Septoria Leaf Blotch
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''Zymoseptoria tritici'', synonyms ''Septoria tritici'', ''Mycosphaerella graminicola'', is a species of filamentous fungus, an
ascomycete Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The def ...
in the family '' Mycosphaerellaceae''. It is a
wheat Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...
plant pathogen causing septoria leaf blotch that is difficult to control due to resistance to multiple
fungicide Fungicides are pesticides used to kill parasitic fungi or their spores. Fungi can cause serious damage in agriculture, resulting in losses of yield and quality. Fungicides are used both in agriculture and to fight fungal infections in animals, ...
s. The pathogen today causes one of the most important diseases of wheat. In 2011, Quaedvlieg et al. introduced a
new combination In Taxonomy (biology), biological taxonomy, a combinatio nova (abbreviated comb. nov. or n. comb.) refers to the formal renaming of an organism's scientific name when it is transferred to a different genus, reclassified within a different specie ...
for this species: ''Zymoseptoria tritici'', as they found that the type strains of both the genus ''Mycosphaerella'' (linked to the anamorph genus ''Ramularia'') and the genus ''Septoria'' (linked to the genus ''Septoria'', an extensive clade of very distinct septoria-like species within the ''Mycosphaerellaceae'') clustered separately from the clade containing both ''Zymoseptoria tritici'' and ''Z. passerinii''. Since 2011, a total of eight ''Zymoseptoria'' species have been described within the genus ''Zymoseptoria''; ''Z. tritici'' (the type of the genus ''Zymoseptoria''), ''Z. pseudotritici, Z. ardabiliae, Z. brevis, Z. passerinii, Z. halophila, Z. crescenta'' and ''Z. verkleyi'' (Named after Gerard J.M. Verkleij, for the contribution that he has made to further the understanding of the genus ''Septoria'').


Description

This fungus causes septoria tritici blotch of wheat, a disease characterized by necrotic blotches on the foliage. These blotches contain asexual ( pycnidia) and sexual ( pseudothecia) fructifications. Asexual state (
anamorph In mycology, the terms teleomorph, anamorph, and holomorph apply to portions of the life cycles of fungi in the phyla Ascomycota and Basidiomycota: *Teleomorph: the sexual reproductive stage (morph), typically a fruiting body. *Anamorph: an ase ...
, asexual stage was previously named as ''Septoria tritici''): Pycnidiospores are hyaline and threadlike and measure 1.7-3.4 x 39-86 μm, with 3 to 7 indistinct septations. Germiniation of pycnidiospores can be lateral or terminal. Cirrhi are milky white to buff. Sometimes in culture nonseptate, hyaline microspores, measuring 1-1.3 × 5-9 μm, occur outside pycnidia by yeastlike budding. File:Mycosphaerella graminicola 2.png, Light stimulates yeast-like growth of ''Zymoseptoria tritici''. Close-up of yeast-like growth of ''Zymoseptoria tritici'' ''in vitro'' on V8
agar Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from " ogonori" and " tengusa". As found in nature, agar is a mixture of two components, t ...
. File:Mycosphaerella graminicola 3.png, ''In vitro'' production of asexual fructifications ( pycnidia; arrow) of ''Zymoseptoria tritici'' on wheat leaf extract agar. File:Mycosphaerella graminicola 4.png, Penetration of a wheat leaf stoma (arrow) by a pycnidiospore germ tube of ''Zymoseptoria tritici''. File:Mycosphaerella graminicola 6.png, Colonization of the mesophyll tissue by an intercellular hypha (arrows) of ''Zymoseptoria tritici'' during the symptomless biotrophic phase of pathogenesis. File:Mycosphaerella graminicola 7.png, Initiation (arrow head) of a pycnidium of ''Zymoseptoria tritici'' in the substomatal cavity of a wheat leaf.
Sexual state (
teleomorph In mycology, the terms teleomorph, anamorph, and holomorph apply to portions of the life cycles of fungi in the phyla Ascomycota and Basidiomycota: *Teleomorph: the sexual reproductive stage (morph), typically a fruiting body. *Anamorph: an ase ...
): Pseudothecia are subepidermal, globose, dark brown, and 68-114 μm in diameter. Asci measure 11-14 × 30-40 μm.
Ascospore In fungi, an ascospore is the sexual spore formed inside an ascus—the sac-like cell that defines the division Ascomycota, the largest and most diverse Division (botany), division of fungi. After two parental cell nucleus, nuclei fuse, the ascu ...
s are hyaline, elliptical, and 2.5-4 × 9-16 μm, with two cells of unequal length.


