Variance Components and Correlations between Doubled Haploid Lines from Two European Flint Landraces and Their Corresponding Testcrosses for Gibberella Ear Rot Resistance, Silking Time, and Plant Height in Maize

Predicting the resistance of hybrids from lines is a relevant approach for accelerating the improvement of disease resistance in hybrid breeding. In this study, genetic variation and covariation among 76 DH lines from two flint landraces, Kemater (KE) and Petkuser (PE), and their corresponding testcrosses (TC) were estimated for the first time for this material for Gibberella ear rot (GER), days to silking (DS), and plant height (PHT). Lines and TC were evaluated in four and two environments, respectively, under artificial infection with GER. TC were, on average, 42% less GER infected than their lines. TC matured 3–4 days earlier and were about 110 cm taller than the lines. GER resistance was 10% higher in KE lines and TC than PE lines and TC. Significant (p < 0.001) genotypic and genotype-by-environment interaction variances were found for all traits. Genotypic variances were generally smaller among TC than lines. Broad-sense heritability estimates were moderate to high for GER severity (0.56–0.82) and high for DS (0.78–0.88) and PHT (0.86–0.94) with higher values always observed in lines. Significant, moderate correlations between TC and line per se performance were found for GER resistance in both KE and PE (r = 0.37 and 0.55, respectively). For the two agronomic traits, correlations were higher (r = 0.59–0.76) than for GER resistance. Genomic prediction accuracies were moderate to high for GER resistance (r = 0.49–0.63) and generally higher for DS and PHT. In conclusion, a pre-selection of DH lines for GER resistance should be feasible; however, TC should be additionally tested on a later selection stage to aim for GER-resistant hybrid cultivars.

Effect of pydiflumetofen on Gibberella ear rot and Fusarium mycotoxin accumulation in maize grain

In Ontario, Canada, Fusarium graminearum Schwabe causes Gibberella ear rot (GER) in maize, resulting in the accumulation of mycotoxins, mainly deoxynivalenol (DON), DON-3-glucoside (DON-3G) and zearalenone (ZEN) in infected kernels. Fungicides can be an important tool for managing GER and DON and other Fusarium mycotoxins in maize. Until recently, all fungicides available to growers were triazoles, thus no resistance management strategy through fungicide use was possible. In this study, a novel carboxamide fungicide active ingredient (pydiflumetofen) was evaluated against conventional triazole fungicides and mixtures for: (1) effectiveness on mycotoxins (2) optimal application timing; and (3) efficacy of application, with and without an insecticide, under natural and inoculated-misted conditions. The best timing for fungicide application was at full silk, resulting in the highest reduction of GER symptoms and lowest accumulation of F. graminearum mycotoxins in harvested grain. DON and DON-3G concentrations were reduced by at least 50% with a fungicide application at full silk. Fungicide treatments did not affect fumonisin concentrations in grain. Pydiflumetofen (94 g active ingredients (AI)/ha) and fungicides containing pydiflumetofen (75-94 g AI/ha) were similar to standard triazole fungicides (prothioconazole at 200 g AI/ha and metconazole at 90 g AI/ha) for reducing GER and F. graminearum mycotoxins under misted-inoculated plots and commercial field conditions; as a result, we expect pydiflumetofen to be competitive with triazole-only chemistries in the marketplace, which should delay the onset of fungicide resistance.

The dominance of Fusarium meridionale over F. graminearum causing Gibberella ear rot in Brazil may be due to increased aggressiveness and competitiveness

In Brazil, Gibberella ear rot (GER) of maize is caused mainly by Fusarium meridionale (Fmer), while F. graminearum (Fgra) is a minor contributor. To test the hypothesis that Fmer is more aggressive than Fgra on maize, six experiments were conducted in the south (summer), and one in the central-south (winter), totaling seven conditions (year ✕ location ✕ hybrid). Treatments consisted of Fgra or Fmer (two isolates of each) inoculated once four days after silk; inoculated sequentially and alternately (Fgra⇾Fmer or Fmer⇾Fgra) six days apart; or (in the central-south) inoculated sequentially without alternating species (Fmer⇾Fmer or Fgra⇾Fgra). Overall, severity was two times greater in the south (37.0%) where summer temperatures were warmer (20 to 25oC) than in central-south. In the south, severity was greatest in Fmer treatments (67.8%); followed by Fmer⇾Fgra (41.1%); then Fgra⇾Fmer (19.4%); and lowest in Fgra (2.1%), suggesting an antagonistic relationship. In the central-south (15 to 20oC), severity was generally higher in the sequential non-alternating inoculation treatments (Fmer⇾Fmer or Fgra⇾Fgra) than when either species was inoculated only once. Only nivalenol (NIV) or deoxynivalenol (DON) were detected when Fmer or Fgra, respectively, were inoculated singly, or sequentially with no alternation. Both toxins were found in grains harvested from the Fmer⇾Fgra treatment, while only NIV was found in kernels from the Fgra⇾Fmer treatment, suggesting that Fmer was more competitive than Fgra in co-inoculations. The dominance of Fmer as a cause of GER in Brazil may be due in part to its higher aggressiveness and competitiveness compared with Fgra.

