Temporal range: Early Eocene to Recent, 52–0 Ma
|Fresh harvest of tomatillos|
Physalis ixocarpa Brot.
The tomatillo (Physalis philadelphica), also known as the Mexican husk tomato, is a plant of the nightshade family bearing small, spherical and green or green-purple fruit of the same name.
The wild tomatillo and related plants are found everywhere in the Americas except in the far north, with the highest diversity in Mexico. In 2017, scientists reported on their discovery and analysis of a fossil tomatillo found in the Patagonian region of Argentina, dated to 52 million years B.P. The finding has pushed back the earliest appearance of the Solanaceae plant family of which the tomatillos are one genus.
Tomatillos were domesticated in Mexico before the coming of Europeans, and played an important part in the culture of the Maya and the Aztecs, more important than the tomato. The specific name philadelphica dates from the 18th century.
The tomatillo (from Nahuatl, tomatl) is also known as husk tomato, Mexican groundcherry, large-flowered tomatillo, or Mexican husk tomato. Some of these names, however, can also refer to other species in the Physalis genus. Other names are Mexican green tomato and miltomate. In Spanish, it is called tomate de cáscara, tomate de fresadilla, tomate milpero, tomate verde (green tomato), tomatillo (Mexico; this term means "little tomato" elsewhere), miltomate (Mexico, Guatemala), or simply tomate (in which case the tomato is called jitomate). Tomatillos are sometimes called "green tomatoes"; unripe tomatoes may go by the same name, though the fruit is in a different genus.
Tomatillos are native to Central America and the south-western region of North America. In Mexico, the plant is grown mostly in the states of Hidalgo and Morelos; it is also grown in the highlands of Guatemala where it is known as "miltomate".
The plant has been exported around the world. In the 1950s, it was exported to India, where it was cultivated in Rajasthan; it is grown and processed also in Queensland (Australia), Polokwane (South Africa), and Kenya. In the United States, tomatillos are grown in California and Iowa, where scientists from Iowa State University promoted a strain of tomatillos for Midwestern farmers they dubbed "jamberry". In 1952, another strain was introduced in Ohio, under the name "jumbo husk tomato".
There is limited information about tomatillo production even though tomatillo is distributed and grown worldwide as home-grown garden plant. Tomatillo is mainly cultivated on outdoor fields in Mexico and Guatemala on a large scale. Smaller crops are planted in many parts of the United States . In Mexico tomatillos are planted within a wide range of altitudes .
In general tomatillo plants are tolerant to many different soil conditions . However, they do best in well-drained, sandy, fertile soil conditions with a pH between 5.5 and 7.3 . Tomatillo plants are cold sensitive . They grow best at 25 to 32 °C. Temperatures at night should not be lower than 15 °C. Below 16 °C, growth is very poor. Tomatillo plants prefer full sun exposure and warm locations .
Transplanting is the most common practice to plant the Tomatillo plant . Transplants are produced in greenhouses or in transplant beds. Germination occurs at 20-27 °C . Transplanting occurs 6 to 8 weeks after seeding and when risk of frost is past. Transplants that were produced indoors need to harden off in a warm, sunny place for a few days before being planted outside. Direct outdoor seeding can only be done if there is no frost risk and soil temperature is > 15 °C . Direct outdoor seeding leads to shortening of the vegetation period . Due to its branching growing pattern a single plant requires enough growing space. Tomatillos are typically grown in rows 0.7 to 1.6 meters apart. In-row plant space should not be less than 40 centimeters. Although tomatillo is a perennial plant, overwintering is hard and it is therefore normally cultivated as an annual plant .
Tomatillo plants can reach heights of 1.5 to 2 meters. Due to its rapid and branching growth it is recommended to stake them. Staking also facilitates later harvesting and prevents the fruit from touching the ground, which reduces damage to fruit and husk . Staking can also reduce disease, as well as slug damages . Fertilization is recommended at a moderate level. An application of 40 – 90 kg/ha of phosphorus is common. Depending on soil type and irrigation, other nutrients and fertilzers (N/ K) may be required.  For non commercial production, regular fertilization is recommended. Even though tomatillo plants becomes more drought tolerant the older they get, regular watering is required . Tomatillo plants require 25–38 mm of water per week . Water can either come from rainfall or irrigation. Irrigation can either be managed by drip, sprinkler, furrow or watering can . Irrigation frequency is depending on weather and crop's growth stage from once or twice a week to daily during hot weather periods . Weeds are a serious challenge in tomatillo production and especially important during the first few weeks. Plastic and organic mulches help to effectively control weeds . Applications of plastic mulches also help to restrict soil water evaporation and modifying microclimate , thereby affecting tomatillo growth and yield .
Tomatillos are harvested when the fruits fill the calyx . This state is normally achieved after 65 to 100 days after transplanting. Fruit production continues for 1 to 2 months or until first frost. Harvesting occurs regularly, typically every day. Harvesting is done by hand. A single plant produces 60 to 200 fruits within a single growing season, with an average yield of about 9 tons per acre.  Tomatillos can be stored up to three weeks in a cold and humid environment .
