Maize Streak Virus
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The maize streak virus ( English , German sometimes Maisstrichel- or -streifenvirus shortly MSV ) is the cause of the Maize Streak Disease ( MSD , German sometimes strip or Strichelkrankheit of maize ), the most destructive viral disease of corn in Africa . It is endemic to sub-Saharan Africa and is a major problem for food security there . It also occurs in Madagascar , Mauritius and Réunion and could spread to other areas.
history
The symptoms of MSD were first described in 1901 by Claude Fuller in Natal, South Africa , although he incorrectly attributed them to a disturbance of the soil. In 1924, HH Storey determined that a virus transmitted via the dwarf leafhopper, Cicadulina mbila , was the cause of the disease. Storey also laid the genetic basis of the transmission is and showed that the resistance of maize against MSD heritable is. In 1974, MSV particles were purified for the first time. In doing so, its previously unknown, twin and quasi- icosahedral shape was discovered. In 1977 it was found out that gemini viruses have a previously unknown, so-called ssDNA structure. The MSV is a type species of the genus Mastrevirus in the family of Gemini viruses ( Geminiviridae ).
Host range and symptoms
In addition to maize, over 80 other grasses are infected with MSV, including economically important species such as wheat , barley and rye .
The first symptoms appear three to seven days after inoculation . Initially, these are circular, pale spots 0.5–2 mm in diameter. Later stages show up in stripes that extend the length of the leaf and can be up to 3 mm wide. Leaves affected in this way can become almost completely chlorotic .
The most severe damage occurs when the time of infection coincides with the emergence of the coleoptile . The loss of yield can be up to 100% with early infection.
Of the nine most important strains identified so far , only MSV-A causes agricultural damage to maize. The other strains (MSV-B to MSV-I) differ from MSV-A by 5–25% in the nucleotide sequence and produce much milder symptoms (or none in MSV-resistant maize varieties).
Diversity and evolution
MSV is closely related to other African fattening viruses, e.g. B. damage sugar cane or millet . The greatest similarity, however, is with a virus of the fingergrass of Vanuatu , with which it corresponds to about 67% in the genome sequence.
The genome sequences of MSV-A show a low diversity , so that two isolates from different locations in Africa are more than 97% similar. This indicates either a low rate of evolution or a rapid spread of variants with higher fitness across the continent. Research has shown that MSV-A has a low rate of evolution, but also a high rate of mutation . Therefore, despite the slow rate of evolution, MSV-A is able to adapt quickly and overcome breeding resistance in maize.
transmission
The transmission of MSV-A via contact or seeds is not possible and relies on several dwarf cicadas of the genus Ciadulina . C. mbila is the most relevant vector because it is the most widespread and the proportion of individuals capable of transmission is greater in C. mbila than in the other species. The cicada in any of its developmental stages can ingest the virus by feeding within an hour; the minimum feeding time is 15 seconds. This is followed by a latency period of 12 to 30 hours, during which no transmission is possible. The virus is then in the vascular system of the insect and can be transmitted back to plants through feeding for a lifetime.
Importance of MSD and control
Although MSD does not occur outside of Africa, it is considered the third most important corn plant disease in the world (after Turcicum leaf drought and leaf spot disease ). Because in Africa MSD is the maize disease with the most serious consequences and, due to the central position of maize in the African diet, it is responsible for more food security problems than any other plant disease.
Using on carbamates based insecticides is an effective control of MSD in crops possible. A variation in the sowing dates can also prevent the greatest possible infestation of leaf hoppers. The problem is that these options are usually not open to smallholders . Resistance breeding therefore appears to be the most promising route for Africa . Virus resistance is related to up to five separate alleles with a mixture of recessive and dominant properties, each of which is insufficient on its own. Despite great advances in research, so far only limited success has been recorded in the field. For example, the environmental conditions in the breeding process differ from those in the field. In addition, there is great agroecological diversity in Africa, which is why a large number of varieties adapted to local conditions have to be developed in order to maximize resistance. Another problem is the fact that natural genetic resistance is often not associated with other desirable traits such as good yields. Most farmers prefer high-yielding varieties with low MSV resistance. Last but not least, the larger number of alleles involved means a breeding process lasting several years.
Efforts are currently underway to introduce resistance genes into maize varieties with the help of green genetic engineering . Genetic engineering offers the advantage of direct transfer of a single resistance gene while avoiding undesirable properties and can be incorporated into many varieties that have already been adapted to local environmental conditions. This strategy is hampered by the negative public perception of genetically modified organisms and the costly and time-consuming risk assessments to ensure safety as food and feed. A resistance was developed at the University of Cape Town in collaboration with Pannar Seed ; it is still in the test phase.
literature
- DP Martin, DN Shepherd, EP Rybicki: Maize Streak Virus. In: Brian WH Mahy, Marc HV van Regenmortel (Eds.): Desk Encyclopedia of Plant and Fungal Virology . Academic Press u. a., Oxford 2009, ISBN 978-0-12-375148-5 , pp. 209-217.
Individual evidence
- ↑ a b c d e ICTV: ICTV Taxonomy history: Maize streak virus , EC 51, Berlin, Germany, July 2019; Email ratification March 2020 (MSL # 35)
- ↑ Peter H. Raven, Ray F. Evert, Susan E. Eichhorn: Biology of plants . 4th, revised edition. Walter de Gruyter, 2006, ISBN 3-11-018531-8 , p. 291 ( limited preview in Google Book search).
- ↑ Kurt Heinze: Phytopathogenic viruses and their transmitters . Duncker & Humblot, Berlin 1959, p. 15 ( limited preview in Google Book search).
- ↑ a b c d e f g h i j k D. P. Martin, DN Shepherd, EP Rybicki: Maize Streak Virus. In: Brian WH Mahy, Marc HV van Regenmortel (Eds.): Desk Encyclopedia of Plant and Fungal Virology . Academic Press, 2009, ISBN 978-0-12-375148-5 , pp. 209-217.
- ↑ Darren P. Martin, Dionne N. Shepherd: The epidemiology, economic impact and control of maize streak disease. In: Food Security. 1 (3), pp. 305-315.
- ↑ Gunjan Sinha: GM Technology Develops in the Developing World. In: Science. 315 (5809) 2007, pp. 182-183.