''Setosphaeria turcica'' (anamorph ''Exserohilum turcicum''; formerly known as ''Helminthosporium turcicum'') is the causal agent of
northern corn leaf blight
Northern corn leaf blight (NCLB) or Turcicum leaf blight (TLB) is a foliar disease of corn (maize) caused by ''Exserohilum turcicum'', the anamorph of the ascomycete '' Setosphaeria turcica''. With its characteristic cigar-shaped lesions, this di ...
in
maize
Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. The ...
. It is a serious fungal disease prevalent in cooler climates and tropical highlands wherever corn is grown. It is characterized by large cigar shaped necrotic lesions that develop on the leaves due to the
polyketide
Polyketides are a class of natural products derived from a precursor molecule consisting of a chain of alternating ketone (or reduced forms of a ketone) and methylene groups: (-CO-CH2-). First studied in the early 20th century, discovery, biosynth ...
metabolite monocerin
Monocerin is a dihydroisocoumarin and a polyketide metabolite that originates from various fungal species. It has been shown to display antifungal, plant pathogenic, and insecticidal characteristics. Monocerin has been isolated from '' Dreschler ...
.
Races
Races of ''S. turcica'' are named for the maize
R gene
Resistance genes (R-Genes) are genes in plant genomes that convey plant disease resistance against pathogens by producing R proteins. The main class of R-genes consist of a nucleotide binding domain (NB) and a leucine rich repeat (LRR) dom ...
s which are not effective against them. For example, the maize gene ''Ht1'' does not confer resistance against a Race 1 isolate, while genes ''Ht2'' and/or ''Ht3'' do. In contrast, genes ''Ht2'' and ''HtN'' do not confer resistance against a Race 2N isolate, while gene ''Ht1'' does. Isolates which do not overcome any known R gene are termed Race 0.
Race 1, which overcomes the ''Ht1'' resistance gene, was originally discovered in Hawaii in 1973. By the time it was first noted in the continental United States, in Indiana in 1980, it was already quite prevalent across the entire state. Genotyping of isolates sampled from the eastern United States later revealed that while Race 0 was predominant in the mid-1970s, Race 1 spread rapidly, becoming the most prevalent race in the region by the mid-1990s.
Mating type
''S. turcica'' is a
heterothallic
Heterothallic species have sexes that reside in different individuals. The term is applied particularly to distinguish heterothallic fungi, which require two compatible partners to produce sexual spores, from homothallic ones, which are capable ...
fungus, meaning that a single isolate cannot mate with itself. Instead, two isolates with complementary mating type genes are required for sexual reproduction. The "perfect stage" (sexual stage or
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 asex ...
) was first described in 1958. A single
mating-type locus The mating-type locus is a specialized region in the genomes of some yeast and other fungi, usually organized into heterochromatin and possessing unique histone methylation patterns. The genes in this region regulate the mating type of the organism ...
was identified the next year
The mating-type locus of ''S. turcica'' follows the same naming convention as other filamentous
ascomycetes: the locus is known as ''MAT1'', while the two idiomorphs (genes at this locus which are not
allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
s, as they do not derive from a common ancestor) are known as ''MAT1-1'' and ''MAT1-2''
Genotyping of ''S. turcica'' populations has shown that sexual reproduction in a given population might be commonplace, extremely rare, or anywhere in between. Genotyping of 264 isolates of ''S. turcica'' from temperate and tropical regions found that tropical populations had very high genetic diversity, an equal proportion of the two
mating type
Mating types are the microorganism equivalent to sexes in multicellular lifeforms and are thought to be the ancestor to distinct sexes. They also occur in macro-organisms such as fungi.
Definition
Mating types are the microorganism equivalent to ...
s, and low amounts of
linkage disequilibrium
In population genetics, linkage disequilibrium (LD) is the non-random association of alleles at different loci in a given population. Loci are said to be in linkage disequilibrium when the frequency of association of their different alleles is h ...
between different genetic loci, all suggestive of frequent sexual mating and recombination. In temperate populations, in contrast, there was low genetic diversity, high amounts of linkage disequilibrium, and a single dominant mating type, which suggests infrequent sexual mating.
[Leonard KJ, Levy Y, Smith DR. 1989. Proposed Nomenclature for pathogen races of ''Exserohilum turcicum'' on corn. Plant Disease 73: 776-777]
References
External links
Index FungorumUSDA ARS Fungal Database
Fungal plant pathogens and diseases
Maize diseases
Pleosporaceae
Fungi described in 1876
{{plant-disease-stub