Mapping and validation of a stable quantitative trait locus conferring maize resistance to Gibberella ear rot

Gibberella ear rot (GER), a prevalent disease caused by Fusarium graminearum (F. graminearum), can result in significant yield loss and carcinogenic mycotoxin contamination in maize worldwide., However, only a few quantitative trait loci (QTLs) for GER resistance have been reported. In this study, we evaluated a Chinese recombinant inbred line (RIL) population comprising of 204 lines, developed from a cross between a resistant parent DH4866 and a susceptible line T877, in three field trial environments under artificial inoculation with F. graminearum. The RIL population and their parents were genotyped with an Affymetrix microarray CGMB56K SNP Array. Based on the genetic linkage map constructed using 1,868 bins as markers, eleven QTLs, including five stable QTLs, were identified by individual environment analysis. Joint multiple environments analysis and epistatic interaction analysis revealed six additive and six epistatic (additive × additive) QTLs, respectively. None of the QTLs could explain more than 10% of phenotypic variation, suggesting that multiple minor-effect QTLs contributed to the genetic component of resistance to GER, and both additive and epistatic effects contributed to the genetic architecture of resistance to GER. A novel QTL, qGER4.09, with largest effect, identified and validated using 588 F2 individuals, was colocalized with genomic regions for FER and Aspergillus ear rot, indicating that this genetic locus likely confers resistance to multiple pathogens, and can be potentially utilized in breeding maize varieties, aimed at improving the resistance not only to GER, but also other ear rot diseases.

Natural Occurrence of Maize Gibberella Ear Rot and Contamination of Grain with Mycotoxins in Association with Weather Variables

Gibberella ear rot (GER) severity (percent area of the ear diseased) and associated grain contamination with mycotoxins were quantified in plots of 15 to 16 maize hybrids planted at 10 Ohio locations from 2015 to 2018. Deoxynivalenol (DON) was quantified in grain samples in all 4 years, whereas nivalenol, 3-acetyldeoxynivalenol, and 15-acetyldeoxynivalenol (15ADON) were quantified only in the last 2 years. Only DON and 15ADON were detected. The highest levels of GER and DON contamination were observed for 2018, followed by 2016 and 2017. No GER symptoms or DON were detected in 2015. Approximately 41% of the samples from asymptomatic ears had detectable levels of DON, and 7% of these samples from 2016 had DON > 5 ppm. Associations between DON contamination and 43 variables representing summaries of temperature (T), relative humidity (RH), rainfall (R), surface wetness, and T-RH combinations for different window lengths and positions relative to R1 growth stage were quantified with Spearman correlation coefficients (r). Fifteen-day window lengths tended to show the highest correlations. Most of the variables based on T, R, RH, and T-RH were significantly correlated with DON for the 15-day window, as well as other windows. For moisture-related variables, there generally was a negative correlation before R1, changing to a positive correlation after R1. Results showed that GER and DON can be frequently found in Ohio maize fields, with the risk of DON being associated with multiple weather variables, particularly those representing combinations of T between 15 and 30°C and RH > 80 summarized during the 3 weeks after R1.

Multi-parent QTL mapping reveals stable QTL conferring resistance to Gibberella ear rot in maize

Maize production is on risk by Gibberella ear rot (GER) caused by Fusarium graminearum. This is one of the most important ear rot diseases in temperate zones as it leads to yield losses and production of harmful mycotoxins. We investigated, for the first time, the potential use of Brazilian tropical maize to increase resistance levels to GER in temperate European flint germplasm by analyzing six interconnected biparental populations. We assessed GER symptoms in Brazil and in Europe in up to six environments (= location × year combinations) during the growing seasons of 2018 and 2019. We conducted multi-parent QTL and biparental QTL mapping, and identified four QTLs with additive gene action, each explaining 5.4 to 21.8% of the total genotypic variance for GER resistance. Among them, QTL q1 was stable across test environments, populations, and between inbred lines and testcrosses. The accuracies of genomic prediction ranged from 0.50 to 0.59 depending on the resistance donor and prediction model. Jointly, our study reveals the potential use of Brazilian resistance sources to increase GER resistance levels by genomics-assisted breeding.