Genetics

''Zymoseptoria tritici'' represents an intriguing model for fundamental genetic studies of plant-pathogenic fungi. It is
haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...
plant-pathogenic fungus. Many fungi are haploid, which greatly simplifies genetic studies. ''Zymoseptoria tritici'' was the first species, in 2002, of the family Mycosphaerellaceae to have a linkage map created. The first report of fully sequenced
genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
of ''Zymoseptoria tritici'' from 2011 was the first genome of a filamentous fungus to be finished according to current standards. The length of the genome is 39.7 Mb, that is similar to other filamentous ascomycetes. The genome contains 21
chromosomes A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most importa ...
, that is the highest number reported among ascomycetes. Furthermore, these chromosomes have an extraordinary size range, varying from 0.39 to 6.09 Mb. A striking aspect of ''Zymoseptoria tritici'' genetics is the presence of many dispensable chromosomes. Eight of chromosomes could be lost with no visible effect on the fungus and thus are dispensable. Dispensable chromosomes have been found in other fungi but they usually occur at a low frequency and typically represent single or a few chromosomes. Dispensable chromosomes have originated by ancient horizontal transfer from an unknown donor, that was followed by extensive
genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryot ...
, a possible mechanism of stealth
pathogenicity In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term ...
and exciting new aspects of genome structure. A surprising feature of the ''Zymoseptoria tritici'' genome compared to other sequenced plant pathogens was that it contained very few genes for
enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
s that break down plant
cell wall A cell wall is a structural layer that surrounds some Cell type, cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, ...
s, which was more similar to
endophyte An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life cycle without causing apparent disease. Endophytes are ubiquitous and have been found in all species of plants studied to date; ...
s than to
pathogen In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...
s. Goodwin et al. (2011) suggested, that the stealth pathogenesis of ''Zymoseptoria tritici'' probably involves degradation of proteins rather than
carbohydrates A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ma ...
to evade host defenses during the biotrophic stage of infection and may have evolved from endophytic ancestors.


Evolution

The fungus ''Zymoseptoria tritici'' has been a pathogen of wheat since host domestication 10,000–12,000 years ago in the
Fertile Crescent The Fertile Crescent () is a crescent-shaped region in the Middle East, spanning modern-day Iraq, Israel, Jordan, Lebanon, Palestine, and Syria, together with northern Kuwait, south-eastern Turkey, and western Iran. Some authors also include ...
. The wheat-infecting lineage emerged from closely related ''Mycosphaerella'' pathogens infecting wild
grass Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family (biology), family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and spe ...
es. It has coevolved and spread with its host globally. ''Zymoseptoria tritici'' shows a significantly higher degree of
host specificity In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include ...
and virulence in a detached leaf assay. The emergence and "co-domestication" of ''Zymoseptoria tritici'' was associated with an
adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the p ...
to wheat and an agricultural environment. Endemic descendants of the
progenitor In genealogy, a progenitor (rarer: primogenitor; or ''Ahnherr'') is the founder (sometimes one that is legendary) of a family, line of descent, gens, clan, tribe, noble house, or ethnic group.. Ebenda''Ahnherr:''"Stammvater eines Geschlec ...
of ''Zymoseptoria tritici'' are still found on wild grasses in the
Middle East The Middle East (term originally coined in English language) is a geopolitical region encompassing the Arabian Peninsula, the Levant, Turkey, Egypt, Iran, and Iraq. The term came into widespread usage by the United Kingdom and western Eur ...
; however these "wild" pathogens show a broader host range than the "domesticated" wheat pathogen. The closest known relative of ''Zymoseptoria tritici'' is named ''Z. pseudotritici'' B. ''Zymoseptoria pseudotritici'' was isolated in
Iran Iran, officially the Islamic Republic of Iran (IRI) and also known as Persia, is a country in West Asia. It borders Iraq to the west, Turkey, Azerbaijan, and Armenia to the northwest, the Caspian Sea to the north, Turkmenistan to the nort ...
from the two grass species '' Agropyron repens'' and ''
Dactylis glomerata ''Dactylis glomerata'' is a species of flowering plant in the grass family Poaceae, known as cock's-foot,Interactive Flora of NW Europ''Dactylis glomerata'' (Cock's-foot)/ref> also colloquially as orchard grass, or cat grass (due to its populari ...
'' growing in close proximity to fields planted to bread wheat (''
Triticum aestivum Common wheat (''Triticum aestivum''), also known as bread wheat, is a cultivated wheat species. About 95% of wheat produced worldwide is common wheat; it is the most widely grown of all crops and the cereal with the highest monetary yield. Ta ...
''). Although ''Z. tritici'' is a frequent pathogen of wheat in Iran, no evidence of
gene flow In population genetics, gene flow (also known as migration and allele flow) is the transfer of genetic variation, genetic material from one population to another. If the rate of gene flow is high enough, then two populations will have equivalent ...
between ''Z. pseudotritici'' and ''Z. tritici'' was detected based on sequence analysis of six nuclear loci.


