There are several Solanaceous species with edible fruit that are popular in Latin America in addition to tomato and chilies. Physalis ixocarpa Brot.; commonly known as the husk tomato and by the Spanish names of tomate de cascara, tomate verde, tomate de fresadilla, tomatillo, and miltomate, is an important vegetable crop in the diets of Mexicans and Central Americans. In Mexico, the fruits are used in the making of chili sauce and dressings for popular dishes such as tacos and enchiladas. P. ixocarpa is gaining ground as a new crop in California due to the increased popularity of Mexican food in the United States (Quiros 1984) and has production potential in the southern United States. In Louisiana, tomatillo imported from Mexico is sold as a fresh fruit in a few grocery stores. There is a potential market for fresh produce and the Louisiana sauce industry may be interested in opening a new ethnic market for their products. Developing a new crop is a difficult and complex process. One way is to import an exotic crop from areas where the crop is already grown or consumed and adapt it to local conditions (Laidig et al. 1983). It appears that P. ixocarpa has potential as a commercial crop in Louisiana. Our interest in tomatillo is to satisfy the regional demand for fresh product and to develop a new processed product for the Louisiana canning and sauce industry
A variety of world-wide, national and local economic factors have combined to cause a reduction in the income of Louisiana farmers. Along with other areas of the country, Louisiana has begun to search for crops that would diversify its agriculture (Hamm 1985). The estimated net return per unit area of land is high in vegetables when the volume of production and sales is sufficient to amortize the required investment Hinson and Cannon (1988) consider that 350,000 to 400,000 ha are suitable for vegetable production in Louisiana. The light-textured alluvial soils along the Mississippi, Red, and Ouachita rivers are highly appropriate for vegetable crops. Climate and soil type are major factors that must be considered for vegetable production. Generally, the rainfall in Louisiana is sufficient for crop production without irrigation; however, the pattern of rain is such that some crops could be destroyed by excessive rainfall unless drainage is provided. In other areas, irrigation after transplanting would be necessary for survival of transplants. The area planted to vegetable crops has increased from 4,300 ha in 1980 to 7,300 ha in 1986. Louisiana clearly has the resources for producing vegetable crops. Both land and water are abundant, although water must be managed. A local supply of labor is available, due to the high unemployment rate (Hinson and Cannon 1988).
Louisiana has a total of 15 commercial fruit and vegetable processing plants. Six of the plants are exclusively canning factories. Eight plants produce pickled products, hot pepper sauce, and other table sauces for seasoning. The remaining processing plants produce products in bulk for further processing. Except for two plants, the processing plants in Louisiana are located in the south central section. Sauce plants are in Saint Martin parish (Acadian Pepper Co., Bruces Food Co., Cajun Chef Products Inc., and Landry Brothers), Iberia parish (Durke-French Foods Inc., McIlhenny Company East, and B.F. Trappey's Sons, Inc.), East Carroll parish (Panola Pepper Co.), and Orleans parish (Baumer Foods Inc.) (Broussard and Hinson 1988).
The total volume of table sauces, pickled, and other items processed in Louisiana is around 22,277,000 kg with an estimated value of $58,427,000. Table sauces accounted for approximately 77% of the total volume (Broussard and Hinson 1988). The increase in table sauce production may be a result of the popularity of cajun food observed across the U.S. Studies have been conducted toward developing tomatoes for processing in Louisiana, but the high summer temperatures cause flower abscision and reduce crop yield. Tomatillo plants are known to grow and set fruit at high temperatures. The Yucatan peninsula, the main area for tomatillo production in Mexico, has daily mean temperatures similar to those in Louisiana during the summers and maximum temperatures are higher in the Yucatan throughout the year than in Louisiana (Fig. 1). The increase in the popularity of Mexican food in the last decade, and the amount of Mexican sauce available in the grocery stores indicates a high potential for tomatillo as an ingredient of canned taco and enchilada sauces. Tomatillo, a main component in traditionally made taco and enchilada sauces in Mexico and Central America could be produced in Louisiana and sold in the fresh vegetable markets of the Southern U.S. or processed by Louisiana companies in sauces.
