Ustilago scitaminea

Sugarcane smut
The black whip produced by sugarcane smut
Common names	smut of sugarcane
Causal agents	Sporisorium scitamineum
Hosts	sugarcane
EPPO code	USTISC

Ustilago scitaminea

Sugarcane smut is a fungal disease of sugarcane caused by the fungus Sporisorium scitamineum. The disease is known as culmicolous, which describes the outgrowth of fungus of the stalk on the cane. It attacks several sugarcane species and has been reported to occur on a few other grass species as well, but not to a critical amount. The most recognizable characteristic of this disease is a black or gray growth that is referred to as a "smut whip". Resistance to sugarcane smut is the best course of action for management, but also the use of disease free seed is important. On smaller scale operations treatments using hot water and removing infected plants can be effective. The main mode of spore dispersal is the wind but the disease also spreads through the use of infected cuttings. Sugarcane smut is a devastating disease in sugarcane growing areas globally.

Sugarcane smut infects all sugarcane species unless the species is resistant. The damage caused depends on the susceptibility of the species. Sugarcane fields are planted using vegetative cuttings from mother plants so they have the same genetic make-up of the parent plant. Seeds are not used in propagation because sugarcane is a multi-species hybrid and therefore is difficult to breed. Sugarcane smuts can also infect some other grass species outside of sugarcane. However, mostly it remains on plants of the genus Saccharum.

Two to four months after the fungus has infected the plant, black whip-like structures, instead of a spindle leaf, emerge from the meristem, or growing point, of the plant. The developing whip is a mixture of plant tissue and fungal tissue. The whip reaches maturity between the sixth and the seventh month. When spores that are contained inside the whip are released, the core of the whip remains behind and is a straw-like color.

Plants infected with the fungus usually appear to have thin stalks and are often stunted. They end up tillering much more than normal and this results in leaves that are more slender and much weaker. They sometimes appear more grass-like than non-infected plants. Less common symptoms of the disease are stem or leaf galls and proliferating buds.

Sugarcane smut is disseminated via teliospores that are produced in the smut whip. These teliospores located either in the soil or on the plant, germinate in the presence of water. After germination they produce promycelium and undergo meiosis to create four haploid sporidia. Sugarcane smut is bipolar and therefore produces two different mating types of sporida. For infection to occur, two sporida from different mating types must come together and form a dikaryon. This dikaryon then produces hyphae that penetrate the bud scales of the sugarcane plant and infect the meristematic tissue. The fungus grows within the meristematic tissue and induces formation of flowering structures which it colonises to produce its teliopores. The flowering structures, usually typical grass arrows, are transformed into a whip like sorus that grows out between the leaf sheaths. At first it is covered by a thin silvery peridium (this is the host tissue) which easily peels back when desiccated to expose the sooty black-brown teliospores. These teliospores are then dispersed via wind and the cycle continues. The spores are reddish brown, round and subovoid and may be smooth to moderately echinulate. The size varies from 6.5 to 8 um. Sugarcane cultivars intended for distribution to other geographical areas should be tested for susceptibility to S. scitamineum populations in each area.