Tomato Crop Germplasm Committee Report, 1996


Tomato (Lycopersicon esculentum Mill.) ranks as the leading fresh and processed vegetable crop in the U.S. with approximately 200 thousand hectares (495,000 acres) planted for commercial production (1). World production, which exceeded 77.5 million metric tons in 1994, occupied approximately 2.85 million hectares (2).

World volume has increased approximately 10% since 1985, reflecting a substantial increase in dietary use of the tomato. Nutritionally, tomato is a significant dietary source of vitamin A and C. Furthermore, recent studies have shown the importance of lycopene, a major component of red tomatoes, to have important antioxidant properties which reduce several cancers.


Public tomato germplasm collections in the U.S. are currently held at two locations. The P.I. collection is maintained at Geneva, N.Y. This collection contains 5,318 accessions with 91.8% being L. esculentum and the rest wild species. Presently 7.3% of the collection is unavailable for distribution. In 1995 342 accessions were requested. Since 1990 an average of 643 accessions were requested per year which amounts to 12% of the collection. The Tomato Genetics Resource Center (TGRC) at Davis, California presently maintains 3209 accessions which includes 1059 wild tomato species, 963 monogenic mutants and 1187 miscellaneous genetic or cytogenetic stocks. In 1994 3501 stocks were sent in response to 256 requests from 196 investigators. The relatively greater use of accessions of the TGRC over the PI collection probably reflects the preponderance of wild species in the former which are used for disease resistance evaluation and the better characterization of the mutant and genetic stocks which can then be utilized for specific studies. The National Seed Storage Laboratory (NSSL) maintains back up stocks of approx. 95% of the TGRC stocks but only 39% of the P.I. Collection.


The present germplasm collections have been extensively used as genetic resources for tomato improvement. Resistance to 42 major diseases have been reported in present world collections and 23 of these resistances have been incorporated into adapted cultivars. New diseases or new strains of some previously known diseases appear on a regular basis and frequently the germplasm collections serve as the only source of resistance to these new pathogens.

Use of the vast genetic resources in Lycopersicon species has been limited by a number of factors including; reliance on chemical controls, a reduction in the number of public tomato breeding

programs over the last 15 years, the lack of a core collection, the lack of evaluation information, the

difficulty in introgression of traits from some wild species - most notably L. peruvianum and L. chilense, and a lack of reliable screening criteria for several traits of interest such as insect resistance and fruit flavor.

A survey of the tomato crop germplasm committee identified a series of tomato problems that could be resolved (or minimized) by genetic methods (Table 1).

(1) Vegetables, 1995 Summary; USDA, Agriculture Statistics Board; Publication Vg1-2.

(2) FAO Agriculture Yearbook, 1995.

Table 1. Tomato problems where genetic improvements would benefit U.S. production.

Type		Priority Description    	Comment 


High Verticillium wilt race 2 Bacterial canker Late blight TYLCV & other geminiviruses

Medium Powdery mildew Fruit rots Corky root rot Bacterial spot

Low Bacterial speck race 2 Spotted wilt CMV PVY Target spot Phytophthora root rot

Insects Protocols important

Medium Silverleaf whitefly Aphids Nematodes


High Cold tolerance Salinity tolerance Protocols important

Medium Heat tolerance


High Soluble solids Sugar type Flavor Need to define components

Medium Antioxidants

Low Color Peelability/dicing Pectin chemistry Nutritional content Blossom-end smoothness


A. Collection

At present the major collections of wild germplasm (USDA at Geneva, NY and TGRC at Davis, CA) contain more than 1,200 accessions of the 9 species of Lycopersicon and 4 related species of Solanum. Each Lycopersicon species is represented by large numbers of accessions, which are generally well distributed over the respective ranges. The collections are, however, deficient in certain restricted areas -- mostly in territory that is of difficult access. Collecting expeditions were sponsored by IBPGR in 1980, 1984, 1986, 1987, and 1988. Additional wild/primitive germplasm was collected in 1985 and 1995 on trips sponsored by other agencies. These forays have covered areas that were poorly represented in existing collections. Drs. Cuartero Nuez and Diaz of the "La Mayora" Experiment Station near Malaga, Spain also collected valuable populations in Peru in 1983. The TGRC assumed responsibility of increasing and maintaining the stocks collected by these expeditions.

