''Gibberella zeae'', also known by the name of its anamorph ''Fusarium graminearum'', is a

fungal A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...

plant pathogen which causes

fusarium head blight Fusarium ear blight (FEB) (also called Fusarium head blight, FHB, or scab), is a fungal disease of cereals, including wheat, barley, oats, rye and triticale. FEB is caused by a range of ''Fusarium'' fungi, which infects the heads of the crop, redu ...

(FHB), a devastating disease on

wheat Wheat is a grass widely cultivated for its seed, a cereal grain that is a worldwide staple food. The many species of wheat together make up the genus ''Triticum'' ; the most widely grown is common wheat (''T. aestivum''). The archaeologi ...

and

barley Barley (''Hordeum vulgare''), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia as early as 10,000 years ago. Globally 70% of barley p ...

. The pathogen is responsible for billions of dollars in economic losses worldwide each year. Infection causes shifts in the

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...

composition of wheat, resulting in shriveled kernels and contaminating the remaining grain with

mycotoxin A mycotoxin (from the Greek μύκης , "fungus" and τοξίνη , "toxin") is a toxic secondary metabolite produced by organisms of kingdom Fungi and is capable of causing disease and death in both humans and other animals. The term 'mycotoxin' ...

s, mainly deoxynivalenol (DON), which inhibits

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

biosynthesis; and

zearalenone Zearalenone (ZEN), also known as RAL and F-2 mycotoxin, is a potent estrogenic metabolite produced by some ''Fusarium'' and ''Gibberella'' species. Specifically, the ''Gibberella zeae ,'' the fungal species where zearalenone was initially detected, ...

, an estrogenic mycotoxin. These toxins cause vomiting, liver damage, and reproductive defects in livestock, and are harmful to humans through contaminated food. Despite great efforts to find resistance genes against ''F. graminearum'', no completely resistant variety is currently available. Research on the biology of ''F. graminearum'' is directed towards gaining insight into more details about the infection process and reveal weak spots in the life cycle of this pathogen to develop fungicides that can protect wheat from scab infection.

Life cycle

''F. graminearum'' is a haploid homothallic ascomycete. The fruiting bodies,

perithecia An ascocarp, or ascoma (), is the fruiting body ( sporocarp) of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores. Ascocarps are mos ...

, develop on the mycelium and give rise to ascospores, which land on susceptible parts of the host plant to germinate. The fungus causes fusarium head blight on wheat, barley, and other grass species, as well as ear rot on corn. The primary inocula are the ascospores, sexual spores which are produced in the perithecia. Spores are forcibly discharged and can germinate within six hours upon landing on the plant surface. The scab disease is monocyclic; after one cycle of infection with ascospores, the fungus produces macroconidia by asexual reproduction. These structures overwinter in the soil or in plant debris on the field and give rise to the mycelium in the next season.

Host and symptoms

The pathogen is capable of causing a variety of diseases: head blight or 'scab' on wheat ('' Triticum''), barley ('' Hordeum'')'','' rice ('' Oryza'')'','' oats (''

Avena ''Avena'' is a genus of Eurasian and African plants in the grass family. Collectively known as the oats, they include some species which have been cultivated for thousands of years as a food source for humans and livestock. They are widespre ...

''), and ''Gibberella'' stalk and ear rot disease on maize ('' Zea''). Additionally, the fungus may infect other plant species without causing any disease symptoms. Maize In ''Gibberella'' stalk rot, the leaves on early-infected plants will turn a dull greyish-green, and the lower internodes will soften and turn a tan to dark-brown. A pink-red discoloration occurs within the stalks of diseased tissue. Shredding of the

pith Pith, or medulla, is a tissue in the stems of vascular plants. Pith is composed of soft, spongy parenchyma cells, which in some cases can store starch. In eudicotyledons, pith is located in the center of the stem. In monocotyledons, it ext ...

