Tomato plants possessing a high level of resistance to the carmine spider mite, Tetranychus cinnabarinus, can be identified in segregating populations by selecting those with the greatest concentration of glandular hairs on their leaves. The glandular hairs were counted by using a stereoscopic microscope, or estimated with the naked eye when the plants were 8 to 10 weeks old. In 18 of 21 F2 populations studied, plants selected for their high concentration of glandular hairs had higher levels of resistance to mites, as determined by ovi-position rate, than did plants selected for having few hairs. The average number of eggs laid per female mite ranged from 6.2 to 50.5% less on the plants selected for a high concentration of hairs than on those with few hairs. Selection with the naked eye was as effective in identifying resistant segregates as was actual glandular hair counts.
The National Plant Germplasm System (NPGS) is responsible for the acquisition, maintenance, evaluation, and distribution of genetic diversity of crop plants important to U.S. agriculture. The NPGS collections currently include more than 425,000 accessions representing more than 8000 species. The curators of the individual active collections face many challenges, including preserving the maximum amount of genetic diversity in active collections, encouraging the use of the germplasm in the collections, and operating with limited resources. During the past 5 years, the NPGS curators and the 40 Germplasm Committees have been evaluating how core subsets can help in meeting these challenges. A set of general guidelines and procedures for developing core subsets has been developed.
U. S. agriculture has an unmatched record for efficiently producing an abundance of high-quality, modestly priced food. At the 1971 ASHS meeting, and in subsequent articles, Wadleigh eloquently described the impact of agricultural research on the production of vegetables and other horticultural crops since 1920 (35, 36). Scientists of many disciplines have contributed to this success story, but much of the credit goes to plant breeders for the development of improved cultivars.
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.
Insects inflict over $185 million in losses to vegetables annually in the United States (Table 1). An additional $100 million or more is spent controlling vegetable insects. Totaled the losses and control costs amount to approximately 18% of the value of vegetables grown in the U.S.
Abscisic acid (ABA) levels in seeds of tomato (Lycopersicon esculentum Mill.) fell about 10-fold during fermentation to remove mucilaginous tissue. Imbibing seeds in 20 µg/ml ABA prevented germination and increased ABA content of the seed 15-fold. Subsequent germination in water averaged greater than 90%.
The United States' National Plant Germplasm System (NPGS) is responsible for the acquisition, preservation, evaluation and distribution of plant genetic resources in the form of seed and clonal germplasm. In order to operate more effectively, the NPGS established a network of 40 Crop Advisory Committees (CACs) to provide analysis, data, and advice about germplasm within a crop or group of related crops of current or future economic importance. CACs are composed of Federal, State and industry scientists representing a variety of agricultural disciplines and geographic areas of importance to the crop. The committees are involved in a variety of activities including: 1) Developing crop descriptors for the collection of standardized characteristic and evaluation data, 2) Determining priorities for germplasm acquisition, evaluation and enhancement, 3) Advising curators on maintenance techniques, and 4) Developing special reports on the status of genetic resources for their crop(s). Twenty-four of the CACs are concerned with horticultural crops.
Tomatoes can be screened for resistance to Drosophila by using a slit-fruit technique, or by trapping adult flies in jars containing sections of fruit of different varieties. These techniques can be used on a turntable to eliminate light effects, or by randomization in a greenhouse or an enclosed room with uniform light. Significant differences were found, both when Drosophila could choose between varieties and when they were restricted to cages containing fruit of only one variety. Due to the large amount of variation encountered when screening tomatoes for Drosophila resistance, large numbers of replications are necessary. The variation can be partially accounted for by differences in fruit maturity and differences between plants of a variety or line.
The leachate from seed of tomato (Lycopersicon esculentum Mill., PI 341984), an accession whose seeds germinate well at low temperatures, promoted the germination of seeds of the same and other tomato cultivars. The leachate from ‘Red Rock’ seed, a cultivar whose seeds germinate poorly at low temperatures, inhibited the germination of seeds of the same and other tomato cultivars. The promotive and inhibitory effects of these leachates on seed germination is apparently highly specific and restricted to tomato seeds. The activity was highest in fresh seeds and the responses were best exhibited at low germination temperatures.
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.