiment. Strains that were effective in the field also expressed in vitro antibiosis to at least two of three Pythium spp.; however, strains expressing the highest levels of antibiosis were not effective in the field. In the field, root rot suppression and yield enhancement were inconsistent across experiments and generally small in magnitude. Therefore, these strains have little potential for commercial use under the conditions in which they were tested.