may reveal small, round, black

on the stalks. ''Gibberella'' (red) ear rot can have a reddish mold that is often at the ear tip. The infection occurs by colonizing

corn silk Corn silk is a common ''Stigma maydis'', the shiny, thread-like, weak fibers that grow as part of ears of corn (maize); the tuft or tassel of silky fibers that protrude from the tip of the ear of corn. The ear is enclosed in modified leaves called ...

and symptoms first occur at the ear's apex. The white mycelium turns from pink to red over time, eventually covering the entire ear. Ears that become infected early don't fully develop the reddish mold near the ear tip, as the mold grows between the husks and ear. Rice ''Gibberella zeae ''can turn affected seeds red and cause brown discoloration in certain areas on the seed or the entire seed surface. The surface of husks develop white spots that later become yellow and salmon or carmine. Infected grains are light, shrunken and brittle. Stem nodes begin to rot and wilt, eventually causing them to turn black and disintegrate when they are infected by the fungal pathogen. Wheat Brown, dark purple-black necrotic lesions will form on the outer surface of the spikelets, what the wheat ear breaks up into. The lesions may be referred to as scabs, but this is not to be confused and associated with other scab diseases such as those with different host and pathogen. Head blight is visible before the spikes mature. Spikelets begin to appear water-soaked before the loss of chlorophyll, which gives a white straw color. Peduncles that are directly under the

inflorescence An inflorescence is a group or cluster of flowers arranged on a Plant stem, stem that is composed of a main branch or a complicated arrangement of branches. Morphology (biology), Morphologically, it is the modified part of the shoot of sperma ...

can become discolored into a brown-purple color. Tissues of the

typically become blighted into a bleached tan appearance, and the grain within it atrophies. The awn will become deformed, twisted and curve in a downward direction. Barley Infections on barley are not always visible in the field. Similar to wheat, infected spikelets show a browning or water-soaked appearance. The infected kernels display a tan to dark brown discoloration. During long periods of wetness, pink to salmon-orange spore masses can be seen on the infected spikelets and kernels. The cortical lesions of infected seeds become a reddish-brown in cool, moist soil. Warm soil can cause head blight to occur after emergence, and crown and basal culm rot can be observed in later plant development.

Infection process

''F. graminearum'' infects wheat spikes from anthesis through the soft dough stage of kernel development. The fungus enters the plant mostly through the flowers; however, the infection process is complex and the complete course of colonization of the host has not been described.

Germ tube A germ tube is an outgrowth produced by spores of spore-releasing fungi during germination. The germ tube differentiates, grows, and develops by mitosis to create somatic hyphae.C.J. Alexopolous, Charles W. Mims, M. Blackwell, ''Introductory My ...

s seem not to be able to penetrate the hard, waxy surface of the lemma and palea which protect the flower. The fungus enters the plant through natural openings such as stomates, and needs soft tissue such as the flowers, anthers and embryo to infect the plant. From the infected floret, the fungus can grow through the rachis and cause severe damage in a short period of time under favorable conditions. Upon germination of the spores on the anthers and the surface of the developing kernel, hyphae penetrate the epicarp and spread through the seed coat. Successively, the different layers of the seed coat and finally the endosperm are colonized and killed.

Management

The control of this disease can be achieved using a combination of the following strategies: fungicide applications, resistance breeding, proper storage, crop rotation, crop residue tillage, and seed treatment. The correct usage of fungicide applications against

Fusarium head blight Fusarium ear blight (FEB) (also called Fusarium head blight, FHB, or scab), is a fungal disease of cereals, including wheat, barley, oats, rye and triticale. FEB is caused by a range of ''Fusarium'' fungi, which infects the heads of the crop, redu ...

