This plant mentioned in "The Legend of Zhen Huan" is gradually dying out

The "cassava flour incident" is a classic scene in "The Legend of Zhen Huan":

Wen Yi: Be careful! Improper consumption of cassava flour can cause discomfort, and improper consumption of cassava can easily lead to poisoning and death.

However, don’t think of “poisoning” when you hear the word “cassava”. It is a “meritorious plant” in human history. It is widely planted in more than 100 countries and regions including Africa, America and Asia. It is drought-resistant and barren-resistant, easy to cultivate and has high yield. It is the world’s sixth largest food crop and is known as the “king of starch”. It feeds nearly 1 billion people in the world.

However, such a plant that has made great contributions to mankind is now gradually going extinct. The reason is that it has encountered a strong enemy - cassava mosaic disease. This disease has "hunted" cassava for 128 years, destroying countless cassava planting bases, and humans still have no good way to save it.

Part 1

Infection only takes 10 minutes?!

Cassava mosaic disease (CMD) is the most serious cassava disease, mainly caused by infection with geminiviruses of the genus bean mosaic virus of the family Synviridae and potato virus group X of the family Betafilioviridae. The disease was first discovered in Tanzania in 1894 and later spread widely throughout 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, and is recognized worldwide as a disease that is devastating to the cassava industry.

Since both Geminivirus and Potato Virus X can cause cassava mosaic disease, the disease also has great differences in "transmission". Among them, the cassava mosaic virus disease caused by Geminivirus is the "main force" that infects cassava and causes cassava mosaic virus disease . The virus mainly exists in the cassava vascular system and can be transmitted through various channels, such as agricultural operations, grafting, seed stems and insect vectors, which can quickly "transmit the virus". Generally, it is transmitted over long distances through the transportation and planting of infected seed stems.

In addition, Geminiviruses will "cooperate" with whiteflies to destroy cassava. In the field, whiteflies feed on diseased plants, and the virus will be contaminated in the insect's mouthparts, and then fly to healthy plants. It takes only 10 minutes at the shortest for the virus particles to enter the vascular bundle of healthy plants with the insect's saliva . From the time the whitefly acquires the virus to the completion of the first transmission, the virus is still present in the body of the whitefly, and its ability to infect can be maintained for more than 9 days, which is a specialized persistent cycle of virus transmission.

Cassava mosaic disease caused by the Potatovirus X group of the Betafilivirus family is relatively rare . The disease has only appeared in South America so far, and there have been no reports of cassava mosaic disease caused by this virus in other regions. According to research, cassava mosaic disease is generally easier to spread through sap friction inoculation, or through seeds, grafting, agricultural operations and infected seed stems. However, vector insects generally have difficulty in spreading cassava mosaic disease, which is a difference from the transmission of geminiviruses.

Part 2

30 million tons of cassava are slaughtered every year

Cassava mosaic disease is very good at "blitzkrieg" and invades very quickly. Once the seed stem is infected with the virus, if the farmer plants it without careful identification, usually about 14 days after planting, the new leaves of the plant will show symptoms of mosaic disease. The main manifestation is that the main veins or lateral veins of the cassava leaf turn green on both sides, forming mosaic symptoms of light green, yellow-green or yellow and normal green. The diseased leaves generally become smaller, and appear curled, wrinkled and other deformities. Young plants are easily infected, and in severe cases they become dwarfed, with few and small tubers or even 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 a total loss of yield.

Today, cassava mosaic disease "slaughters" nearly 30 million tons of cassava worldwide every year, destroying many cassava plantations and directly affecting the food security of more than 500 million people. In the 1990s, Uganda, which relies on cassava for its livelihood, suffered a serious outbreak of cassava mosaic virus disease, with a yield loss of up to 86%, forcing farmers in many parts of the country to give up growing cassava, and the country once suffered a serious famine. Tanzania, also in Africa, has also suffered a serious attack of mosaic disease. Thousands of farmers have been affected by the famine and can only rely on government food aid, which has even caused serious social security problems.

