| Sporisorium sorghi | |
|---|---|
Scientific classification | |
| Kingdom: | Fungi |
| Division: | Basidiomycota |
| Class: | Ustilaginomycetes |
| Order: | Ustilaginales |
| Family: | Ustilaginaceae |
| Genus: | Sporisorium |
| Species: | S. sorghi |
| Binomial name | |
| Sporisorium sorghi | |
| Synonyms | |
Cintractia sorghi-vulgaris(Tul. & C.Tul.)G.P.Clinton (1897) | |
Sporisorium sorghi, commonly known assorghum smut,[1] is a plant pathogen that belongs to the Ustilaginaceae family. This fungus is the causative agent of covered kernel smut disease and infects sorghum plants all around the world such asSorghum bicolor (S. vulgare) (sorghum),S. sudanense (Sudan grass),S. halepense (Johnson grass) andSorghumvulgare var.technichum (broomcorn).[2] Ineffective control ofS. sorghi can have serious economic and ecological implications.
A collection ofS. sorghi on sorghum grass allowedEhrenberg to establish the genusSporisorium in 1825. This genus was replaced bySphacelotheca, a genus established byde Bary. Soon after,G. P. Clinton transferredSporisorium to this genus instead, giving it the nameSphacelothecasorghi.[3]Ontogeny studies provide evidence thatSporisorium is restricted toparasitic smut fungi. This support allowed for the reinstatement of theSporisorium genus byLangdon & Fullerton and for the reclassification ofS. sorghi into this genus.[3] Other members ofSphacelotheca were also renamed toSporisorium after proving they were harmful to grasses.[4]Ustilago andSporisorium genera are closely related, but descriptions made byLink and Langdon & Fullerton allow for a taxonomic boundary of these genera based on variation incolumellae, sterile cells, and spore balls.[5]Vánky transferred species withinUstilago toSporisorium following this differentiation.[4]
Susceptible hosts ofSporisorium sorghi infection consist of all groups ofsorghum plants. Main hosts include Sorghum bicolor, Sorghum caffrorum, Sorghum dochna, and Sorghum sudanense.[6] Incidences of infection inperennial grasses have also been reported.[7]S. sorghi is the causative agent of sorghumsmut or covered kernel smut disease.
Symptoms of infection byS. sorghi are typically expressed as the plant matures and begins tohead. At this time, grain kernels are replaced by fungalspore-producing structures known as sori. Smut sori are covered by aperidium, a tough membrane that appears grayish-brown in color.[6] The shape of sori, themselves, are generally oval orconical. Oftentimes sori appear as an elongated sorghum seed. Variations in the size of sori range anywhere from 0.4-1.3 cm long and 0.2-0.4 cm wide.[8]Glumes, or tiny leaves, will sometimes cover very small sori. They appear white, gray, or brown in color. In some instances, sori may have a striped appearance. Dark brown, powder-like masses of sori may be concentrated on a particular region of an infected head, or in some instances, all kernels of a smutted head can be destroyed.[8] Missing or distortedspikelets covered in sori on the sorghumpanicles also indicate signs of infection in rare instances.[1]
Planting of a sorghum kernel infected withS. sorghi leads to the development of fungal structures that occur alongside the growth of the plant. These seedborne structures are known asteliospores, and theygerminate as the sorghum plant matures by colonizing plant tissues and theapical meristem.[7] No signs of fungal growth arise until plant maturation, or heading. At harvest time, the membranes surrounding the sori of infected kernels rupture. This releases the teliospores inside, allowing them to adhere to the surface of healthy seeds on other plants.[8] Teliospores that do this willoverwinter, where they remain in adormant state and resume their pathogenicity when conditions resume being favorable. Some teliospores, when released will go on to contaminate soils. Because spores must be seed-borne in order to cause infection, soilborne teliospores are insignificant in terms of seedling infections.[8]
Conditions suited for delayed germination of sorghum seedlings provide optimum conditions forS. sorghi infection. Because spore formation occurs as seedlings mature,S. sorghi attempts to avoid environments that allow the plants to grow rapidly and escape infection.[9] Temperatures ranging from 20-30 degrees Celsius allow maximum spore production.[6] Warm, wet soils outside of this temperature range (15.5-33.2 degrees C) have been shown to decrease the incidence of seedling infection. A variety of other factors such ashost variability and depth of seedlingsowing also affect the prevalence at which infection occurs.[6]
Sorghum grains are found in all regions of the world as they act as a major food crop for both humans and livestock in areas with little precipitation and high temperatures.[10] Covered kernel smut disease has been reported in all continents, and is found to be the most common smut disease in areas where untreated seeds are planted. Asia and Africa combined account for more than 80% of sorghum production in the world, making these two continents especiallysusceptible to S.sorghi infection.[10] In sorghum-growing states of India,S. sorghi infection has become one of the most serious diseases.[6]
To effectively manage the spread of disease caused byS. sorghi, the presence ofS. sorghi infection must be detected early on in sorghum development. Symptoms of infection don't present themselves until heading occurs, but research has demonstrated thatmicroscopy andPCR techniques are useful in differentiating healthy from infected sorghum seedlings early on in their life cycle.[7] Knowing this information will be beneficial for the early detection ofS. sorghi and the implementation of more effective control measures.
Currently, covered kernel smut andS. sorghi are controlled by using protectantfungicides to treat infected seeds. This method is very effective, reliable, and simple as it prevents the fungi from being introduced into an uninfected field of sorghum. However, in some less developed countries, this method is not sustainable as this practice is expensive and oftentimes not available.[9] In these instances, cultural methods may be used where seeds are soaked in water for four hours and then dried in the sun. Such methods keep intact seedviability while destroying spores.[11]S. sorghi spores can live in the soil for long periods of time so rotation of crops every four years is another method of control.[10] The burning of sorghum plants before the release of teliospores may be effective, although this does reduce crop yield. Planting the sorghum kernels in 15.5-32 degree Celsius soil further serves as a preventative measure that limitsS. sorghi germination.[10]
The fungal spores ofS. sorghi enter and grow within theovary of sorghum plants. Release of teliospores following sori membrane rupturing causes spores to contaminate other plants and nearby soils. Spores are long-lived structures that are difficult to eliminate. Without the use of seed treatments,S. sorghi infection can have serious economic and ecological impacts.[8]Small-scale farms anddeveloping countries tend to have more restricted use of fungicides, and therefore see a greater incidence ofS. sorghi infection. More than 100 million people insub-Saharan Africa depend on sorghum as a staple food source, and over 500 million people are dependent on sorghum in Africa and Asia combined.[10] In the early 1900s,S. sorghi infection was responsible for a loss of 3 million dollars across the U.S.[1] Loss of this plant byS. sorghi infection can be drastic in these parts of the world along with others.