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, whereas F. graminearum is a minor contributor. To test the hypothesis that F. meridionale is more aggressive than F. graminearum 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 F. graminearum or F. meridionale (two isolates of each) inoculated once 4 days after silk, inoculated sequentially and alternately (F. graminearum → F. meridionale or F. meridionale → F. graminearum) 6 days apart, or (in the central-south) inoculated sequentially without alternating species (F. meridionale → F. meridionale or F. graminearum → F. graminearum). Overall, severity was two times greater in the south (37.0%), where summer temperatures were warmer (20 to 25°C) than in central-south. In the south, severity was greatest in F. meridionale treatments (67.8%); followed by F. meridionale → F. graminearum (41.1%), then F. graminearum → F. meridionale (19.4%), and lowest in F. graminearum (2.1%), suggesting an antagonistic relationship. In the central-south (15 to 20°C), severity was generally higher in the sequential nonalternating inoculation treatments (F. meridionale → F. meridionale or F. graminearum → F. graminearum) than when either species was inoculated only once. Only nivalenol (NIV) or deoxynivalenol was detected when F. meridionale or F. graminearum, respectively, was inoculated singly, or sequentially with no alternation. Both toxins were found in grains harvested from the F. meridionale → F. graminearum treatment, whereas only NIV was found in kernels from the F. graminearum → F. meridionale treatment, suggesting that F. meridionale was more competitive than F. graminearum in coinoculations. The dominance of F. meridionale as a cause of GER in Brazil may be due in part to its higher aggressiveness and competitiveness compared with F. graminearum.

Trichothecene Genotype Composition of Fusarium graminearum Not Differentiated Among Isolates from Maize Stubble, Maize Ears, Wheat Spikes, and the Atmosphere in New York.

In order to test the hypothesis that the trichothecene genotype composition of local populations of Fusarium graminearum is structured by specific habitats, a collection of 1,407 isolates was obtained from overwintered maize stubble, mature maize ears and wheat spikes, and the atmosphere 1.5 m aboveground during the flowering stage of these crops. These isolates were sampled at three diverse agricultural locations in New York State: namely, Aurora (sampled in 2012 and 2013) in central New York, Belmont (sampled in 2013) in southwestern New York, and Willsboro (sampled in 2013) in northeastern New York. Approximately 100 isolates of F. graminearum from each habitat were collected within a 10-mile2 area in each location. Polymerase chain reaction assays were used to identify three main B-trichothecene genotypes--3-acetyldeoxynivalenol (3-ADON), 15-ADON, or nivalenol (NIV)--based on amplification of portions of Tri3 and Tri12 genes. All but the NIV genotype were detected. The 15-ADON genotype predominated in most locations; frequencies were 92% (652/709) at Aurora, 78% (332/379) at Belmont, and 53% (167/319) at Willsboro. Frequencies of any genotype did not differ in general among the four habits in each location. An exception was in Aurora 2012, where only 5 in 24 3-ADON isolates were found in samplings from the air and grains of both crops. As viewed by the composition of trichothecene genotypes, local populations of F. graminearum appear not to be structured by these four habitats inclusive of pathogenic and saprophytic phases of the fungus life cycle. The similar frequency of 3-ADON and 15-ADON in eastern New York (Willsboro), which is less than 400 km away from the Aurora sampling location in the central area of the state, suggests that regional populations may be differentiated based on selection associated with climatic or landscape features not currently identified.

Regional and field-specific factors affect the composition of fusarium head blight pathogens in subtropical no-till wheat agroecosystem of Brazil.

A multiyear survey of >200 wheat fields in Paraná (PR) and Rio Grande do Sul (RS) states was conducted to assess the extent and distribution of Fusarium graminearum species complex (FGSC) diversity in the southern Brazilian wheat agroecosystem. Five species and three trichothecene genotypes were found among 671 FGSC isolates from Fusarium head blight (FHB)-infected wheat heads: F. graminearum (83%) of the 15-acetyldeoxynivalenol (15-ADON) genotype, F. meridionale (12.8%) and F. asiaticum (0.4%) of the nivalenol (NIV) genotype, and F. cortaderiae (2.5%) and F. austroamericanum (0.9%) with either the NIV or the 3-ADON genotype. Regional differences in FGSC composition were observed, with F. meridionale and the NIV type being significantly (P<0.001) more prevalent in PR (>28%) than in RS (≤9%). Within RS, F. graminearum was overrepresented in fields below 600 m in elevation and in fields with higher levels of FHB incidence (P<0.05). Species composition was not significantly influenced by previous crop or the stage of grain development at sampling. Habitat-specific differences in FGSC composition were evaluated in three fields by characterizing a total of 189 isolates collected from corn stubble, air above the wheat canopy, and symptomatic wheat kernels. Significant differences in FGSC composition were observed among these habitats (P<0.001). Most strikingly, F. meridionale and F. cortaderiae of the NIV genotype accounted for the vast majority (>96%) of isolates from corn stubble, whereas F. graminearum with the 15-ADON genotype was dominant (>84%) among isolates from diseased wheat kernels. Potential differences in pathogenic fitness on wheat were also suggested by a greenhouse competitiveness assay in which F. graminearum was recovered at much higher frequency (>90%) than F. meridionale from four wheat varieties inoculated with an equal mixture of F. graminearum and F. meridionale isolates. Taken together, the data presented here suggest that FGSC composition and, consequently, the trichothecene contamination in wheat grown in southern Brazil is influenced by host adaptation and pathogenic fitness. Evidence that F. meridionale and F. cortaderiae with the NIV genotype are regionally significant contributors to FHB may have significant implications for food safety and the economics of cereal production.

Fusarium graminearum Isolates from Wheat and Maize in New York Show Similar Range of Aggressiveness and Toxigenicity in Cross-Species Pathogenicity Tests.

This study aimed to assess whether pathogenic Fusarium graminearum isolates from wheat and maize were more aggressive on their host of origin and whether aggressiveness was influenced further by B-trichothecene chemotype. Fifteen isolates were selected from a contemporary collection of isolates surveyed in New York in 2011 to 2012 to represent diversity of host of origin and chemotype. Three pathogenicity assays were used to evaluate and compare these isolates. Fusarium head blight (FHB) severity and trichothecene production in wheat, and maize seedling blight were evaluated in greenhouse inoculation experiments, and Gibberella ear rot (GER) severity and trichothecene production were evaluated in maize ears inoculated in the field. Our results showed among F. graminearum isolates a wide variation in aggressiveness and mycotoxin production toward wheat and maize and these isolates could not be structured by their host of origin or by chemotype. Moreover, aggressiveness rank order changed according to the host/organ evaluated. This indicates that relative susceptibility at the seedling stage may not predict susceptibility of ears. Significant correlations were observed of total trichothecenes (deoxynivalenol [DON] and its acetylated derivatives) produced with FHB and GER severity on wheat and maize, respectively. One isolate did not produce DON or ADON in wheat or maize kernels, yet was aggressive on both hosts. Nine of the fifteen isolates produced small amounts of zearalenone (ZON) in maize kernels, but not in wheat kernels, and ZON level was not correlated with GER severity. F. graminearum isolates from New York showed wide variation in aggressiveness and mycotoxin production toward susceptible wheat and maize. Neither host of origin nor trichothecene chemotype appeared to structure the populations we sampled.