Tomatillo has been known to botanists for nearly 400 years as P. philadelphica Lam. Francisco Hernandez in 1651 described two varieties from numerous plant types called tomate by the Aztecs. Botanists have suggested that the small-fruited miltomate is a wild-type plant, whereas, the tomatillo is a domesticated plant that derives from plants similar, if not identical, to miltomate (Hudson 1986). The specific boundaries in Physalis are poorly defined with some duplication of names and many changes in the nomenclature during the last 50 years. The complexity of the genus is caused mainly by the wide range of genetic variability present presumably resulting from interspecific hybridization (Menzel 1951, 1957; Waterfall 1958) and also by the ambiguity of the earlier taxonomic descriptions (Raja-Rao 1979). For example, P. aequata Jacq. and P. capscicifolia Rydb are considered synonymous with P. ixocarpa.
To clarify the taxonomic classification of Physalis, Menzel (1951, 1957) and Waterfall (1967) made extensive cytologic and taxonomic studies of the genus. Menzel reduced P. philadelphica to synonymy under the variable P. ixocarpa Brot. a name that had to come to be widely used for the domesticated tomatillo (Hudson 1986). The only apparent difference between the two species was the length of the peduncle, with the peduncle of P. ixocarpa shorter than that of P. philadelphica. Waterfall (1958) accepted this nomenclature when studying the species of North Mexico, but he reversed himself when he analyzed Physalis spp. from Mexico and Central America (Waterfall 1967). He incorporated the small-flowered P. ixocarpa within the broader limits of P. philadelphica. Fernandes (1974) made a thorough investigation of this nomenclatural problem and concluded that P. ixocarpa is a distinct species, different from P. philadelphica based on previous cytological evidence, the distinctive sigma, and the small flowers of the type. Chromosome morphology has recently been used to understand the interspecific relationships in the genus. Gottschalk (1954), Raja-Rao (1979), Venkateswarlu and Raja-Rao (1977, 1979a, b), and Raja-Rao and Lydia-Prasad (1984) studied the morphology of chromosomes during the pachytene stage with most important Physalis spp. and demonstrated cytological differences between the species. Nevertheless, the taxonomic complexity of the genus is not yet clarified, especially between P. ixocarpa and P. philadelphica.
Tomatillo seedlings form a single shoot which has three to five internodes above the cotyledons. The last internode ends with a flower, one leaf and two lateral ramifications. Each ramification has one node which terminates in the same pattern, one end flower, one leaf and two branches. This pattern continues until senescence, "with the exception that when two leaves are formed there is no further branching (Fig. 2). One characteristic of the main branches is that the internodes differ in length and have many adventitious roots. When these roots contact soil, they grow into the soil and are independent of the main root system.
The number of fruits set is variable, but generally fruit are set until the 10th and 11th weeks after emergence. Tomatillo is similar to tomato plants, in that the biggest fruit are from the first flowers on the main branches. The lateral and sublateral branches produce more flower buds but they abscise and do not produce harvestable fruit. Mulato-Brito et al. (1985) reported that the greater total number of nodes on the lateral and sublateral branches produce more fruits than the main branches, but that those fruits rarely reach commercial size. Maximum fruit production is reached by 11 weeks after emergence. The high number of fruits present at this time compete with each other for the available nutrient supply Most of the fruit on lateral branches are dropped or they do not reach commercial size (Cartujano-Escobar et al. 1985b).
A breeding program should select for a determinant plant type suitable for mechanical harvesting of tomatillo. The elimination of sublateral branch production and reduction of internode number in lateral branches would be very important for restricting fruit production to a short period of time.
The seed germinates in 7-10 days, followed by elongation of the primary shoot for 4 weeks. The first flower bud is formed before the elongation of the primary shoot end, which is around the 3rd or 4th week after emergence, and flowering continues until senescence of the plant. The first flower appears 4 to 5 weeks after emergence and the first fruit appears one week later, reaching 3 cm in diameter at 8 weeks. The symptoms of senescence are visible after 12 weeks, with the plant reaching total senescence at 14 or 15 weeks after emergence (Fig. 3).
The growing period for tomatillo is short (3 to 4 months) and several overlapping crops could be produced in Louisiana. The only limit to plant growth is low temperatures. The growth of tomatillo is poor at temperatures of 16-18°.C or less. Plants grown in Louisiana during the hottest part of the summer produced marketable size fruits (up to 7 cm in diameter).
