Although most of the research available published in English relates only to Europe, globally, it's been estimated that a startling 75% of crop genetic diversity was lost between 1910 and 2010. It very hard to state exactly why individual varieties were lost, however, so how much is due to the world wars is really up for debate.

Part of the problem is naming; a variety grown by just a handful of people might not have a name at all, so there may well be no record of it being grown or lost, and any name given might be only used locally- the same name might mean something completely a few towns over. Although the first known plant variety name in England at last dates back to 1699 (the Cupani sweet pea) it wasn't until most people were buying their seed instead of saving it or swapping within the local area that variety names really began to standardise. Now countries maintain databases of varieties (as breeder protection, and to make sure growers aren't sold magic beans), but the names we know many old varieties by might not have been the ones they were known by 100 years ago. In fact, for many crops, it would be inaccurate to describe what was grown as a variety- instead they were what is known as a landrace; a genetically diverse mixture, defined as much by the area they were grown in as their appearance or taste. A landrace will produce a crop in almost any conditions, often with little or no input of fertiliser, but they never produce a large crop- useful for those who need to ensure they produce enough grain for the family to survive, not so much for those hoping to sell the crop. Modern uniform crops, by contrast, can produce a far larger crop under ideal circumstances, but also risk complete failure in adverse weather.

Due to the lack of definition of a variety and especially a landrace, although it's extremely likely that many of them were lost directly, due to movement, death of people, and individual farms, villages and towns being destroyed, it's hard to tie down examples. It's often hard to even determine when they have actually been lost. Is the bean you bought today really the same variety that was grown 200 years ago, or has it gradually been selected into something else? Was it even the same one that had the old name in the first place? We often have no good descriptions, and no way to tell for sure. And for a landrace it's even harder; is it lost when 20% of the diversity is lost? 50%? 100%?

Regardless; there were definitely multiple factors leading to variety loss due to the world wars, but not necessarily in ways you might expect. One of the biggest factors was the production of ammonia on an industrial scale, initially for explosives, but production continued post-war, adopted for fertiliser use. This availability of cheap fertiliser, combined with a reduced workforce and the need to feed a population as efficiently as possible when imports were difficult and expensive, led to widespread use of artificial fertiliser, and the adoption of specific varieties which responded well to these, resulting in many older varieties being quietly abandoned. DDT also came into widespread use during WWII for treating human lice; although far from safe, DDT is much less dangerous to humans than the expensive arsenic and lead based insecticides available earlier. Insect damage was a huge problem for monocultures before the development of this. The combination of cheap fertiliser and pesticides completely changed the way crops were grown, a smaller number of farmers grew a smaller range of crops for sale. These crops were generally not those passed down and saved, they were bought in, chosen for high yield response to the new conditions.

There were more niche impacts as well; in Victorian England the upper classes would often have a large estate, with a gardening team. As both a practical food source and a status symbol, the team would be responsible for providing food and decoration for the house. To demonstrate the skill involved the team could produce a huge range of complicated to grow plants - pineapples being the classic example, but tropical flowers and other crops as well. The death of so many younger men meant labour costs rose, and it was no longer practical for many wealthy families to keep on a gardening team- food was bought in instead. Many higher labour specialist varieties went along with them. We have better records of some of these niche decorative plants or exotic fruits than we do for those grown by poor farmers, unsurprisingly.

All in all, we know that the way crops were grown changed drastically over the world war period, and we know that many of the factors leading to this were due, directly or indirectly, to the wars, but it's rarely possible to nail down exactly what led to the loss of an individual variety.