Tomatillos are a key ingredient in fresh and cooked Mexican and Central-American green sauces. The green color and tart flavor are the main culinary contributions of the fruit. Purple and red-ripening cultivars often have a slight sweetness, unlike the green- and yellow-ripening cultivars, and are therefore generally used in jams and preserves. Like their close relatives, cape gooseberries, tomatillos have a high pectin content. Another characteristic is they tend to have a varying degree of a sappy sticky coating, mostly when used on the green side out of the husk.
Ripe tomatillos keep refrigerated for about two weeks. They keep even longer with the husks removed and the fruit refrigerated in sealed plastic bags. They may also be frozen whole or sliced.
The tomatillo can be harvested at different stages of its development. For the famous salsa verde, it is harvested early, when the fruit is rather sour and has a light taste. For a sweeter taste, it can be picked later, when the fruit is seedier. In this stage, it could be suitable as a tomato substitute.
The tomatillo flavour is different to popular european vegetables, but not very exotic, therefore the fruit could gain european mainstream acceptance but also be a possibility for new culinary experiences. It is already used in the fusion cuisine, which tries to blend flavours from Latin American dishes with those of European and North America. But the different usage possibilities, linked to different harvesting points and drying, remain largely unexploited in Europe.
Native Americans have used the fruits as a medicine against headache and stomach trouble and used it as a dressing for wounds. Recent studies have found withanolides in plant and fruit tissue which have strong anti-proliferative activity. In 2006 Choi et al. evaluated the anti-proliferative properties of withanolides extracted from tomatillos on cancer cells. They came to the conclusion that the withanolides, especially Ixocarpalactone A, may have cancer chemopreventive properties.
In 2015 Gallagher et al. achieved a 60% volume reduction in triple negative breast cancer in a mouse experiment. This indicates that Physalis longifolia could serve as a dietary supplement to fight against tumor diseases.
P. philadelphica grow up to 15 to 60 cm and have few hairs on the stem. The leaves have acute and irregularly separated dents on the side. They are typically about one meter in height, and can either be compact and upright or prostrate with a wider, less dense canopy. The leaves are typically serrated and can either be smooth or pubescent.
The tomatillo is a member of the genus Physalis, erected by Carl Linnaeus in 1753. Jean-Baptiste de Lamarck described the tomatillo under the name Physlis philadelphica in 1786. Other species like Physalis aeuata and Physalis violacea were described later. The tomatillo is also often classified as P. ixocarpa BROT. However, Physalis philadelphica is the most important species economically. The nomenclature for the Physalis changed since the 1950s. Physalis philadelphica was at one time classified as a variety of P. ixocarpa. Later the classification of P. ixocarpa was revised under the species of P. philadelphica. Today, the name P. ixocarpa is commonly used for the domestic plant and P. philadelphica for the wild one.
Flowers come in several colors including white, light green, bright yellow, and sometimes purple. Flowers may or may not have purple spots toward the center of the corolla. The anthers are typically dark purple to pale blue. Tomatillo plants are highly self-incompatible, and two or more plants are needed for proper pollination. Thus, isolated tomatillo plants rarely set fruit.
The tomatillo fruit is surrounded by an inedible, paper-like husk formed from the calyx. As the fruit matures, it fills the husk and can split it open by harvest. The husk turns brown, and the fruit can be several colors when ripe, including yellow, green, or even purple. The freshness and greenness of the husk are quality criteria.
There are several varieties of tomatillos, with a variety of tastes and traits. (They ripen to be the following colors: green, yellow, and purple.) The Pineapple tomatillo, for instance, is yellow and has hints of pineapple flavor; Rio Grande Verde is very large, and green, by comparison. Like tomatoes, tomatillos may be either indeterminate or determinate, although they are sometimes thought to be generally indeterminate. Here is a list of tomatillo varieties:
Tomatillos carry self-incompatible traits. The plant, i.e. the fertile hermaphrodite, is not able to produce zygotes after self-pollination occurrs. This limits the ability to improve tomatillo production regarding the seed quality and the production of varieties.
The self-compatibility gene is situated in the chromosomes of the tomatillo and is not inherited through cytoplasm. Only heterozygous plants can be self-compatible as the trait is controlled by a dominant gene. Tomatillo can thus produce seeds through self-pollination due to the involvement of self-compatibility traits but the germination viability is different throughout the produced seeds. This suggests that not only incompatible pollen is involved but also inviability at the seedling stage.
Tomatillo is generally a very resistant crop, as long as its climatic requirements are met. However, as with all crops, mass production brings with it exposure to pests and diseases. As of 2017, two diseases affecting tomatillo have been documented, namely tomato yellow leaf curl virus and turnip mosaic virus. Symptoms of tomato yellow leaf curl virus, including chlorotic margins and interveinal yellowing, were found in several tomato and tomatillo crops in Mexico and Guatemala in 2006. After laboratory tests, the virus was confirmed. Symptomatic plants were associated with the presence of whiteflies, which were likely the cause for this outbreak.
Turnip mosaic virus was discovered in several tomatillo crops in California in 2011, rendering 2% of commercially grown tomatillo plants unmarketable, with severe stunting and leaf distortion. The green peach aphid is a common pest in California, and since it readily transmits the turnip mosaic virus, this could be a threat to tomatillo production in California.
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