Life cycle

''Zymoseptoria tritici'' overwinters as fruiting bodies on crop debris, mostly as pseudothecia (sexual fruiting bodies) but sometimes also some pycnidia (asexual fruiting bodies). The sexual spores are quantitatively the more significant spores involved in primary inoculum of the disease, while the asexual spores are more significant in the secondary cycle. In early spring,
ascospore In fungi, an ascospore is the sexual spore formed inside an ascus—the sac-like cell that defines the division Ascomycota, the largest and most diverse Division (botany), division of fungi. After two parental cell nucleus, nuclei fuse, the ascu ...
s, the sexual spores of the fungus, are released from the pseudothecia. Ascospores are wind-dispersed and eventually land on the leaves of a host plant (bread wheat or durum wheat). Unlike most other plant pathogens, ''Zymoseptoria tritici'' uses a germ tube to enter the host leaf through
stoma In botany, a stoma (: stomata, from Greek language, Greek ''στόμα'', "mouth"), also called a stomate (: stomates), is a pore found in the Epidermis (botany), epidermis of leaves, stems, and other organs, that controls the rate of gas exc ...
ta rather than by direct penetration. There is a long latent period of up to two weeks following infection before symptoms develop. The fungus evades host defenses during the latent phase, followed by a rapid switch to necrotrophy immediately prior to symptom expression 12–20 days after penetration. The period between infection and formation of sporulating structures (latent period) was estimated to be 20.35 ± 4.15 days for ''Zymoseptoria tritici'' in Northern Germany and decreased with increasing temperature. Such a switch from biotrophic to necrotrophic growth at the end of a long latent period is an unusual characteristic shared by most fungi in the genus ''Mycosphaerella''. Very little is known about the cause or mechanism of this lifestyle switch even though ''Mycosphaerella'' is one of the largest and most economically important genera of plant-pathogenic fungi. Primary inoculum requires wet conditions and cool temperatures of 50-68 °F. Under appropriate environmental conditions, lesions are able to develop on infected leaves, and soon pycnidia begin to develop on the lesions. The pycnidia appear as small dark dots on the lesions. From the pycnidia, conidiospores, the asexual spores of the fungus, are released. These asexual spores are dispersed via rain splash and are response for the secondary inoculum of this polycyclic disease cycle. When the conidiospores are splashed onto leaves, they act similarly to ascospores and cause the development of foliar lesions. In addition to pycnidia, pseudothecia also develop within these lesions. Pycnidia and pseudothecia are the structures in which the fungus overwinters, and the cycle begins again.


