Head smut is a systemic disease of maize caused by Sphacelotheca reiliana. The pathogen affects the tassels and the ears, causing characteristic symptoms that are easily recognized. Head smut can cause extensive economic damage in areas where maize is cultivated frequently and the growing season is characterized by dry and hot conditions. Incidence of head smut as high as 80% has been documented.

Pathogen: Sphacelotheca reiliana (syn. Sorosporium reilianum, Ustilago reiliana)

Symptoms

Characteristic symptoms of head smut are visible once the maize plant has reached the tasseling and reproductive growth stages. Tassels and ears are typically malformed, overgrown, and replaced by smut sori. Smut sori are usually covered by a thin tissue, which ruptures to revela masses of dry, black spores (teliospores) that are wind disseminated. A tangled mass of vascular threads are often observed in ruptured sori. Sori can infect a few spikelets in the tassel or can infect the entire tassel leading to abnormal growth and structures. Infected ears are round, lacking in silks, and the ear is replaced by masses of teliospores that leave the vascular bundles exposed and shredded. Under less severe infection, only partial sections of the ear may be smutted and some normal kernels may still form. Occasionally sori can form on leaves where they develop as narrow stripes. Infected plants are also frequently dwarfed and may tiller.

Confirmation

Teliospores of Sphacelotheca reilians are dark colored, ranging from rusty brown to black. Teliospores are 9-14μm in diameter, circular, spiny, and are produced abundantly in sori. Teliospores germinate from basidia and sporidia that are single-celled, hyaline and 7 to 15 μm in diameter. S. reiliana population is composed of various races that are pathogenic to different hosts. Races pathogenic to maize are rarely pathogenic to other hosts. Head smut can be differentiated from common smut by the absence of a prominent white/grey periderm. Additionally, unlike other Ustiliginaciae species such as Ustilago maydis, S. reiliana only infects maize via the plant root.

Incidence and factors favoring disease severity

Head smut has been documented from most regions of the world where maize is cultivated intensely. It is recognized as economically important in dry, hot maize growing areas in the tropics, as well as many temperate maize growing regions.

Figure 1. Geographic distribution of head smut

Dry and cool weather at the start of the season favors the disease as maize seedling growth is slow. Later in the season dry and warm conditions (21-30°C) favor disease severity. Soil moisture content of 15-25% W/w is optimal for seedling infection. Head smut has been recorded as being more severe under nitrogen deficiency.

Host range

S. reiliana is known to infect maize, sorghum, Sudan grass, Johnson grass, and teosinte. Various pathogenic races of S. reiliana that are pathogenic to different hoses have been documented. Generally, races that are pathogenic to one host do not infect others.

Life cycle

Teliospores are produced abundantly in the sori of smutted maize. They are wind disseminated once the sori rupture and overwinter in the soil until temperature and soil moisture levels are optimal for germination, able to survive in the soil for over three years. Teliospores germinate to form four-celled basidia. Each basidium germinates into several haploid basidiospores which bud in a similar fashion to yeast cells, forming haploid sporidia of opposing mating types (+ and -). Mating types are generally produced in equal numbers. Sporidia of opposing mating types fuse to produce dikaryotic hyphae that are infectious to maize seedlings. Infective mycelium invades the meristemic tissue of seedlings and undifferentiated floral tissue, giving rise to characteristic symptoms once the plant reaches the reproductive growth stages. Head smut is predominantly a soil borne disease. Seed borne inoculum has been documented but is not considered important.

Damage

• Mechanism of damage: Damage is caused predominantly by the replacement of hte ear with a sorus. Infection of the tassel also leads to reduced pollen production.
• When damage is important: Damage is the highest when susceptible hybrids are cultivated and when temperature (21-30°C) and soil moisture (15-25% W/w) favor disease severity. Low soil nitrogen levels increase disease severity. Usually, if the tassel is smutted then the entire ear will consist of sori, resulting in extensive crop loss.
• Economic importance: Although head smut is widely documented, resistant maize varieties are widely available and extensive losses are rare. Nonetheless, where susceptible varieties have been grown disease incidence as high as 80% has been documented.

Management principles

Host resistance

• Resistant varieties are available and should be cultivated in regions where head smut is a problem. Both qualitative and quantitative resistance has been documented.
• Varieties with rapid seedling emergence may escape disease infection.

Cultural methods

• As teliospores can survive for several years in the soil, crop rotation is of limited use.
• Planting maize during seasons that are unfavorable to teliospore germination will reduce infection.
• Ensuring that the soil is well-fertilized can reduce disease severity, as head smut is reported to be more serious under nitrogen stress.
• Removing and destroying sori before they rupture can limit the spread of inoculum.

Fungicides

• Foliar application of fungicide has not been effective for the control of head smut.
• Seed treatment with systemic fungicides and in-furrow treatments with fungicides have been used to effectively control head smut.
• Fungicide treatment in-furrow may not be economical under low disease pressure or for grain production.

Prepared by Biswanath Das

References

Al-Sohaily, I.A., C.J. Mankin, and G. Semeniuk. 1963. Physiologic specialization of Sphacelotheca reiliana to sorghum and corn. Phytopathology 53: 723-6.

CAB International. 1978. Distribution Maps of Plant Diseases. Edition 5 (October), Map 69. Wallingford, UK: CAB International.

CIMMYT. 2004. Maize diseases: A guide for field identification. 4th Edition. Mexico, D.F.: CIMMYT.

Martinez, C., C. Roux, A. Jauneau, and R. Dargent. 2002. The biological cycle of Sporisorium reilianum f. sp. zeae: an overview using microscopy. Mycologia 94: 505-14.

Matyac, C.A. and T. Kommedahl. 1985. Factors affecting the development of Head Smut caused by Sphacelotheca reiliana on corn. Phytopathology 75: 577-81.

Pataky, J.K. 1999. Head Smut. In Donald G. White (ed.), Compendium of Corn Diseases. St. Paul, MN: The American Phytopathology Society. Pp. 33-5.

University of Illinois Extension. 1990. Report of Plant Disease: Corn Smuts. RPD No. 203. Department of Crop Sciences, University of Illinois at Urbana-Champaign. http://web.aces.uiuc.edu/vista/pdf_pubs/203.pdf (27 August 2007).