Many cassava-growing countries, seeing the various evil deeds of cassava mosaic virus disease in Africa that "compete with people for food", have included it in the list of pests for entry quarantine and strictly quarantine incoming plants. However, despite all the precautions, there is always a loophole. In 2015, cassava mosaic disease caused by the Sri Lankan cassava mosaic virus strain occurred in the border area between eastern Cambodia and southern Vietnam and quickly spread to surrounding areas. Three years later, the disease spread widely in many Southeast Asian countries such as Thailand, 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 rampant spread of cassava mosaic disease in Southeast Asia, cassava mosaic disease was found in many cassava plantations in Hainan in July 2018. Subsequently, the disease was found in Fujian, Yunnan, Guizhou, Guangxi and Guangdong, and it could spread and become a disaster at any time.

Part 3

Humanity has tried every possible means, but still fails

Faced with the cassava mosaic disease, which threatens the food and clothing of hundreds of millions of people, many scientists have joined the campaign to " arrest" the mosaic disease. However, various methods have been used for hundreds of years, but with little success. The main reason is that the virus that causes the mosaic disease has a simple structure and is very easy to adapt to new environments and new varieties. It often recombines with hosts and other viruses, thus mutating into different strains. There are even genetic variations between different strains. The frequency of gene recombination is high and the evolution speed is extremely fast, which makes it easy for the cultivated varieties to lose disease resistance.

Cassava mosaic virus disease has mutated and recombined to produce multiple pathogenic strains in the years of "killing potatoes", such as 9 strains of the genus Bean Golden Mosaic Virus of African Cassava Mosaic Virus, Sri Lanka Cassava Mosaic Virus and Indian Cassava Mosaic Virus, and 5 strains of the Potato X Virus Group. Many cassava varieties have basically no resistance to the invasion of different pathogenic strains and can only passively take the beating. For example, my country once sent the main cultivated varieties such as "South China 205", "South China 6068", "South China No. 9", "South China Series" and "Guire Series" to Uganda and Cambodia for planting, but they were all found to be severely attacked by mosaic disease.

In order to save cassava and give it a chance to survive, scientists have come up with a variety of rescue methods, among which the better ones are heat therapy and meristem culture. However, these methods only treat the symptoms and not the root cause. To truly defeat cassava mosaic disease, disease-resistant varieties are still needed . So some scientists used CRISPR/Cas9 gene editing technology to target African cassava mosaic virus, hoping to give cassava immunity to mosaic virus. Many people have great expectations for this method, believing that it may be possible to cultivate cassava varieties that are truly resistant to mosaic disease.

However, dreams are beautiful, but reality is cruel. The relevant experiment failed in 2019 because CRISPR/Cas9 transgenic plants failed to acquire effective resistance to geminiviruses, and the African cassava mosaic virus evolved a heritable mutant virus strain under the selection pressure of CRISPR/Cas9 gene editing. This mutant virus can escape the cutting effect of the CRISPR/Cas9 system, thereby accelerating the evolution of geminiviruses, which may bring major biosafety risks.

At this point, there is still a long way to go to save cassava, and the road ahead is still bumpy and difficult.

Part 4

Cassava rescue operation requires three-dimensional prevention and control

Since there is no good "cassava rescue" method, the current prevention and control of cassava mosaic disease mainly adopts comprehensive prevention and control measures based on quarantine, mainly including the following points:

1. Strengthen quarantine and cut off external sources of infection

There are now multiple pathogenic strains of cassava mosaic disease, and only Sri Lankan cassava mosaic virus is rampant in China, and other viruses have not yet been seen. In order to prevent more strains from invading, it is forbidden to carry or import diseased seedlings and tubers from epidemic areas. If there is a special need, the number of imports can be limited, and the exporting country must issue a certificate and the import can only be carried out after trial planting and observation in an insect-proof greenhouse.

2. Strengthen field management to reduce virus occurrence

Select healthy seedlings for planting, regularly inspect and monitor the field planting situation, and remove diseased plants in a timely manner if found; in addition, strengthen water and fertilizer management to improve the resistance of cassava plants to diseases. Pay attention to field cleaning after harvesting, and spray virus control drugs such as chitosan, copper acetate and Ningnanmycin.

3. Kill whiteflies and cut off the transmission channels

As an important "partner" in the transmission of cassava mosaic disease, strengthening the killing of whiteflies and cutting off the transmission channels can reduce the occurrence of cassava mosaic disease. Therefore, whiteflies can be controlled with pesticides such as cyantraniliprole, flonicamid and imidacloprid. In addition, whiteflies are sensitive to yellow and have a strong tendency to move towards yellow light, so they can be killed with yellow sticky insect traps.

Source: Science Institute