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Vegetables
 
                   
 
 

The history

Interesting fact
Sowing and harvesting

Popular carrot cultivars in New Zealand

Pests and Diseases
Pests

Carrot rust fly

Diseases
Field root knot nematode
Leaf blights or spots of carrot
Recipe
 

Carrots

 

Carrots (Daucus carota ) belong to the Umbelliferae family of plants along with parsley, celery and dill. They are biennial plants (live for 2 years), producing and storing sugars during the first year of growth in single large roots called taproots and then using the stored sugars to produce flowers and seeds in the second year. However, flowers and seeds are rarely observed in home gardens because carrots are usually harvested after their first year of growth.

The history of carrots

Carrots are root vegetables that have probably been used for thousands of years, especially as medicinal herbs. They were most likely first grown by farmers in the region of modern day Afghanistan, but by the 13 th and 14 th centuries AD, their popularity had spread across the plains of Asia right through to the eastern Mediterranean, and then towards central and northern Europe. The earliest carrots were generally purple or yellow, and varied in taste and size.

Almost 2000 years later, purple carrots can still be found in some parts of the world, although they have been completely overshadowed by the familiar orange carrot. However, Food Science students at Massey University are currently promoting purple carrots to growers and supermarkets throughout New Zealand .

The origin and history of the orange carrot is a mystery. Dutch paintings of 16th century kitchens show long purple and yellow carrots with the gradual appearance of yellow, orange and intermediate colours in later years. Interestingly, it was not until the 1700s that orange carrots were first grown in the Netherlands and white carrots in Europe. There is also evidence that orange and white carrots first appeared as a result of selection from yellow ones or hybridisation with wild carrots.

Orange carrots gradually, became dominant. They have a wide range of shapes and sizes, and reach maturity at different rates. However, there is now a move back to breeding a range of coloured carrots from purple to red, yellow and creamy white, all with the same 'carroty' aroma and flavour. The Vegetable Research Station at Wellesbourne in England holds a large selection of these coloured carrots for use in breeding. Breeders in Vilmorin, France, are producing some of the best breeds. Interestingly, Vilmorin is named after French breeder, Henri Vilmorin, who is credited for turning wild 'branched' carrots that looked like Queen Anne's lace into the single rooted orange carrots we see today.

Interesting fact
The largest carrot ever recorded was grown by John Evans in the United States. In 1998, he grew a carrot weighing in at 8.61 kg. It was measured and weighed at the 1998 Alaska State Fair and is listed in the 2003 Guinness Book of World Records.

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Sowing and harvesting

Carrots can be sown year round throughout much of New Zealand. However, research shows that July to March in temperate zones, and August to February in colder zones are usually the most reliable periods for sowing carrots. Late autumn or winter sowing may cause plants to go to seed without forming roots. Scientists have determined that carrots usually grow best on deep, well drained, aerated, sandy loam soils with good structure. A well-maintained level of soil organic matter is also important. An important factor when sowing carrots is to avoid compact layers of stones or lumps, which restrict root penetration. Bulky organic manures should only be used if they can be incorporated evenly rather than buried in thick layers; otherwise branched carrot roots are produced..

Fine compost can be hoed into the soil when preparing seedbeds for carrots. Adding a handful of superphosphate to the soil before cultivating will also provide an additional boost for the young carrot seedlings. Keeping the soil moist during hot weather with light irrigation can prevent seedling burn-off.

The desirable soil pH for many vegetables is about 6.5, and this is also ideal for carrots. Weeding between carrots is difficult so it is best to prepare the seedbed a few weeks ahead of sowing to give weeds a chance to germinate before hoeing them out. It is also a good idea to follow the instructions on seed packets for tips on sowing and thinning.

Carrots do not need a lot of nitrogen because excessive amounts promote unnecessary leaf growth. A light dressing of a complete and balanced fertiliser is adequate. However, nitrogen and sulphur deficiency can cause pale yellow leaves.

