cab can be a devastating disease affecting all classes of wheat and other small grains. This fungal disease, also called Fusarium head blight (FHB), has the ability to completely destroy a potentially high-yielding crop within a few weeks of harvest. Lush, green fields become blighted seemingly overnight (Figs. 1 and 2). Frequent rainfalls, high humidities, and/or heavy dews that coincide with the flowering and early kernel-fill period of the crop favor infection and development of the disease. Damage from head scab is multifold: reduced yields, discolored, shriveled “tombstone” kernels (Figs. 3 to 5), contamination with mycotoxins, and reduction in seed quality. The disease also reduces test weight and lowers market grade. Difficulties in marketing, exporting, processing, and feeding scabby grain are experienced. In North America, Fusarium graminearum Schwabe (teleomorph Gibberella zeae (Schwein.) Petch; synonym = G. saubinetti) predominates among several Fusarium species that can cause scab (4,5,8,40,48,60). Other species may predominate in cooler climates or where crops other than wheat and corn are dominant (8,40,48,60). F. graminearum also is associated with stalk and ear rot of corn and may cause a root rot of cereals. The fungus persists and multiplies on infected crop residues of small grains and corn. The chaff, light-weight kernels, and other infected head debris of wheat and barley, returned to the soil surface during harvest, serve as important sites of overwintering of the fungus. Continued moist weather during the crop growing season favors development of the fungus, and spores are windblown or water-splashed onto heads of cereal crops. Wheat and barley are susceptible to head infection from the flowering (pollination) period up through the soft dough stage of kernel development. Spores of the causal fungus may land on the exposed anthers of the flower and then grow into the kernels, glumes, or other head parts. Excellent descriptions of the disease cycle and spore stages of the causal fungi have been published (4,8,21,40,48). Mycotoxins are frequently associated with the growth and invasion of cereal grains by scab fungi. The most common toxin associated with F. graminearum– infected grain is vomitoxin (deoxynivalenol). Vomitoxin is known to cause vomiting and feed refusal in nonruminant animals and poses a threat to other animals and humans if exposure levels are high (45). The presence of mycotoxins in infected grain further exacerbates the losses that scab can cause. Recent articles have reviewed the epidemiology, management, and history of scab outbreaks in the United States, Canada, Europe, Asia, and South America (5,40,45,48). As these papers indicate, numerous research and survey reports have described the worldwide occurrence and epidemic levels of scab during the past century. Yield loss reports have not always been based on replicated research trials, but extensive surveys of producers’ fields have provided assessments of head blighting severity, which were translated into yield loss estimates. In the United States, scab was found in 31 of 40 states surveyed in 1917, with losses estimated at 288,000 metric tons (10.6 million bushels), primarily in Ohio, Indiana, and Illinois (4). Scab caused an estimated loss of 2.18 million metric tons (80 million bushels) of winter and spring wheat throughout the United States in 1919 (14). Extensive field surveys

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Scab of wheat and Barley : A re-emerging disease of devastating impact

SUMMARY
The rapid global re-emergence of Fusarium head blight disease of wheat and barley in the last decade along with contamination of grains with mycotoxins attributable to the disease have spurred basic research on the fungal causal agent. As a result, Fusarium graminearum quickly has become one of the most intensively studied fungal plant pathogens. This review briefly summarizes current knowledge on the pathogenicity, population genetics, evolution and genomics of Fusarium graminearum.

Taxonomy:  Based on the sexual state Gibberella zeae (Schwein.) Petch: Superkingdom Eukaryota; Kingdom Fungi; Phylum Ascomycota; Subphylum Pezizomycotina; Class Sordariomycetidae; Subclass Hypocreomycetidae; Order Hypocreales; Family Nectriaceae; Genus Gibberella.

