The latter part of the 19th and the first several decades of the 20th century can be described as a “golden age” for plant exploration and collecting. During the initial years of this period, agricultural scientists from the United States and elsewhere devoted considerable resources to collecting potential new crops for farmers as well as superior plants or cultivars of the species that farmers were already growing. Over time, there was a shift toward collecting unadapted germplasm, or raw material that possessed traits that plant breeders and other scientists could use for cultivar improvement and other types of research. Although many institutions and individuals were involved in plant collecting during this period, the creation of the U.S. Department of Agriculture (USDA) Office of Seed and Plant Introduction in 1898, resulted in the largest single program devoted to plant exploration. This office employed many individuals, including David Fairchild, P.H. Dorsett, Frank Meyer, Walter Swingle, and Wilson Popenoe. These and many other individuals collected—and introduced into the United States—seeds and plants of thousands of fruits, vegetables, nuts, ornamentals, cereals, forages, oilseeds, and other types of crops. Although the mission of most of the plant explorations during this period was to collect any plants that appeared interesting or potentially useful, others focused on collecting targeted species. Much of the material collected during this era is still maintained by the U.S. National Plant Germplasm System (NPGS), and much more of it shows up in the pedigrees of cultivars grown by farmers and gardeners today. In addition to collecting plants for immediate and future use, scientists of this era, such as Nicolai I. Vavilov and Jack Harlan, contributed greatly to the understanding of the evolution of plants and plant genetic diversity, and the interdependence of plants and civilization.

Leaf disks of tomato, Lycopersicon esculentum Mill, and L. hirsutum C. H. Mull, subjected to feeding by the Colorado potato beetle, Leptinotarsa decemlineata (Say), were altered by insect age, sex, moisture level, and plant age. Feeding response was altered by temperatures of 21.1°C or 26.7° in young adult beetles, but not in larvae or older adults. Resistance to feeding by the insect was found with L. hirsutum lines PI 134417 and PI 134418.

When cultivars and accessions of the cultivated tomato, Lycopersicon esculentum Mill., and related species, were evaluated for resistance to a leaf miner, Liriomyza munda Frick, in greenhouse and field cage tests, the screening tests revealed several lines of L. esculentum having genes for adult nonpreference or larval antibiosis or both. All accessions of L. hirsutum Humb. & Bonpl. and L. hirsutum f. glabratum C. H. Mull, were virtually immune to attack in both greenhouse and field tests. Lycopersicon pimpinellifolium (Jusl.) Mill, demonstrated a considerable level of antibiosis in the greenhouse, but was not promising in the field. All accessions of L. peruvianum var. dentatum Dun. and L. glandulosum C. H. Mull, tested were susceptible. The commercial cv. VF 145B was nonpreferred in the field.