In view of these circumstances, the status of wild tomato germplasm is considered good and vastly better than that of many other crop plants. Certain remote areas in the watershed of R o Mara on are known to harbor populations of L. peruvianum and possibly other species. Without evaluation of these populations, it is impossible to assess the need to mount expeditions into the difficult and rather inaccessible terrain of their habitats.

Another aspect of collection is the acquisition of germplasm for research projects that have terminated or are anticipated to terminate in the near future. A great wealth of tomato germplasm exists in such holdings, consisting of cultivars, breeding lines, genic and chromosomal variants, and other stocks. In the recent past it has been possible to acquire the valuable items in certain collections; unfortunately, in others, collections were discarded before useful items could be salvaged. It is suggested that the Tomato CGC survey collections that could be in jeopardy now or in the near future and request that caretakers assist the NPGS in acquiring the most valuable items. The amount of funds to be budgeted for such activities to cover costs of correspondence, publicity, and acquisition should be minor.

Of particular interest from both terminated and active programs are enhanced stocks from wild species. This is especially true for stocks from the peruvianum complex since it is so difficult to obtain hybrids and the first backcross. Breeders who introgress genes select for those of interest, but the early generation lines may contain valuable genes which were not selected for. Having such lines available for other researchers could save them years of time and a lot of work. The collection and maintenance of these stocks could be a major input to the Geneva center and may require additional funding.

B. Maintenance and Preservation

1. PI Collections:

The PI collection contains nearly 5000 accessions. About 500 of these need to be regrown because of poor germination and/or (rarely) quantity on hand. The seeds of both collections (Davis and Geneva) are maintained at 4-5oC and 30-35% relative humidity. Field increases with no pollination control seem to be suitable for L. esculentum accessions, all L. parviflorum and L. cheesmanii, and about half of the L. pimpinellifolium accessions. The other species must be protected against outcrossing by isolation or hand-pollinations in the greenhouse.

Twenty-four plants are used for regrows of the self-pollinating accessions, up to 100 for the outcrossing lines.

2. Tomato Genetics Stock Center

As mentioned, the Tomato Genetics Stock Center, Davis, CA, has 3,145 accessions fairly equally divided into: wild tomato species, monogenic mutants, and miscellaneous genetic/cytogenetic stocks.

a. Wild species. It is of no avail to acquire new, valuable accessions of tomato species unless they are maintained properly and the seeds stored under optimal conditions. Experience gained in managing various collections served to instruct how these operations can be best conducted. In respect to maintenance, an understanding of the genetic composition and natural mating system of a given accession is vitally important to proper management of increasing seed stocks. The wild species most closely related to the tomato are classified as follows in respect to their mating systems:

Autogamous: L. cheesmanii, L. esculentum, L. parviflorum


Self-compatible: L. chmielewskii, L. pimpinellifolium
Self-compatible/incompatible: L. hirsutum, L. pennellii, L. peruvianum

Allogamous (entirely self-incompatible): L. chilense, S. juglandifolium, S. lycopersicoides, S. ochranthum, S. sitians

A further breakdown is necessary within the facultative species because their subspecific groups can differ radically in their maintenance requirements. Thus, in L. pimpinellifolium the situation varies from the extremes of nearly complete autogamy at the margins of the distribution to high levels of outcrossing in the central region, intermediate situations being found in parts of the intervening territory. The extent of outcrossing very closely parallels the extent of genetic variation within accessions. L. hirsutum is an example of a facultative species in which populations of the central area are self-incompatible, hence obligatorily allogamous and highly polymorphic, whilst the northern and southern peripheries are populated by self-compatible, highly uniform colonies.

Methods of maintenance must obviously be regulated according to mating system and extent of genetic variation in order to maintain integrity of the accessions. The highly autogamous accessions (nearly always genetically uniform) need to be increased from no more individuals than necessary to produce the desired seed quantity. At the other extreme, the obligate outcrossing accessions with very high levels of genetic variability need to be increased from as many individual plants as possible.