Host range:  The pathogen is capable of causing head blight or ‘scab’ on wheat (Triticum), barley (Hordeum), rice (Oryza), oats (Avena) and Gibberella stalk and ear rot disease on maize (Zea). The fungus also may infect other plant species without causing disease symptoms. Other host genera cited for Gibberella zeae or F. graminearum sensu lato (see below) are Agropyron, Agrostis, Bromus, Calamagrostis, Cenchrus, Cortaderia, Cucumis, Echinochloa, Glycine, Hierochloe, Lolium, Lycopersicon, Medicago, Phleum, Poa, Schizachyrium, Secale, Setaria, Sorghum, Spartina and Trifolium.

Disease symptoms and signs:  For wheat, brown, dark purple to black necrotic lesions form on the exterior surface of the florets and glume (Fig. 1). Although these lesion symptoms sometimes are referred to as scab, they are not formally related to the hyperplasia and hypertrophic epidermal growth associated with other scab diseases such as apple scab. Peduncles immediately below the inflorescence may become discoloured brown/purple. With time, tissue of the inflorescence often becomes blighted, appearing bleached and tan, while the grain within atrophies. Awns often become deformed, twisted and curved downward. In barley, infections are not always readily apparent in the field. Infected spikelets may show a browning or water-soaked appearance. Infected barley kernels show a tan to dark brown discolouration that can be similar to that caused by other kernel blighting organisms. During prolonged wet periods, pink to salmon-orange spore masses of the fungus are often seen on infected spikelets, glumes and kernels in both wheat and barley. For maize ear rot, infection occurs by way of colonizing silk and thus symptoms first appear at the ear apex. White mycelium, turning pink to red with time, colonizes kernels and may progress basipetally, covering the entire ear.



Figure 1. 

Field-grown wheat inflorescence showing symptoms of Fusarium head blight. The third spikelet from the bottom shows a darkened necrotic lesion (‘scab’) whereas the second and fifth spikelets demonstrate tissue bleaching (‘blight’) symptoms. Photograph courtesy of Jacki Morrison, USDA ARS Cereal Disease Laboratory.


Useful websites: http://www.broad.mit.edu/annotation/fungi/fusarium/mips.gsf.de/genre/proj/fusarium/

http://www.cdl.umn.edu/scab/gz-consort.html

http://www.scabusa.org/

Heading for disaster: Fusarium graminearum on cereal crops.

Fusarium species, particularly Fusarium graminearum and F. culmorum, are the main cause of trichothecene type B contamination in cereals. Data on the distribution of Fusarium trichothecene genotypes in cereals in Europe are scattered in time and space. Furthermore, a common core set of related variables (sampling method, host cultivar, previous crop, etc.) that would allow more effective analysis of factors influencing the spatial and temporal population distribution, is lacking. Consequently, based on the available data, it is difficult to identify factors influencing chemotype distribution and spread at the European level. Here we describe the results of a collaborative integrated work which aims (1) to characterize the trichothecene genotypes of strains from three Fusarium species, collected over the period 2000–2013 and (2) to enhance the standardization of epidemiological data collection. Information on host plant, country of origin, sampling location, year of sampling and previous crop of 1147 F. graminearum, 479 F. culmorum, and 3 F. cortaderiae strains obtained from 17 European countries was compiled and a map of trichothecene type B genotype distribution was plotted for each species. All information on the strains was collected in a freely accessible and updatable database (www.catalogueeu.luxmcc.lu), which will serve as a starting point for epidemiological analysis of potential spatial and temporal trichothecene genotype shifts in Europe. The analysis of the currently available European dataset showed that in F. graminearum, the predominant genotype was 15-acetyldeoxynivalenol (15-ADON) (82.9%), followed by 3-acetyldeoxynivalenol (3-ADON) (13.6%), and nivalenol (NIV) (3.5%). In F. culmorum, the prevalent genotype was 3-ADON (59.9%), while the NIV genotype accounted for the remaining 40.1%. Both, geographical and temporal patterns of trichothecene genotypes distribution were identified.