(FHB) can reduce the disease by 50 to 60 percent. ''Fusarium'' refers to a large genus of soil fungi that are economically important due to the profound effects they have on crops. Application of fungicides is necessary at early heading date for barley and early flowering for wheat, where the early application can limit the infection of the ear. Barley and wheat differ in fungicide application because of their differences in developmental traits. Some

biofungicide Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality, ...

s control FHB. Scaglioni ''et al.'', 2019 extract

phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it ...

s from Spirulina spp. and demonstrate growth retardation by 25% (per weight). The disease generally develops late in the season or during storage, so fungicide use is only effective in the early season. Management against insect pests such as ear borers, for corn, will also reduce the infection of the ear from wounds caused by insect feeding. Cultivating a variety of hosts that are resistant to FHB is one of the most evidence-based and cost-effective ways to manage the disease. Using varieties that have looser tusks that cover the ear are less vulnerable to FHB. Once the crop has been harvested, it is essential to store it at low moisture, below 15%, as this will reduce the appearance of ''Gibberella zeae'' and ''Fusarium'' species in storage. Avoiding the planting of small grain crops following other small grain crops or corn and tillage of crop residue minimizes the chances of FHB in environmentally favorable years. The rotation of small grains with soybean or other non-host crops has proven to reduce FHB and mycotoxin contamination. Crop rotation with the tillage of residue prevents crops from remaining to infect on the soil surface. Residues can provide an overwintering medium for ''Fusarium'' species to cause FHB. As a result, the chances of infection are greatly improved in the succeeding small grain crop. If minimal or no tillage occurs, the residue spreads and allows the fungus to overwinter on stalks and rotted ears of corn and produce spores. The seeds (kernels) that colonize with the fungus have less resistance because of poor germination. Planting certified or treated seeds can reduce the amount of seedling blight, which is caused by the seeds colonized with the fungus. If it is necessary to replant seeds that were harvested from a FHB infected field, then the seeds should be treated to avoid reoccurrence of the infection.

Importance

The loss of yield and contamination of seed with

s, alongside reduced seed quality, are the main contributions to the impact of this disease. Two mycotoxins, the trichothecene deoxynivalenol (DON), a strong biosynthesis inhibitor, and

, an estrogenic mycotoxin, can be found in grains after FHB epidemics. DON is a type of vomitoxin and, as its name states, is an antifeedant. Livestock that consume crops contaminated with vomitoxin become sick and refuse to eat anymore.

Zearalenone Zearalenone (ZEN), also known as RAL and F-2 mycotoxin, is a potent estrogenic metabolite produced by some ''Fusarium'' and ''Gibberella'' species. Specifically, the ''Gibberella zeae ,'' the fungal species where zearalenone was initially detected, ...

is a phytoestrogen, mimicking mammals' estrogen. It can be disastrous if it gets into the food chain, as

causes abortions in pregnant females and feminization of males. In 1982, a major epidemic affected of the

spring wheat Winter wheat (usually '' Triticum aestivum'') are strains of wheat that are planted in the autumn to germinate and develop into young plants that remain in the vegetative phase during the winter and resume growth in early spring. Classification ...

and

growing in the northern Great Plains of

North Dakota North Dakota () is a U.S. state in the Upper Midwest, named after the indigenous Dakota Sioux. North Dakota is bordered by the Canadian provinces of Saskatchewan and Manitoba to the north and by the U.S. states of Minnesota to the east, So ...

South Dakota South Dakota (; Sioux: , ) is a U.S. state in the North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Lakota and Dakota Sioux Native American tribes, who comprise a large porti ...

, and

Minnesota Minnesota () is a state in the upper midwestern region of the United States. It is the 12th largest U.S. state in area and the 22nd most populous, with over 5.75 million residents. Minnesota is home to western prairies, now given over to ...

. The yield losses exceeded worth approximately $826 million, with total losses related to the epidemic near one billion dollars. Years that followed this epidemic, reported losses that have been estimated between $200-$400 million annually. Losses in barley because of FHB are large in part due to the presence of DON. Barley prices from 1996 in Minnesota fell from $3.00 to $2.75 per bushel if the mycotoxin was present and another $0.05 for each

part per million In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they ...

of DON present. DON chemotypes of ''F. graminearum'' include .

References

External links

Interactive Science Experiment Showcasing the Growth of ''Gibberella zeae'' (GCSE/A-level)

Index Fungorum

USDA ARS Fungal Database
{{Taxonbar, from=Q288349 zeae Fungal plant pathogens and diseases Barley diseases Wheat diseases Fungi described in 1822 Taxa named by Lewis David de Schweinitz