''Alternaria alternata'' is a fungus which has been recorded causing
leaf spot
A leaf spot is a limited, discoloured, diseased area of a leaf that is caused by fungal, bacterial or viral plant diseases, or by injuries from nematodes, insects, environmental factors, toxicity or herbicides. These discoloured spots or lesion ...
and other diseases on over 380 host species of plant. It is an opportunistic pathogen on numerous hosts causing leaf spots, rots and blights on many plant parts.
It can also cause
upper respiratory tract
The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa.
Air is breathed in through the nose to ...
infections
and
asthma
Asthma is a long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, c ...
in humans with compromised immunity.
Hosts and symptoms
''Alternaria alternata'' has many different hosts depending on its
forma specialis
''Forma specialis'' (plural: ''formae speciales''), abbreviated f. sp. (plural ff. spp.) without italics, is an informal taxonomic grouping allowed by the International Code of Nomenclature for algae, fungi, and plants, that is applied to a para ...
. In this review, only ''Alternaria alternata'' f. sp. ''lycopersici'' (AAL) is going to be assessed. This pathogen infects only certain cultivars of tomato plants and is often referred to as Alternaria stem canker of tomato.
AAL's main symptom is cankers in the stem. It resides in seeds and seedlings, and is often spread by spores as they become airborne and land on plants. It can also spread throughout other plants.
Under severe infection, lesions enlarge and become coalesced causing blighting of the leaves. This symptom progression occurred in research done in Pakistan: the symptoms on affected tomatoes started with yellowing and browning of the lower leaves, then began developing on the leaf tips and along the margins of the leaf petiole. This progression continued until the entire leaves were covered in diseased tissue and then fell off. In addition to necrotic leaves and petioles, plants are found to have severe defoliation, with considerable yield losses when it occurs before flowering.
The tomato fruit can also be infected as well, with brown cankers dotting them and making them inedible. Once the disease has spread to a certain point, little can be done to save the tomato plant.
There are several host factors that affect disease development. For example, various signaling pathways in tomato plants affect their susceptibility to AAL. Salicylic acid promotes resistance to AAL and antagonizes the ethylene response. Ethylene controls the synthesis of jasmonic acid, which is a necessary pathway for susceptibility. Independently of each other, salicylic acid, ethylene, and jasmonic acid can influence the susceptibility of tomato to AAL. Diagnosis of AAL is often from observing signs and symptoms from this fungal pathogen. In addition, a tomato cultivar's resistance to a toxin produced by AAL also affect disease development.
Environment
In order to survive, ''Alternaria alternata'' needs a moist warm environment. It is often found in areas with humid climates, or where there has been significant rainfall.
The fungus lives in seeds and seedlings, and is also spread by spores. This disease flourishes in dead plants that have been left in gardens over winter. Additionally, when dead infected debris is added to compost pile it can spread to other vegetables throughout the garden.
There are no insect vectors for this disease. This means that using insecticides has no effect on the spread of this pathogen. However, there are several cultural practices that can be followed to suppress this fungal pathogen's impact. The disease first occurs in the host's exposed leaves. Plants planted with rows in an east–west direction have more severe disease than do plants planted north–south. This implies that if one plants tomato plants in a north–south manner they will be less susceptible. It is also suggested to highly monitor plants in April through June. This is when the pathogen is most prevalent. If monitoring indicates the presence of AAL, it is suggested to begin late-spring treatments of fungicide about mid-April. However, if a garden has a history of disease, it is advised to take extra measures. This can be done by treating tomatoes in mid to late April and 2 to 3 weeks later by applying a fungicide.
Because this is a fungal pathogen that thrives off of wet environments, overhead irrigation is never advised when irrigating. This causes the moisture to remain on the leaf tissue and increase susceptibility to the disease and provides an optimal environment for the fungus to survive and grow. Furrow irrigation or drip irrigation systems allow the plant to remain dry.