Disease Management

''Zymoseptoria tritici'' is a difficult fungus to control because populations contain extremely high levels of
genetic variability Genetic variability is either the presence of, or the generation of, genetic differences. It is defined as "the formation of individuals differing in genotype, or the presence of genotypically different individuals, in contrast to environmentally ...
and it has very unusual biology for a pathogen. ''Z. tritici'' has an active sexual cycle under natural conditions, which is an important driver of septoria tritici blotch
epidemics An epidemic (from Ancient Greek, Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of Host (biology), hosts in a given population within a short period of time. For example ...
and results in high genetic diversity of populations in the field. The most effective, economical, and simple method of ''Z. tritici'' management is planting resistant cultivars. Twenty-one resistant genes have been named, mapped, and published. Mikaberidze and McDonald 2020 found a fitness tradeoff between genes for ''Septoria'' tolerance and ''Septoria'' resistance in wheat. Some cultivars are resistant in one region but susceptible in another; it depends on the local pathogen population. All varieties of bread wheat and durum wheat are susceptible to the disease to some extent, but planting varieties that have at least partial resistance to the local population of ''Zymoseptoria tritici'' can greatly improve yield. There are also cultural management strategies that may be effective, including regular rotation of crops, deep plowing, and late planting. More specifically, rotating a recently infected field to any non-host crop can be useful in minimizing the amount of fungus present in the field. Planting winter wheat after the first ascospore flights in September is a way to reduce primary inoculum of winter wheat. Fungicide use often simply is not economical for Septoria Leaf Blotch. The rapid evolution of pathogen resistance to fungicides is a major barrier. ''Zymoseptoria tritici'' has resistance to multiple fungicides, because it has number of substitutions of CYP51. CYP51 substitutions include Y137F which confers resistance to triadimenol, I381V which confers resistance to tebuconazole and V136A that confers resistance to prochloraz. Chemical control of the pathogen (using
fungicide Fungicides are pesticides used to kill parasitic fungi or their spores. Fungi can cause serious damage in agriculture, resulting in losses of yield and quality. Fungicides are used both in agriculture and to fight fungal infections in animals, ...
s) now relies on the application of SDHIs,
azole Azoles are a class of five-membered heterocyclic compounds containing a nitrogen atom and at least one other non-carbon atom (i.e. nitrogen, sulfur, or oxygen) as part of the ring. Their names originate from the Hantzsch–Widman nomenclature. Th ...
fungicides which are
demethylase Demethylases are enzymes that remove methyl (CH3) groups from nucleic acids, proteins (particularly histones), and other molecules. Demethylases are important epigenetics, epigenetic proteins, as they are responsible for transcriptional regulation ...
inhibitors that inhibit lanosterol 14 alpha-demethylase (
CYP51 In enzymology, a sterol 14-demethylase () is an enzyme of the cytochrome P450 (CYP) superfamily. It is any member of the CYP51 family. It catalysis, catalyzes a chemical reaction such as: :obtusifoliol + 3 O2 + 3 NADPH + 3 H+ \rightleftharpoons 4 ...
) activity. The last method of control for ''Zymoseptoria tritici'' is biological control using bacteria. ''
Bacillus megaterium ''Priestia megaterium'' (''Bacillus megaterium'' prior to 2020) is a rod-like, Gram staining, Gram-positive, mainly aerobic, endospore, spore forming bacterium found in widely diverse habitats.De Vos, P. ''et al.'' Bergey's Manual of Systematic B ...
'' has been shown to cause about an 80% decrease in disease development in the trials done so far.
Pseudomonads The Pseudomonadaceae are a family of bacteria which includes the genera '' Azomonas'', ''Azorhizophilus'', ''Azotobacter'', '' Mesophilobacter'', ''Pseudomonas'' (the type genus), and '' Rugamonas''. The family Azotobacteraceae was recently rec ...
are also a promising bacterial control option. A benefit to using pseudomonads or bacillus is that they are not harmed by most fungicides, so they can be used in combination with chemical controls. However, a lack of commercial availability limits the use of biological controls.