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A European Database of Fusarium graminearum and F. culmorum Trichothecene Genotypes

The Fusarium species predominantly found associated with Fusarium head blight (FHB) in wheat and other small-grain cereals all over Europe are F. graminearum, F. avenaceum and F. culmorum. Among the less frequently encountered species are several others which are less pathogenic or opportunistic, but also toxigenic. These include F. poae, F. cerealis, F. equiseti, F. sporotrichioides, F. tricinctum and, to a lesser extent, F. acuminatum, F. subglutinans, F. solani, F. oxysporum, F. verticillioides, F. semitectum and F. proliferatum. The species profile of FHB is due to several factors, primarily climatic conditions, particularly rain and the temperature at flowering stage, but also agronomic factors, such as soil cultivation, nitrogen fertilization, fungicides, crop rotation, and host genotype. The most frequently encountered Fusarium mycotoxins in FHB in Europe has proved to be deoxynivalenol and zearalenone produced by F. graminearum and F. culmorum, with the former more common in southern (warmer) and the latter in northern (colder) European areas. Nivalenol was usually found associated with deoxynivalenol and its derivatives (mono-acetyldeoxynivalenols), together with fusarenone-X, formed by F. graminearum, F. cerealis, F. culmorum and, in northern areas, by F. poae. Moreover, from central to northern European countries, moniliformin has been consistently reported, as a consequence of the widespread distribution of F. avenaceum, whereas the occurrence of T-2 toxin derivatives, such as T-2 toxin and HT-2 toxin, and diacetoxyscirpenol have been recorded in conjunction with sporadic epidemics of F. sporotrichioides and F. poae. Finally, beauvericin and various enniatins have recently been found in Finnish wheat colonized by F. avenaceum and F. poae.

Toxigenic Fusarium species and mycotoxins associated with head blight in small-grain cereals in Europe

During the past decade, numerous studies have been published on molecular mapping of Fusarium head blight (FHB) resistance in wheat. We summarize the relevant findings from 52 quantitative trait loci (QTL) mapping studies, nine research articles on marker-assisted selection and seven on marker-assisted germplasm evaluation. QTL for FHB resistance were found on all wheat chromosomes except chromosome 7D. Some QTL were found in several independent mapping studies indicating that such QTL are stable and therefore useful in breeding programmes. We summarize and update current knowledge on the genetics of FHB resistance in wheat resulting from QTL mapping investigations and review and suggest FHB breeding strategies based on the available information and DNA markers.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1439-0523.2008.01550.x

QTL mapping and marker-assisted selection for Fusarium head blight resistance in wheat: A review

This review of Fusarium ear blight (scab) of small grain cereals has shown that up to 17 causal organisms have been associated with the disease, which occurs in most cereal-growing areas of the world. The most common species were Fusarium graminearum (Gibberella zeae), F. culmorum, F, avenaceum (G, avenacea), F, poae and Microdochium nivale (Monographella nivalis). The disease was recorded most frequently under hot, wet climatic conditions where significant yield losses and mycotoxin accumulation in grain were reported. Possible sources of inoculum were reported as crop debris, alternative hosts and Fusarium seedling blight and foot rot of cereals. The mode of dispiersal of inoculum to ears remains unclear, but contaminated arthropod vectors, systemic fungal growth through plants, and wind and rain-splash dispersal of spores have been proposed. Infection of wheat ears was shown to occur mainly during anthesis, and it has been demonstrated that fungal growth stimulants may be present in anthers. Despite the importance of the disease, particularly during epidemic years, control methods are limited. Much effort has gone into breeding resistant wheat varieties and into improving our understanding of the possible mechanisms and genetic basis of resistance, with only moderate success. There are also surprisingly few reports of successful fungicidal or biological control of the disease in the field.

L. McLEOD

Papers

Fusarium ear blight (scab) in small grain cereals—a review