Corn stunt: addition of Spiroplasma kunkelii to the EPPO Alert List
Corn stunt was first described from the Rio Grande Valley of Texas (US) in 1945. The disease is now considered as a major limiting factor in the production of maize in Central America and parts of South America. It has periodically invaded maize fields in parts of the Southern USA, and disease incidence has increased during the last decade, in particular in California. The disease is mainly transmitted by Dalbulus maidis (Homoptera: Cicadellidae) and may correspond to a complex of different pathogens. The three major components of corn stunt are Spiroplasma kunkelii, maize bushy stunt phytoplasma and Maize rayado fino virus (Marafivirus). S. kunkelii is considered to be the most important of the three pathogens because it is more often detected and causes high levels of damage to maize crops throughout Latin America. Highest infection levels have been observed in Central America, Argentina and Peru. Considering the damage recently reported from Argentina, the Panel on Phytosanitary Measures suggested that S. kunkelii could be added to the EPPO Alert List.
Spiroplasma kunkelii (corn stunt spiroplasma)
Why: Corn stunt is considered a major disease of maize (Zea mays) in the Americas. Although it is probably caused by a complex of pathogens, Spiroplasma kunkelii seems to be the major component of this disease. S. kunkelii is transmitted by leafhoppers in the field (mainly by Dalbulus maidis, Homoptera: Cicadellidae). As the importance of corn stunt is increasing in several American countries, the EPPO Panel on Phytosanitary Measures suggested the addition of S. kunkelii to the EPPO Alert List.
Where:
EPPO region: absent.
North America: Mexico, USA (California, Louisiana, Mississippi, Texas).
Central America: El Salvador, Honduras, Jamaica, Nicaragua.
South America: Argentina, Bolivia, Brazil, Colombia, Paraguay, Peru, Venezuela.
The main insect vector, D. maidis is a subtropical species which occurs throughout the Americas in all areas where maize is grown. D. maidis is not known to occur in the EPPO region.
On which plants: The main host of S. kunkelii is maize (Zea mays), but the pathogen has also been detected in teosintes (e.g. Euchlaena mexicana, Z. perennis). The insect vector, D. maidis feeds on cultivated or wild species of the genus Zea (and occasionally on species of the closely-related genus Tripsacum).
Damage: Infected maize plants are stunted and show chlorotic stripes on the leaves. They have much shorter internodes with a proliferation of secondary shoots, thus giving the plants a short and bushy appearance. Infected plants often show reddish purple leaf margins. Ears are small and poorly filled. Symptoms may vary according to climatic conditions, maize cultivar and the presence of other pathogens. Pictures of the symptoms can be viewed on the Internet: http://www.ipmimages.org/browse/bimages.cfm?SUB=10498&area=72
S. kunkelii is reported as a major disease limiting maize production and causing economic damage in Central and South America. In recent years, its incidence has increased in many tropical and sub-tropical maize-growing areas. For example studies carried out in Argentina from 1991 to 2001, showed that the disease has largely spread across the northern part of the country, since its initial discovery in the north-east. In the Tucumán province, it was showed that S. kunkelii caused yield reductions varying from 50% to 90% (with an average of 70%) and that infected plants produced 3 times less grain (in weight). In the USA where the disease was formerly considered sporadic, corn stunt has been observed every year in the central valley of California since 1996, and in 2001, an outbreak there was reported to have caused economic losses of more than 5 million USD.
Transmission: In maize fields, S. kunkelii is transmitted by leafhoppers, mainly by D. maidis but D. elimatus has also been reported as a vector. Other species have been found capable of transmitting the pathogen in experimental conditions (D. tripsacoides, D. gelbus, D. guevarai, D. quinquenotatus, D. tripsaci, Exitianus exitiosus, Graminella nigrifons and Stirellus bicolor). Spiroplasmas overwinter within the adult leafhoppers, and when they emerge from overwintering in early spring, they can be infective. S. kunkelii is also pathogenic to its insect vectors, in particular it has been shown to shorten the longevity of D. maidis. It is important to note that S. kunkelii is not seed-transmitted.
Pathway: Over long distances, pathways for introducing S. kunkelii into the EPPO region are very limited because S. kunkelii is not seed-transmitted, its insect vectors are not likely to be associated with plants other than maize, maize is not vegetatively multiplied (only grown from seeds) and not normally traded as plants for planting.
Possible risks: Maize is a major crop in many European countries and S. kunkelii causes significant economic damage in its area of origin. Disease control is difficult in the field, and there is little information about effective methods against the insect vector or about the availability of resistant/tolerant maize varieties. Data is also lacking on the potential of establishment of the pathogen and its vector in the EPPO region, as both of them occur in tropical and subtropical regions there may be some climatic constraints in more temperate areas. No data is available on the occurrence of potential insect vectors in the EPPO region (the main vector, D. maidis does not occur in Europe). Finally, data is also lacking on the possible pathways for the introduction of the pathogen via infected plants or insect vectors into the EPPO region.
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EPPO RS 2008/012
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Panel review date: -
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Entry date 2008-01
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Sources
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