Would you possibly clear this issue?

[Cyrus the Great]

; and the spread of monoculture and use of high-yielding varieties of crops have been accompanied by the disappearance of old varieties of food plants which might have provided some insurance against pests or diseases in future.

Would anyone kindly explain me readily or simply the following parts?

Why do food plants have provided some insurance against pests or diseases in future?

In addition, I can not the meaning of this one: use of high-yielding varieties of crops have been accompanied by the disappearance of old varieties of food plants

..............

Source:

www.teachers.cambridgeesol.org/ts/digitalAssets/115016_Academic_Reading_sample_task_-_Matching_headings_2.pdf

Many thanks

MEAMAR

[Acme]

As farmers switch to growing new varieties they stop growing old varieties and so they stop collecting seeds of old varieties and so there is no seed stock for the old varieties. Without the old varieties (often called 'heirloom' varieties) there is a potential loss to get back to the original plants and so what was valuable about them to begin with. Part of that value is the food aspect, part the genetic qualities that 'naturally' evolved.

[chadn737]

Landraces, which are local varieties, have been adapted to the regions of their origins and so often possess genetic diversity that may offer desirable traits such as disease resistance. However, most elite varieties planted by farmers today come from a handful of founder lines, so there is reduced genetic diversity in the gene pool. Most landraces are not as high-yielding, however, there is growing interest in using them for breeding unique traits into the elite varieties.

[CharonY]

To add to the insurance part, higher genetic diversities protects from diseases as some may be more resistant against them. However if every plant is genetically the same, then if e.g. a virus can infect one plant, all plants are in danger.

[overtone]

Three examples:

 

The original domesticators and cultivators of the potato in its South American homeland planted a wide variety of types. When the Conquistadors brought them back to Europe they left most of the varieties behind, and brought only the largest and sweetest and most productive and most easily grown kinds. In consequence, the Irish came to depend on a single basic type of potato as the dietary staple of eight million people. It turned out that this type was vulnerable to a certain fungus, which was endemic in the South American highlands but did not do great harm because it only afflicted some potato types some of the time. In Ireland, when it got there and the weather turned favorable for it, it killed the entire potato crop for three years running. Millions of people starved, millions fled the country in desperation, the population of the entire country crashed.

 

The original domesticators and cultivators of maize in its Mexican homeland planted a wide variety of types. In the US in the 1970s a new kind of high-yield hybrid field maize was developed from crossing a couple of strains - it took over the market, and soon 75% of the US maize crop was some minor variation on this one hybrid. It turned out that this type was vulnerable to a certain fungus, which was endemic to the region in Mexico but did not do great harm because it only afflicted some maize types some of the time. In the US, when it got there and the weather turned favorable for it, it came close to wiping out 2/3 of the US maize crop. Only emergency measures and quick action by alert agricultural experts at the publicly funded universities saved the US from serious trouble - that and luck, as it turned out some farsighted agricultural scientists had in their large research seedbanks included a couple of varieties that were resistant to the fungus, and rapid crossbreeding via modern scientific methods bred that resistance into the commercial seed.

 

The original domesticators and cultivators of rubber trees in their Amazon homeland tapped and tended a wide variety of types. When the Europeans commercialized the crop in South America, they chose for their plantations a single high yielding and easily tended type. When this valuable plant was transported to European botanical gardens, and from there to suitable locations in colonial provinces where there was cheap labor and land, the transported plants were a small selection of the already genetically narrow high yielding plantation trees rather than a selection from the many wild types. Currently almost all the rubber trees grown anywhere on earth - including the massive acreages of Chinese bred and refined high yield trees planted in the regions bordering China - are of this narrow selection from this one type. It turns out this type is vulnerable to a certain fungus - it has pretty much destroyed the monocultural rubber plantations in South America, where it is endemic but does little harm to the traditional semi-domesticated forest trees of the many types, most of them resistant.

 

Nobody knows when this fungus will get to SE Asia. When it does, it is capable of killing every single rubber tree on the Asian subcontinent and Indonesian archipelago within a couple of years. Unlike corn, or potatoes, rubber trees take a few years to grow, a few years to breed. There is currently no reasonably priced substitute for natural rubber in many important uses, such as landing gear for large airplanes. So cross your fingers.