Overall, AAL thrives in moist warm environments. Cultural practices for preventing this disease include planting tomatoes in a row north to south, monitoring plants heavily April through June, and using a drip irrigation system to keep as much plant tissue dry and free of favorable environments for this pathogen.
While
γ-aminobutyric acid (GABA) has no direct
fungicidal
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 ...
activity on ''A. alternata'',
it does
induce resistance in
Solanum lycopersicum
The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as the tomato plant. The species originated in western South America, Mexico, and Central America. The Mexican Nahuatl word gave rise to the Spanish wor ...
.
Some or all of that
resistance
Resistance may refer to:
Arts, entertainment, and media Comics
* Either of two similarly named but otherwise unrelated comic book series, both published by Wildstorm:
** ''Resistance'' (comics), based on the video game of the same title
** ''T ...
is by activation of the tomato's own
antioxidant
Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricants ...
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
s.
Disease cycle
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 ...
of ''Alternaria alternata'' is thought to be ''Clathrospora diplospora'': this has yet to be confirmed.
As a result, this pathogen propagates itself via asexual spores called
conidia
A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an asexual, non-motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also called mitospores due to th ...
.
These conidia are produced in lesions on mature or dying leaves.
Their production can begin in as few as ten days after the first symptoms appear, and can continue for to up to fifty days.
[Dewdney, M. M. "Alternaria Brown Spot1." EDIS New Publications RSS. Web. 22 October 201]
/ref> A. alternata's conidia disperse via air currents, and their release from the lesions can be triggered by rainfall, or even just a sudden drop in humidity. When the conidium lands on a leaf, it will wait until the nighttime dew, and then germinate. It can either enter through the stomata, or penetrate directly through the top of the leaf, using its appressorium, infecting the leaf within 12 hours.
Pathogenesis
At the cellular level, toxins are produced by AAL that are essential for pathogenicity on tomato. This host specific mycotoxin is called fumonisin B1. It was identified and confirmed by research conducted on fast atom bombardment and ion spray mass spectrometry. Thus, tomatoes that are resistant to this pathogen may be resistance to this specific toxin. Resistance to the pathogen in tomato is inherited as a single gene expressing complete dominance. However, sensitivity to the fumonisin B1 gene is controlled by a single locus with two alleles expressing incomplete dominance when heterozygous. In addition to resistance to the specific gene, resistance can be found from signaling pathways.
At the organismal level, AAL grows very slowly. This makes it so its presence is often not known until seedlings become larger and are transplanted into the garden. A fungicide may be used to save the plants once they are infected; however, the disease cannot be completely eradicated.
Ultimately, pathogenicity of this organism depends on a specific tomato cultivar's resistance to a fumonisin B1 gene. Additionally, the best way to prevent this pathogen from producing disease on tomato plants is to ensure the tomatoes are resistant cultivars.
Identification
Teleomorph
Thought to be ''Clathrospora diplospora'' (to be confirmed).
Anamorph
Conidiophore
* Pale brown to olive brown
* 25–60 x 3–3.5 μm
* Straight or flexuous
* Individual conidiophores arise directly from substrate forming bushy heads consisting of 4–8 large catenate conidia chains
* Secondary conidiophores are generally short and 1-celled
Conidia
* Pale brown to light brown
* Obclavate to obpyriform orellipsoid, short conical beak at the tip, or beakless
* Surface smooth to verruculose
* Size
** 20–63 x 9–18 μm in size
** (on PCA) mature conidia typically 10–30 x 5–12 μm
* Septa
** Several vertical and −8 transverse septa
** (on PCA) 3–7 transepta, 1–5 longisepta
* Chains
** Produced in an often branched, long chain more than 5 conidia.
** (on PCA) individual chains of 5–15 conidia, complex of branching chains may contain up to 50–60 conidia
References
External links
Index Fungorum
USDA ARS Fungal Database
{{Taxonbar, from=Q2704244
alternata
Cereal diseases
Fungal grape diseases
Fungal plant pathogens and diseases
Fungal citrus diseases
Fungi described in 1817