1898- Russia to the United States: Experts were predicting food shortages and famine because increasing population would overtake our ability to grow sufficient wheat by 1931. They may have been right - except 1898 was also the year U.S. Department of Agriculture special agent Mark A. Carleton was sent on his first plant exploration trip to Russia.  He brought back new durum and hard red wheat varieties to grow in the United State.  Five years after the introduction of wheat from Russia, wheat production in the United States exploded from 60,000 to 20 million bushels a year.  Not only did the drought tolerance of these new varieties open up the Great Plains and the Northwest for wheat growing, the durum wheat tasted better in pasta, and the hard red wheat made better bread.

1898- Avocados that created a California industry were brought back from Mexico.

1905-1908- " Before [Frank Meyer, a USDA agricultural explorer] went to China in 1905, only eight varieties of soybeans were grown in the United States, and these were for animal forage.  Between 1905 and 1908, Meyer added 42 new soybeans, which have parented thousands of varieties over the years. Among the soybeans that he collected was the one that gave rise to soybean oil production, and industry worth billions of dollars today...Meyer was ahead of his time in the early 1900s, when he advocated that the United States should pick up on an Asian soybean industry and begin producing a food called tofu.

1948- Jack R. Harlan and Osman Tosun collect wheat in Fakiyan Semdinli, Turkey.  'At the time the sample was taken, the wheat looked terrible - it lodged, had no winter hardiness, and was susceptible to leaf rust...It was a hopelessly useless wheat but dutifully conserved,' Harlan wrote of the wheat that was listed simply as Plant Introduction 178383.  But 15 years later, when U.S. wheat breeders were looking for a way to breed resistance to a stripe rust outbreak, PI 178383 was found to have resistance...Today, PI 178383 appears in the pedigree of virtually all the wheat grown in the Pacific Northwest.

1920- When wilt and blight became virulent and threatened to kill off the Virginia spinach industry in 1920, the genes for resistance to these diseases were found in a spinach that had been collected 20 years earlier in Manchuria by a USDA plant explorer.

1920- A spinach collected in Manchuria in 1900 saved the Virginia spinach industry from blight and disaster in 1920.

1930s- Siberian and Chinese elms are brought back from Asia.  When the drought of the 1930s began to turn the prairie states into the Dust Bowl, [the] elms formed a large part of the 17,000 miles of shelterbelts that were created to reduce wind erosion. These tree-lined windbreaks planted between 1935 and 1942 helped conserve millions of tons of soil.

1960- A wild oat found in Israel in the 1960s had genes that helped breeders develop one of the world's most disease-resistant oat varieties.
 
 

Source: "Conserving the world's plants."  Agricultural Research from a 1998 article by J. Kim Kaplan
Picture from USDA Pomological Watercolor Collection
Map from Mapquest.com