Exploiting genetic diversity in two European maize landraces for improving Gibberella ear rot resistance using genomic tools

Key message High genetic variation in two European maize landraces can be harnessed to improve Gibberella ear rot resistance by integrated genomic tools. Abstract Fusarium graminearum (Fg) causes Gibberella ear rot (GER) in maize leading to yield reduction and contamination of grains with several mycotoxins. This study aimed to elucidate the molecular basis of GER resistance among 500 doubled haploid lines derived from two European maize landraces, “Kemater Landmais Gelb” (KE) and “Petkuser Ferdinand Rot” (PE). The two landraces were analyzed individually using genome-wide association studies and genomic selection (GS). The lines were genotyped with a 600-k maize array and phenotyped for GER severity, days to silking, plant height, and seed-set in four environments using artificial infection with a highly aggressive Fg isolate. High genotypic variances and broad-sense heritabilities were found for all traits. Genotype-environment interaction was important throughout. The phenotypic (r) and genotypic ($${r}_{g}$$ r g ) correlations between GER severity and three agronomic traits were low (r = − 0.27 to 0.20; $${r}_{g}\hspace{0.17em}$$ r g = − 0.32 to 0.22). For GER severity, eight QTLs were detected in KE jointly explaining 34% of the genetic variance. In PE, no significant QTLs for GER severity were detected. No common QTLs were found between GER severity and the three agronomic traits. The mean prediction accuracies ($$\rho $$ ρ ) of weighted GS (wRR-BLUP) were higher than $$\rho $$ ρ of marker-assisted selection (MAS) and unweighted GS (RR-BLUP) for GER severity. Using KE as the training set and PE as the validation set resulted in very low $$\rho $$ ρ that could be improved by using fixed marker effects in the GS model.

Stability of Hybrid Maize Reaction to Gibberella Ear Rot and Deoxynivalenol Contamination of Grain

Trials were conducted to quantify the stability (or lack of G × E interaction) of 15 maize hybrids to Gibberella ear rot (GER; caused by Fusarium graminearum) and deoxynivalenol (DON) contamination of grain across 30 Ohio environments (3 years × 10 locations). In each environment, one plot of each hybrid was planted and 10 ears per plot were inoculated via the silk channel. GER severity (proportion of ear area diseased) and DON contamination of grain (ppm) were quantified. Multiple rank-based methods, including Kendall’s concordance coefficient (W) and Piepho’s U, were used to quantify hybrid stability. The results found insufficient evidence to suggest crossover G × E interaction of ranks, with W greater than zero for GER (W = 0.28) and DON (W = 0.26), and U not statistically significant for either variable (P > 0.20). Linear mixed models (LMMs) were also used to quantify hybrid stability, accounting for crossover or noncrossover G × E interaction of transformed observed data. Based on information criteria and likelihood ratio tests for GER and DON response variables, the models with more complex variance-covariance structures—heterogeneous compound symmetry and factor-analytic—provided a better fit than the model with the simpler compound symmetry structure, indicating that one or more hybrids differed in stability. Overall, hybrids were stable based on rank-based methods, which indicated a lack of crossover G × E interaction, but the LMMs identified a few hybrids that were sensitive to environment. Resistant hybrids were generally more stable than susceptible hybrids.

Fusarium meridionale is more aggressive and competitive than F. graminearum as a causal agent of Gibberella Ear Rot of maize in Brazil

In Brazil, Gibberella ear rot (GER) of maize is caused mainly by Fusarium meridionale (Fmer), while F. graminearum (Fgra) is a minor contributor. To test the hypothesis that Fmer is more aggressive than Fgra on maize, three experiments were conducted in the south (subtropical summer), and one in the central-south (tropical winter) of Brazil, totaling seven conditions (year ✕ location ✕ hybrid). Treatments consisted of Fgra or Fmer (two isolates of each) inoculated once four days after silk; alternately inoculated (Fgra⇾Fmer or Fmer⇾Fgra) six days apart; and (in central-south only) sequentially inoculated without alternating (Fmer⇾Fmer or Fgra⇾Fgra). Overall GER severity was higher in the south (37.0%) compared with the central-south (16.9%), likely due to the higher summer temperatures (20 to 25oC). In the south, severity was highest in Fmer treatments (67.8%); followed by Fmer⇾Fgra (41.1%); then Fgra⇾Fmer (19.4%); and lowest in Fgra (2.1%), suggesting an antagonistic relationship. In the central-south during winter (15 to 20oC), severity was generally higher (7.3 to 11.6%) in sequential inoculation treatments than when either species was inoculated alone. Only nivalenol (NIV) or deoxynivalenol (DON) were detected when Fmer or Fgra, respectively, were inoculated singly and sequentially with no alternation. Both toxins were found in grains harvested from the Fmer⇾Fgra treatment, while only NIV was found in kernels from the Fgra⇾Fmer treatment, suggesting that Fmer was more competitive than Fgra in co-inoculations. The dominance of Fmer as a cause of GER in Brazil may be due in part to its higher aggressiveness and competitiveness than Fgra.