"The Cassava Flour Incident" is a classic episode in " Legend of Zhen Huan ":
Wen Yi: Pay attention! Improper consumption of cassava flour will cause discomfort, while improper consumption of cassava can easily lead to death by poisoning.
However, don't think of "poisoning" as soon as you hear "cassava". It is a "merited plant" in human history. It is widely grown in more than 100 countries or regions in Africa, America and Asia. With high cultivation and yield, it is the sixth largest food crop in the world, known as the "starch king", and feeds nearly 1 billion people in the world.
, however, is such a plant that has made great contributions to human beings, but now it is dying step by step. There is no other reason, that is, it has encountered a strong enemy - cassava mosaic disease. This disease "chased" cassava for 128 years, destroying countless cassava planting bases, and human beings still have no good rescue method.
contagion only takes 10 minutes? !
Seed stem virus infection causes cassava mosaic disease symptoms at seedling stage Image source: The website of the Ministry of Agriculture and Rural Affairs of the People's Republic of China
Cassava mosaic disease (CMD) is the most serious cassava disease, mainly caused by conjoined Caused by infection by geminiviruses of the genus Golden Mosaic Virus of the viridae family and the potato X virus group of the Betafloviridae family. The disease was first identified in Tanzania in 1894 and later spread widely across the African continent. Due to the development of world trade, cassava mosaic virus disease has now spread to dozens of countries in Africa, Asia and South America.
Because both gemini virus and potato X virus can cause cassava mosaic disease, the disease is also very different in "virus transmission". Among them, the cassava mosaic virus disease caused by gemini virus is the "main force" of cassava mosaic virus disease caused by infecting cassava. The virus mainly exists in the vascular system of cassava and can be transmitted through various ways, such as agricultural Manipulation, grafting, seed stems and insect mediators can all quickly "transmit the virus", and are generally spread over long distances through the transfer and planting of infected seed stems.
In addition, the gemini virus will "cooperate" with Bemisia tabaci to kill cassava together. In the field, Bemisia tabaci feeds on infected plants. At this time, the virus will be infected in the insect's mouthparts, and then fly to the healthy plants. It only takes 10 minutes for to enter the virus particles with the insect's saliva. into the vascular bundles of healthy plants. Bemisia tabaci is still toxic in the body from the time of acquiring the virus to the first transmission of the virus, and its infectivity can be maintained for more than 9 days, which belongs to the specialized persistent circulation mode of transmission.
and The common mosaic disease of cassava caused by the infection of the Betafloviridae potato X virus group is relatively rare , the disease only occurs in South America at present, and no common mosaic of cassava caused by this virus has occurred in other regions. disease reports. It has been found that the common mosaic disease of cassava is generally easier to spread through sap friction inoculation, or through seeds, grafting, farming operations and infected diseased stems. However, it is generally difficult for vector insects to transmit common mosaic disease of cassava, which is a difference from gemini virus in transmission.
"slaughter" 30 million tons of cassava every year Blitz ", the invasion speed is extremely fast, once the seed stems are infected with the virus, if farmers do not plant them carefully, usually about 14 days after planting, the new leaves extracted from the plants can show symptoms of mosaic disease. It is mainly manifested as chlorosis of the main veins of cassava or both sides of the veins, forming light green, yellow-green or yellow and normal green mosaic symptoms. The affected leaves generally become smaller, and deformities such as curling and shrinking appear. Young plants are easily infected, dwarfed in severe cases, and have few or no tubers, resulting in a serious reduction in yield. The average yield loss of diseased cassava fields is 30-40%, and in severe cases it can even lead to production failure.
Today, the cassava mosaic disease "slaughtered" nearly 30 million tons of cassava every year around the world, and many cassava orchards were destroyed, directly affecting the food security of more than 500 million people. In the 1990s, , Uganda, , which lived on cassava, had a severe outbreak of cassava mosaic virus disease, with a yield loss of up to 86%, which led to farmers in many parts of the country being forced to abandon cassava cultivation. severe famine. Tanzania, which is also in Africa, also suffered a severe attack from mosaic disease. Thousands of farmers were affected by famine and could only rely on government food assistance. As a result, serious social security problems even occurred.
Many cassava-growing countries have seen all kinds of vicious acts of cassava mosaic virus disease "grabbing" with people in Africa, and they have included it in the list of pests for entry quarantine, and strictly quarantine imported plants. However, every precaution, after all, is still a hundred secrets. In 2015, the cassava mosaic disease caused by the cassava mosaic virus strain in Sri Lanka occurred in the junction area of Cambodia east and Vietnam south and spread rapidly to the surrounding areas. Three years later, the disease occurred in Thailand and other Southeast Asia. It has spread widely in many countries, causing the price of cassava in Southeast Asia to double.
China listed cassava mosaic virus in the list of pests for entry quarantine in 2007. However, affected by the raging spread of cassava mosaic disease in Southeast Asia, cassava mosaic disease was found in several cassava orchards in Hainan in July 2018. Subsequently, the disease was discovered in Fujian, Yunnan, Guizhou, Guangxi, Guangdong and other places, and it may spread to disaster at any time.
