Maize Streak Virus

The maize streak virus (English, German sometimes Maisstrichel or strips virus, short MSV ) is the cause of maize streak disease ( MSD, German sometimes strip or Strichelkrankheit of corn), the most devastating virus disease of maize in Africa. It is endemic in sub-Saharan Africa and is there a big problem for food security dar. In addition, it appears Madagascar, Mauritius and Réunion to 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 erroneously attributed to a disturbance of the soil. In 1924, HH Storey found that a virus that is transmitted through the Zwergzikadenart Cicadulina Mbila, the cause of the disease. Storey also explained the genetic basis of transmission and showed that the resistance of maize against the MSD is heritable. 1974 MSV particles were first purified. Yet his hitherto unknown, twins and quasi- icosahedral form was discovered. In 1977, it concluded that geminiviruses possess a previously unknown, known ssDNA structure. The MSV belongs to the genus Mastrevirus in the family of geminiviruses.

Host range and symptoms

In addition to corn over 80 other grasses are infected with MSV, which include economically important species such as wheat, barley and rye.

Initial symptoms occur three to seven days after inoculation. First, it involves circular, pale spots with 0.5-2 mm diameter. Show later stages in strips extending along the length of the sheet and can be up to 3 mm wide. Such infested leaves can be almost completely chlorotic.

The most severe damage occurs when the time of infection is accompanied by the emergence of the coleoptile. The yield losses can be up to 100 % in early infection.

Of the previously identified nine major tribes only MSV -A caused agricultural damage in maize. The other strains ( MSV -B to MSV -I) differ from MSV -A to 5-25 % in the nucleotide sequence and produce much milder symptoms (or none at MSV -resistant maize varieties ).

Diversity and Evolution

MSV is closely related to other African mastreviruses, for example, Cane or Rispenhirsen damage. The greatest similarity is, however, a virus of finger millet from Vanuatu, with which it corresponds to about 67 % in the genome sequence.

The genome sequences of MSV -A have low diversity, so that two similar isolates from different places in Africa to more than 97%. This suggests either suggests a low rate of evolution or the rapid spread of variants with higher fitness over the continent. Research has shown that MSV -A low rate of evolution, but has a high mutation rate at the same time. Therefore, MSV -A is capable of despite the slow rate of evolution to adapt quickly and to overcome resistance breeding in maize.

Transmission

The transmission of MSV -A via contact or seed is not possible and rely on several leafhoppers of the genus Ciadulina. C. Mbila is the most relevant intermediaries, as it is the most widespread and the share transfer capable individuals of C. Mbila is greater than in the other species. The cicada may include in each of its stages the virus by feeding within one hour; the minimum feeding time is 15 seconds. This is followed by a latency time of 12 to 30 hours, during which no transmission is possible. Thereafter, the virus in the vascular routes of the insect and can be transferred for a lifetime by feeding back to plants.

Importance of MSD and combat

Although MSD does not occur outside of Africa, it is considered the third most important plant disease of corn in the world ( after Turcicum leaf blotch and leaf spot disease). In Africa, however, MSD is the most serious disease of maize, and the central position of maize in the African diet is responsible for more problems of food security than any other plant disease.

Use of insecticides based on carbamates is possible an effective control of MSD in crops. Also can prevent the maximum cicada infestation varying the sowing dates. The problem is that small farmers are not open these options usually. Therefore, the most promising way for Africa resistance breeding appears. The virus resistance is associated with up to five separate alleles with a mixture of recessive and dominant properties, seen separately, are not sufficient. Despite major advances in research have so far had limited success in the field recorded. For example, different environmental conditions in the breeding process from those in the field. In addition, a large agro-ecological diversity in Africa, which is why many are adapted to local conditions varieties need to be developed to maximize the resistance. Another problem is the fact that natural genetic resistance often associated with other desirable properties such as good yields. Most farmers prefer high-yielding varieties with low MSV resistance. Last but not least is the greater number of involved alleles a multi-year breeding process.

Currently running efforts to bring the help of green genetic engineering resistance genes in maize. Genetic engineering offers the advantage of direct transfer of a single resistance gene while avoiding undesirable properties, and can be incorporated into many already adapted to local environmental conditions varieties. This strategy is hampered by the negative perception of genetically modified organisms in the public as well as the costly and time -intensive risk assessments in order to ensure the safety as food and feed. At the University of Cape Town resistance was developed in collaboration with Pannar Seed; it is still in the testing phase.