Seed stocks are increased and placed under optimal storage conditions as soon as possible after the original accession is received because under travel/collecting conditions seeds may be exposed to conditions unfavorable for viability. The planting for increase is made by sampling seeds from each wild plant in order to obtain as much of the original variation as possible. Our general practice with this group is to crowd as many plants per container as possible without interfering with individual seed producing capacity. These accessions must be grown in the fall-winter greenhouse to prevent cross-contamination and to have appropriate day lengths for flowering. Pollen is collected by a mechanical vibrator after all plants are in flower, aliquots taken from each plant in the population. After the pollen mass is collected it is well shaken and used for hybridizing flowers of all plants. Two such pollinations at weekly intervals usually suffice to produce adequate seed supplies. When ripe, fruits are harvested from each plant and seeds extracted and thoroughly blended to provide as adequate as possible representation of the original variation of the accession in question.

b. Genic and chromosomal stocks. Maintenance procedures vary considerably between subcategories. Many items reproduce well by automatic selfing; male steriles and other sterile types must be propagated via heterozygotes; autotetraploids and many genic variants require hand pollination; trisomics must be selected from mixed populations and are mated with diploids; weak genotypes must be maintained in greenhouses and may require special feeding, grafting, or other care; and stocks of certain other accessions that produce defective seeds must be replenished frequently. In certain instances, even with optimal care, very low seed yields are obtained. Otherwise, maintenance procedures do not differ from the bulk of the tomato accessions.

C. Evaluation:

A preliminary evaluation proposal for tomato germplasm was developed by the Tomato CGC in 1979. The coordinated system of tomato germplasm evaluation that was established listed 3 objectives: (1) Obtain data on 29 high priority descriptors using a uniform evaluation system; (2) Identify suitable sites and cooperators for the evaluation of each descriptor; (3) Compile data, enter it in the database, and publicize its availability. The plan was established with the overall focus of providing information on important characters to the user community and thus increase utilization of the collections.

Little has been done on evaluation due primarily to a lack of funds. Many quantitative traits such as soluble solids, necessitate evaluation in important tomato production areas, possibly more than one, to lend credence to the results. This may be more expensive and more cumbersome to handle but would be of more value to the user community. It is also important that evaluation for disease resistances that do not have a proven artificial screening technique be carried out in regions where there is adequate disease pressure so accurate data can be obtained. Simple metric traits such as fruit color, plant habit, and pedicel type etc. could be collected in Geneva or elsewhere.

Often data are obtained by researchers on traits of interest, but reports are not given directly to the germplasm centers. Users could often provide data on traits if they were aware such traits were of interest. Efforts should be made to facilitate this by correspondence from the germplasm centers. Possibly the data sheets outlining desired characteristics could be distributed along with requested seed.

D. Enhancement

The role of the public sector in tomato improvement has declined substantially in the past two decades. At least 12 breeding positions have been eliminated. In view of the long term nature of tomato improvement efforts which entail use of germplasm collections, it would appear likely that expanded use of these collections would be encouraged by expanded funding of public germplasm enhancement efforts. Certainly enhancement efforts are needed for difficult areas such as insect resistance and salt tolerance where wild species have been known to have tolerance for years but little in the way of improved germplasm has been developed.



  1. A Specific Cooperative agreement between the USDA and the TGRC runs through 1997. The annual allocation is $45,000 and is derived from Geneva based funds. This agreement should be continued and the allocation increased as evidenced by the high usage of TGRC accessions.
  2. Evaluation. The current collections would be evaluated for traits listed in Table 1 with emphasis placed according to the priorities listed. This should include studies on research methods where appropriate.
  3. Access materials available in terminated public and private programs and add this material into the Geneva collection.


  1. Establish a core collection.
  2. Access core materials from terminated public programs while retired personnel are available to assist.
  3. Access early generation lines introgressed from the peruvianum complex in both terminated and active programs. This could be done on a worldwide basis.
  4. Coordinate stock maintenance of the TGRC, the PI collection at Geneva , and the National Seed Storage Laboratory to eliminate unneeded redundancy.
  5. Update evaluation procedures so scientists can report existing information and so any future funding will be utilized with maximum efficiency.

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Updated June 24, 1996.