Humans have tried their best, but still failed In the operation of "arresting" mosaic disease, various methods have been used for hundreds of years, but still have little effect. The main reason for is that the virus that causes mosaic disease has a simple structure, and it is easy to adapt to new environments and the replacement of new varieties. There are also genetic variations, the high frequency of genetic recombination, and the extremely rapid evolution, resulting in bred varieties that easily lose disease resistance .
Cassava mosaic virus disease has mutated and recombined in many years of "killing potatoes" to produce multiple pathogenic strains, such as African cassava mosaic virus, Sri Lankan cassava mosaic virus and Indian cassava mosaic virus golden yellow mosaic leaves 9 strains including genus virus, and 5 strains including potato X virus group. Many cassava varieties are basically incapable of resisting the invasion of different pathogenic strains, and can only be passively beaten . For example, my country once sent major varieties such as "Hua Nan 205", "Hua Nan 6068", "Hua Nan No. 9", "Hua Nan Series" and "Gui Re Series" to Uganda and Cambodia for planting, but all of them were found to be infected with mosaic disease. Severely invaded.
In order to save cassava and give cassava a way of life, scientists have come up with a variety of rescue methods, among which the better are heat therapy and meristem culture. However, these methods only treat the symptoms and not the root cause. To truly defeat the cassava mosaic disease, still needs a disease-resistant variety . Therefore, some scientists used CRISPR/Cas9 gene editing technology to target African cassava mosaic virus, and hoped to give cassava immunity to mosaic virus . Many people have great expectations for this method and believe that it will be possible to cultivate A cassava variety that is truly resistant to mosaic disease.
However, the dream is beautiful, and the reality is very skinny. Relevant experiments failed in 2019 because CRISPR/Cas9 transgenic plants failed to acquire effective geminivirus resistance, and the African cassava mosaic virus evolved a heritable genetically modified under the selective pressure of CRISPR/Cas9 gene editing. This mutant virus can escape the cleavage of the CRISPR/Cas9 system, thereby accelerating the evolution of geminiviruses, which may pose significant biosafety risks.
So far, there is still a long way to go to save cassava, and the road ahead is still bumpy and difficult.
tapiocaRescue operations require three-dimensional prevention and control of cassava mosaic disease symptoms at the adult stage Image source: website of the Ministry of Agriculture and Rural Affairs of the People's Republic of China To take comprehensive prevention and control measures mainly based on quarantine, mainly include the following points:
1. Strengthen quarantine and cut off the source of foreign infection , the rest of the virus has not yet been seen. In order to avoid the invasion of more strains, it is forbidden to carry or import diseased seedlings and tubers from the epidemic area. If there are special needs, the imported quantity can be limited, and the exporting country is required to issue a certificate, and it can be introduced after the insect-proof greenhouse trial observation.
2. Strengthen field management to reduce virus occurrence
Select healthy seedlings for planting, regularly inspect and monitor field planting conditions, and find diseased plants that need to be removed in time; in addition, strengthen water and fertilizer management to improve cassava plants' resistance to diseases, pay attention to after harvesting Field cleaning was carried out, and virus control drugs such as chitosan , copper acetate and ningnanmycin were sprayed.
3. Kill Bemisia tabaci, cut off the transmission channel
Bemisia tabaci, as an important “partner” of cassava mosaic disease, strengthen the hunt and kill Bemisia tabaci and cut off the transmission channel, which can reduce the incidence of cassava mosaic disease occur. Therefore, cyantraniliprole, flonicamid and imidacloprid can be used to control Bemisia tabaci. In addition, Bemisia tabaci is sensitive to yellow and has a strong yellow light habit, which can be trapped and killed by yellow sticky insect boards.
Reference:
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[2] Shi Tao, Wang Guofen, Li Chaoping, Lin Zhaowei, Huang Guixiu. The first report of cassava mosaic disease in China[J].Tropical Agricultural Science, 2018,38(10 ): 99-100.
[3] Shi Tao, Cai Jimiao, Huang Guixiu. The damage of cassava mosaic disease and its control measures[J].China Tropical Agriculture, 2015(05):38-40.
[4] T.Kaiser Walter, Zhang Kaiming.Thermotherapy of cassava mosaic disease[J].Tropical Crops Translation Cong,1983(05):46-48.
[5] K. K. Kartha,Zhang Yixian.Tissue of cassava resistant to mosaic disease Cultivation[J].Tropical Crops Translation Cong, 1985(05):52-54.
[6] Research on Natural Rubber and Cassava Disease Monitoring.【Knowledge Sharing】Cassava Mosaic Disease.[EB/OL ].2021.08.13
[7] iplants. CRISPR gene editing promotes the evolution of cassava virus, need to re-examine the safety of transgenic plants?
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[8] Ye Xintong. Using CRISPR/Cas9 to target Sri Lankan cassava mosaic virus to create disease-resistant cassava materials [D]. Zhejiang University, 2021. Science. 284, 1751 (1999).
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