Fusarium graminearum Species Complex: A Bibliographic Analysis and Web-Accessible Database for Global Mapping of Species and Trichothecene Toxin Chemotypes.

Fusarium graminearum is ranked among the five most destructive fungal pathogens that affect agroecosystems. It causes floral diseases in small grain cereals including wheat, barley, and oats, as well as maize and rice. We conducted a systematic review of peer-reviewed studies reporting species within the F. graminearum species complex (FGSC) and created two main data tables. The first contained summarized data from the articles including bibliographic, geographic, methodological (ID methods), host of origin and species, while the second data table contains information about the described strains such as publication, isolate code(s), host/substrate, year of isolation, geographical coordinates, species and trichothecene genotype. Analyses of the bibliographic data obtained from 123 publications from 2000 to 2021 by 498 unique authors and published in 40 journals are summarized. We describe the frequency of species and chemotypes for 16,274 strains for which geographical information was available, either provided as raw data or extracted from the publications, and sampled across six continents and 32 countries. The database and interactive interface are publicly available, allowing for searches, summarization, and mapping of strains according to several criteria including article, country, host, species and trichothecene genotype. The database will be updated as new articles are published and should be useful for guiding future surveys and exploring factors associated with species distribution such as climate and land use. Authors are encouraged to submit data at the strain level to the database, which is accessible at https://fgsc.netlify.app.

Aggressiveness and Mycotoxin Production by Fusarium meridionale Compared with F. graminearum on Maize Ears and Stalks in the Field.

Fusarium meridionale and F. graminearum both cause Gibberella ear rot (GER) and Gibberella stalk rot (GSR) of maize in Brazil, but the former is much more common. Recent work with two isolates of each from maize suggested this dominance could be caused by greater aggressiveness and competitiveness of F. meridionale on maize. We evaluated pathogenicity and toxigenicity of 16 isolates of F. graminearum and 24 isolates of F. meridionale recovered from both wheat and maize. Strains were individually inoculated into ears of four maize hybrids in field trials. GER severity varied significantly between isolates within each species. Although ranges overlapped, the average GER severity induced by F. meridionale (25.2%) was two times as high overall as that induced by F. graminearum (12.8%) for isolates obtained from maize but was similar for those isolated from wheat (19.9 and 21.4%, respectively). In contrast, severity of GSR was slightly higher for F. graminearum (22.2%) than for F. meridionale (19.8%), with no effect of the host of origin. Deoxynivalenol and its acetylated form 15ADON were the main mycotoxins produced by F. graminearum (7/16 strains), and nivalenol toxin was produced by F. meridionale (17/24 strains). Six isolates of F. graminearum and three of F. meridionale also produced zearalenone. Results confirmed that F. meridionale from maize is, on average, more aggressive on maize but also suggested greater complexity related to diversity among the isolates within each species and their interactions with different hybrids. Further studies involving other components of the disease cycle are needed to more fully explain observed patterns of host dominance.

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.

Diversity and Cross-Infection Potential of Colletotrichum Causing Fruit Rots in Mixed-Fruit Orchards in Kentucky.

Fungi in the genus Colletotrichum cause apple, blueberry, and strawberry fruit rots, which can result in significant losses. Accurate identification is important because species differ in aggressiveness, fungicide sensitivity, and other factors affecting management. Multiple Colletotrichum species can cause similar symptoms on the same host, and more than one fruit type can be infected by a single Colletotrichum species. Mixed-fruit orchards may facilitate cross-infection, with significant management implications. Colletotrichum isolates from small fruits in Kentucky orchards were characterized and compared with apple isolates via a combination of morphotyping, sequencing of voucher loci and whole genomes, and cross-inoculation assays. Seven morphotypes representing two species complexes (C. acutatum and C. gloeosporioides) were identified. Morphotypes corresponded with phylogenetic species C. fioriniae, C. fructicola, C. nymphaeae, and C. siamense, identified by TUB2 or GAPDH barcodes. Phylogenetic trees built from nine single-gene sequences matched barcoding results with one exception, later determined to belong to an undescribed species. Comparison of single-gene trees with representative whole genome sequences revealed that CHS and ApMat were the most informative for diagnosis of fruit rot species and individual morphotypes within the C. acutatum or C. gloeosporioides complexes, respectively. All blueberry isolates belonged to C. fioriniae, and most strawberry isolates were C. nymphaeae, with a few C. siamense and C. fioriniae also recovered. All three species cause fruit rot on apples in Kentucky. Cross-inoculation assays on detached apple, blueberry, and strawberry fruits showed that all species were pathogenic on all three hosts but with species-specific differences in aggressiveness.

Standard Area Diagrams for Aiding Severity Estimation: Scientometrics, Pathosystems, and Methodological Trends in the Last 25 Years.

Standard area diagrams (SAD) have long been used as a tool to aid the estimation of plant disease severity, an essential variable in phytopathometry. Formal validation of SAD was not considered prior to the early 1990s, when considerable effort began to be invested developing SAD and assessing their value for improving accuracy of estimates of disease severity in many pathosystems. Peer-reviewed literature post-1990 was identified, selected, and cataloged in bibliographic software for further scrutiny and extraction of scientometric, pathosystem-related, and methodological-related data. In total, 105 studies (127 SAD) were found and authored by 327 researchers from 10 countries, mainly from Brazil. The six most prolific authors published at least seven studies. The scientific impact of a SAD article, based on annual citations after publication year, was affected by disease significance, the journal's impact factor, and methodological innovation. The reviewed SAD encompassed 48 crops and 103 unique diseases across a range of plant organs. Severity was quantified largely by image analysis software such as QUANT, APS-Assess, or a LI-COR leaf area meter. The most typical SAD comprised five to eight black-and-white drawings of leaf diagrams, with severity increasing nonlinearly. However, there was a trend toward using true-color photographs or stylized representations in a range of color combinations and more linear (equally spaced) increments of severity. A two-step SAD validation approach was used in 78 of 105 studies for which linear regression was the preferred method but a trend toward using Lin's correlation concordance analysis and hypothesis tests to detect the effect of SAD on accuracy was apparent. Reliability measures, when obtained, mainly considered variation among rather than within raters. The implications of the findings and knowledge gaps are discussed. A list of best practices for designing and implementing SAD and a website called SADBank for hosting SAD research data are proposed.