Mexico. Tomatillo is cultivated in Mexico from the second week of May to the middle of December, which would correspond to temperatures from mid-May to mid-October in Louisiana. It is cultivated with and without irrigation. Tomatillo in central Mexico follows the culture of sugarcane (Saccharum officinarum L.) or rice (Oryza sativa L.). These are also major crops in Louisiana. Thus, the crop rotation and climatic conditions would be similar in Louisiana. Tomatillo requires a well prepared soil, generally with furrows 25-cm deep, and intensive tillage to allow good development of the root system (Grazon-Tizanado and Garay-Alvarez 1978).
The number of harvests in Mexico varies with the plant type and quality of the product. Four to six harvests is normal. Fruit removal should begin when three or four fruit are mature on each plant which is around 55 to 70 days after transplanting. A fruit is considered mature when the berry fills the husk and in some cases breaks it. The size of husk and fruit, the color, and the flavor of the fruit is variable. The fruit can be green to yellowish-green or even purple and the flavor can range from sweet to acid sweet (Villanueva and Loya-Ramirez 1976). The criollas types of tomatillo yield about 15,000 kg/ha in Mexico. The cultivar `Rendidora' yields about 25,000 kg/ha. `Rendidora' had about 35% of the total production in Mexico with large (5 to 7 cm) size fruit and 85% of the fruit were of commercial quality (Saray-Meza et al. 1978).
Tomatillo is self-incompatible, so all plants are hybrids. Pollination is by insects. Cross pollination with other cultivars or other Physalis spp. would be possible if the plants are closer than 500 m. All seed production must be carried out in isolation. Saray-Meza et al. (1978) reported that 10 kg of fruit yields 100 to 200 g of seeds. Plant viruses can reduce tomatillo yields by 30 to 40%. Delgado-Sanchez (1986) described a complex of at least three different viruses affecting tomatillo.
Louisiana. Plants of P. ixocarpa were grown in the greenhouse in 1986 with seeds from a single fruit. Seeds were germinated in petri plates with wet filter paper. The plantlets were transferred to 7.5 cm pots and placed in the greenhouse. When plants reached 4 or 5 leaves (4 weeks), they were transplanted to the field. The field was ploughed twice at 25-30 cm deep, fertilized with 50 kg/ha 15-15-15 (NPK) and covered with black plastic mulch before transplanting. Rows were 120 cm apart with 60 cm between plants. Tomatillo plants were transplanted to the field on June 6, June 25, July 15, and August 1. Insecticide was applied at 15 day intervals. The first harvest was made after 6 weeks and harvesting continued at 10 day intervals for a total of seven harvests during the plant cycle. The estimated yield was 13,450 kg/ha. There was variation between plants in size, leaf shape, fruit size and shape, and yield. Fruit damage by lepidopterous insects was severe, probably reducing the yield by 20 to 30%. No major diseases were observed.
For micropropagation, single nodes from sterile regenerated plants were subcultured in Magenta GA7 vessels containing a medium consisting of basal MS, B5 vitamins, 0.25% Gelrite, and 10 g/liter sucrose. These plated nodes readily proliferated shoots from buds. Surface sterilized stem pieces from seed-grown plants also produced numerous shoots from buds when plated on this medium. Micropropagation will be useful in maintaining lines for hybridization for seed production and for producing large numbers of cloned plants from high yielding, horticulturally superior plants.
Tomatillo is genetically highly variable. To become a viable commercial crop it will be necessary to develop plants with uniform fruit size suitable for mechanical harvesting. Mechanical harvesting requires a determinate plant with most of the fruit maturing at about the same time. The husk should be loose at maturity and the fruit should detach easily from the pedicel. Breeding programs should give these characteristics priority
Fig. 1. Daily mean temperatures (top), and maximum and minimum temperatures (bottom) of Yucatan, Mexico (period 1931-1960) and Morgan City, Louisiana (period 1951-1980). Sources: National Climatic Data Center (1985), and Mosino and Garcia (1974).
|Fig. 2. Diagram of the tomatillo plant in full development showing fruit setting over different branches. Modified from Cartujano-Escobar et al. (1985a).|
|Fig. 3. Growth curve and phenological stages of tomatillo in Morelos Mexico. Modified from Mulato-Brito et al. (1985) and Cartujano-Escobar et al. (1985a).|
|Fig. 4 . Tomatillo fruit at maturity. The fruit break open the enveloping husk. The fruit are greenish-yellow, with a slightly sticky surface, high in solids and containing many seeds.|