Disease Importance

The ascomycete fungus ''Zymoseptoria tritici'' causes septoria tritici blotch, a foliar disease of
wheat Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...
that poses a significant threat to global
food production The food industry is a complex, global network of diverse businesses that supplies most of the food consumed by the World population, world's population. The food industry today has become highly diversified, with manufacturing ranging from sm ...
. It is the primary foliar disease of
winter wheat Winter wheat (usually ''Common wheat, 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. C ...
in most western European countries. ''Zymoseptoria tritici'' infects wheat crops throughout the world and is also currently a big problem in Iran, Tunisia, and Morocco. Severe epidemics of the disease have decreased wheat yields by 35-50%. In the United States, Septoria leaf blotch is a very important disease in wheat, second only to wheat rust. An estimated $275 million is lost per year in the US due to this disease. In Europe the annual losses are equivalent to over 400 million USD. Different areas of the world are currently trying different management strategies. For example, in the Nordic-Baltic region, one of the largest wheat-producing regions of the world, the use of fungicides has substantially increased wheat yields. The fungicides that have been shown to be effective include quinone outside inhibitors (QoIs), which, like most fungicides, are expensive to apply in large quantities. As
climate change Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...
begins to increase temperatures around the globe, ''Zymoseptoria tritici'', along with many other fungal pathogens, is likely to show increased overwintering survival and therefore more substantial primary inocula. The need for effective management techniques will become even more important as the prevalence of Septoria leaf blotch increases with climate change. File:Mycosphaerella graminicola 9.png, Typical infection caused by ''Zymoseptoria tritici'' of the primary leaf of a resistant
cultivar A cultivar is a kind of Horticulture, cultivated plant that people have selected for desired phenotypic trait, traits and which retains those traits when Plant propagation, propagated. Methods used to propagate cultivars include division, root a ...
. Note the low fungal density in the
apoplast The ''apoplast '' is the extracellular space outside of plant cell membranes, especially the fluid-filled cell walls of adjacent cells where water and dissolved material can flow and diffuse freely. Fluid and material flows occurring in any extr ...
(arrow) and the response of the mesophyll cells (arrow head), particularly the chloroplasts, to the presence of intercellular
hyphae A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...
. File:Mycosphaerella graminicola 10.png, (upper image) Typical symptoms of ''Zymoseptoria tritici'' on a primary seedling leaf of a highly susceptible wheat cultivar. (lower image) Typical response to ''Zymoseptoria tritici'' on a primary leaf of a highly resistant wheat cultivar. File:Mycosphaerella graminicola 14.png, Symptoms of ''Zymoseptoria tritici'' on a naturally infected adult plant flag leaf of wheat.


References

This article incorporates CC-BY-2.5 text from referencesStukenbrock E.H., Jørgensen F.G., Zala M., Hansen T.T., McDonald B.A. & Schierup M.H. (2010). "Whole-Genome and Chromosome Evolution Associated with Host Adaptation and Speciation of the Wheat Pathogen ''Mycosphaerella graminicola''". ''PLoS Genetics'' 6(12): e1001189. Wittenberg A.H.J., van der Lee T.A.J., Ben M'Barek S., Ware S.B., Goodwin S.B., et al. (2009). "Meiosis Drives Extraordinary Genome Plasticity in the Haploid Fungal Plant Pathogen ''Mycosphaerella graminicola''". ''PLoS ONE'' 4(6): e5863. .Goodwin S.B., Ben M'Barek S., Dhillon B., Wittenberg A.H.J., Crane C.F., et al. (2011). "Finished Genome of the Fungal Wheat Pathogen ''Mycosphaerella graminicola'' Reveals Dispensome Structure, Chromosome Plasticity, and Stealth Pathogenesis". ''PLoS Genetics'' ''7''(6): e1002070. Mullins J. G. L., Parker J. E., Cools H. J., Togawa R. C., Lucas J. A., et al. (2011). "Molecular Modelling of the Emergence of Azole Resistance in ''Mycosphaerella graminicola''". '' PLoS ONE'' 6(6): e20973. .


External links


USDA ARS Fungal Database
* * Orton E. S., Sian Deller S. & Brown J. K. M. (2011). "''Mycosphaerella graminicola'': from genomics to disease control". ''
Molecular Plant Pathology ''Molecular Plant Pathology'' is a monthly open access peer-reviewed scientific journal published by Wiley-Blackwell on behalf of the British Society for Plant Pathology. It was established in January 2000 by Gary D. Foster, University of Bristol ...
'' 12(5): 413-424. . {{Taxonbar, from1=Q104837794, from2=Q138622 Mycosphaerellaceae Fungal plant pathogens and diseases Wheat diseases Fungi described in 1842 Taxa named by John Baptiste Henri Joseph Desmazières Fungus species