Sporisorium sorghi, commonly known as sorghum smut, 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 as Sorghum bicolor (S. vulgare) (sorghum), S. sudanense (Sudan grass), S. halepense (Johnson grass) and Sorghum vulgare var. technichum (broomcorn). Ineffective control of S. sorghi can have serious economic and ecological implications.
Taxonomy
A collection of S. sorghi on sorghum grass allowed Ehrenberg to establish the genus Sporisorium in 1825. This genus was replaced by Sphacelotheca, a genus established by de Bary. Soon after, G. P. Clinton transferred Sporisorium to this genus instead, giving it the name Sphacelotheca sorghi. Ontogeny studies provide evidence that Sporisorium is restricted to parasitic smut fungi. This support allowed for the reinstatement of the Sporisorium genus by Langdon & Fullerton and for the reclassification of S. sorghi into this genus. Other members of Sphacelotheca were also renamed to Sporisorium after proving they were harmful to grasses. Ustilago and Sporisorium genera are closely related, but descriptions made by Link and Langdon & Fullerton allow for a taxonomic boundary of these genera based on variation in columellae, sterile cells, and spore balls. Vánky transferred species within Ustilago to Sporisorium following this differentiation.
Hosts and symptoms
Susceptible hosts of Sporisorium sorghi infection consist of all groups of sorghum plants. Main hosts include Sorghum bicolor, Sorghum caffrorum, Sorghum dochna, and Sorghum sudanense. Incidences of infection in perennial grasses have also been reported. S. sorghi is the causative agent of sorghum smut or covered kernel smut disease.
Symptoms of infection by S. sorghi are typically expressed as the plant matures and begins to head. At this time, grain kernels are replaced by fungal spore-producing structures known as sori. Smut sori are covered by a peridium, a tough membrane that appears grayish-brown in color. The shape of sori, themselves, are generally oval or conical. 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. 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. Missing or distorted spikelets covered in sori on the sorghum panicles also indicate signs of infection in rare instances.
Life cycle
Planting of a sorghum kernel infected with S. sorghi leads to the development of fungal structures that occur alongside the growth of the plant. These seedborne structures are known as teliospores, and they germinate as the sorghum plant matures by colonizing plant tissues and the apical meristem. 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. Teliospores that do this will overwinter, where they remain in a dormant 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.
Conditions suited for delayed germination of sorghum seedlings provide optimum conditions for S. 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. Temperatures ranging from 20-30 degrees Celsius allow maximum spore production. 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 as host variability and depth of seedling sowing also affect the prevalence at which infection occurs.