Pathogen

Spiroplasma kunkelii (syn. Corn Stunt Spiroplasma).

VECTOR: Corn stunt is transmitted primarily by the cicadellid leafhoppers Dalbulus maidis (corn leafhopper) and by D. elimatus (Mexican corn leafhopper). Other less important Dalbulus, Graminella, Exitianus, Stirellus and Euscelidius leafhopper species may also transmit S. kunkelii.

The vectors are also known to transmit other pathogens (including maize rayado fino virus, maize dwarf mosaic virus, maize stripe virus, maize mosaic virus and maize bushy stunt phytoplasma) and mixed infections are common.

Symptoms

Infected plants show a range of symptoms depending on maize genotype. Typically the first symptoms include chlorosis of leaf margins, followed by reddening or purpling of older leaves. Chlorotic stripes often form at the base of younger leaves, which also turn purple at the tips. Chlorotic stripes may also enlarge to cover the entire leaf. At low temperature, chlorosis and purpling may be restricted to the base of upper leaves. Foliar symptoms appear close to tasseling.

Infected plants are stunted due to shortening of internodes and numerous, barren ear shoots develop. Often, plants tiller and root excessively and tassels are sterile. Plants can be completely barren or suffer from poor seed set and reduced ear size. Under severe infection, plants can die prematurely.

Confirmation

S. kunkelii is a motile prokaryote that lacks true cell walls. Morphology ranges from helical filaments that measure 0.1-0.15 × 3-10µm to spherical cells that are 0.3-0.8µm in diameter. S. kunkelii is visible under phase contrast microscopy of plant juice or abdominal smears from vectors. S. kunkelii is a facultative anaerobe and is phloem-limited. It is highly resistant to penicillin in vitro. S. kunkelii can be readily cultured on several media where optimal growth occurs at 30-32°C and with a pH of 7.2-7.4.

Identity of S. kunkelii can be confirmed using ELISA tests. Additionally, stomata of epidermal strips of infected leaves often have rounded guard cells, while subsidiary cells often fuse into adjoining epidermal cells.

Why and where it occurs

Corn stunt is common throughout the hot and humid lowlands of Central and South America, the Caribbean and the southern USA. The disease has also been recorded from elevations more than 2000 m above sea level. The disease is prevalent where maize is cultivated continuously and where leafhoppers are able to overwinter in mild climatic conditions.

Geographic distribution

Host range

S. kunkelii infects plants of the genus Zea including Z. maydis (maize), Z. perennis, Z. mays mexicana, Z. diploperennis, and Z. luxurians. Euchlaena mexicana, E. perennis, Vinca roseas and broad bean have also known as experimental hosts.

Life cycle

S. kunkelii overwinters in infected leafhoppers. Infected leafhoppers are reported to survive in greater numbers than uninfected ones. Once acquired by the leafhopper following feeding on infected maize plants, S. kunkelii undergoes an incubation period of 2 to 3 weeks in the vector.

Damage

• Mechanism of damage: Damage is caused by crop stunting, physiological deformities, and formation of barren or smaller ears which directly result in yield loss.   
• When damage is important: Damage is most critical when plants are infected early and when climatic conditions favor the emergence and proliferation of the leafhopper vectors. Optimum temperatures of leafhopper development are between 20 and 30°C.
• Economic importance:Corn stunt is considered to be an economically important disease of maize in much of the neotropical lowlands. Yield losses as high as 70% have been documented in Argentina.

Global distribution

The disease is confined to the New World (Figure 1) and is predominantly a problem in tropical lowlands of Central and South America and the Caribbean.

Management principles

Host resistance

• Development of resistant varieties offers the most practical means of disease management. However, development of host resistance has been hindered by little information about the genetics of host resistance.
• Cultivation of varieties resistant to leafhoppers will reduce the risk of infection.

Management of insect vectors

• Management of insect vectors during the season will reduce rates of transmission. However, insecticide use is generally not considered to be effective at leafhopper management as the insects are widely distributed in affected areas.

Cultural control

• Management of volunteer plants during the off-season by removing infected plants or discing will reduce potential sources of inoculum.
• Planting early will avoid periods later in the season when vectors are more abundant and disease severity is high.

References

Bradbury, J.F. 1991. Spiroplasma kunkelii. IMI Descriptions of Fungi and Bacteria (No. 105) Sheet 1047. Surrey, UK: CABI Bioscience.

Carpane, P., I.G. Laguna, E. Virla, S. Paradell, L. Murua and M.P. Gimenez-Pecci. 2006. Experimental transmission of corn stunt spiroplasma present in different regions of Argentina. Maydica 51:461-8.

Chen, T.A. and C.H. Liao. 1975. Corn stunt spiroplasma: Isolation, cultivation and proof of pathogenecity. Science 188:1015-17.

CIMMYT. 2004. Maize Diseases: A guide for Field Identification. 4th Edition. Mexico, D.F.: CIMMYT.

Claflin, L.E.. 1999. Mollicutes. In Donald G. White (ed), Compendium of Corn Diseases. St. Paul, Minnesota: The American Phytopathology Society. Pp. 8-9.

Tsai, J.H. and J.W.  Miller. 1995. Corn stunt spiroplasma. Plant Pathology Circular No. 373. Florida Agriculture and Consumer Services, Division of Plant Industry.  http://www.doacs.state.fl.us/pi/enpp/pathology/pathcirc/pp373.pdf (5 September 2007).

Virla, E.G., C.G. Diaz, P. Carpane, I.G. Laguna, J. Ramallo, L. Geronimo Gomez and M.P. Gimenez-Pecci. 2004. Preliminary evaluation about corn yield losses caused by ‘Corn stunt spiroplasm’ (CSS) in Tucuman, Argentina. Boletin de Sanidad Vegetal - Plagas 30: 403-413.

Contributor: Biswanath Das