Too much sunlight on exposed crowns can cause the tops of carrot taproots to turn green so it is important that root crowns are covered with soil until the carrots are harvested. As a side note, harvested carrots that are stored over winter may develop fine roots and go to seed in early spring. To avoid this, store carrots in boxes filled with moist sand. As a general rule of thumb, use carrots damaged by carrot flies first.

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Popular carrot cultivars in New Zealand

Short-root cultivars: grow well in shallow soils and suit for home gardening.

  • King Chantenay and Early Chantenay - ideal for home gardening, excellent colour.
  • Early Horn - matures early, sweet and, pale.
  • Express Hybrid - cylindrical, excellent colour.
  • Manchester Table - can be sown in winter, cylindrical, excellent colour.
  • Tom Thumb - slightly elongated with ball-shaped roots, excellent colour.


Long-root cultivars:

  • Top Weight - can be sown in winter
  • Egmont Gold

Baby carrot - matures early, suitable for sowing in late autumn, and can be grown in pots.

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Pests and Diseases

The most common pests are carrot aphids, carrot rust flies, and field root-knot nematodes. The most common disease is leaf blight or spots of carrots.

These are described in detail below followed by information on Integrated Garden Management (IGM) techniques, which are considered best practice control methods that benefit the environment and reduce risks to human health.

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Pests
Carrot aphid ( Cavariella aegopodii )

Characteristics:
Aphids ('plant lice') are introduced insects. Over 120 species of aphids are spread throughout New Zealand, although not all of them are pests. However, aphids are small, approximately 1-5 mm long, pear-shaped, and soft-bodied. They are slow moving insects with long antennae and legs, and are usually found in large numbers or colonies on the undersides of leaves or on stems, feeding on foliage in summer. Carrot plants, like many others, can tolerate moderate levels of aphid infestation. However, some forms of aphids carry plant diseases that affect carrot plants such as the motley dwarf virus. The source of this virus is believed to be wild carrots and related plants such as parsnips. Aphids transmit viruses from these plants to carrot crops.

The carrot aphid also infests parsnips and willows ( Salix species) and is considered to be a major pest in the warmer parts of New Zealand, especially in the North Island, Canterbury and Marlborough. Numbers are usually low during January, February, May and October, moderate in March and December, and highest in April and November.

A colony of carrot aphids consists entirely of females. They can be either wingless (apterous) or winged (alate). The first aphids to arrive and colonise the carrot plants are usually winged females, but eventually wingless female aphids become the dominant type. Winged females have black heads, dusky green thoraxes (middle part of the body), and dark patches on their abdomens ( rear section of the body ) , while wingless females have yellowish-green heads and abdomens.

Aphids feed on their host plants by extracting sap from the host plant tissue using their needle-like mouthparts. They also secrete excess sap as honeydew, a sweet, sticky, shiny substance, which is basically sap that is ingested by the insects and passed through their bodies. Honeydew coats the surfaces of plants but is not usually obvious until there is a high level of aphid infestation. In addition, honeydew attracts fungi, which grow on it. The fungi tend to darken the honeydew, making affected plants look dirty. The fungus does not damage the affected plants but may interfere with photosynthesis when the aphid infestation is at its peak.

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Life cycles:
Most species of aphids survive winter as eggs. During autumn, male and female aphids fly to their over-wintering hosts, i.e. willow plants, to mate and lay eggs that stay dormant over winter. The eggs hatch in spring, coinciding with the emergence of spring plant foliage, to produce a new generation of aphids. As aphid colonies become over crowded, winged female aphids begin to reproduce parthenogenetically (reproduction without fertilisation). They then migrate to new host plants (usually carrots and parsnips) to begin new colonies. Whenever an aphid colony becomes overcrowded on a particular host plant, winged females develop and the migration cycle continues throughout spring and summer. In autumn, at the end of the growing season when stressful environmental conditions threaten their survival, winged males and egg bearing females develop and once again fly to willows to mate and lay eggs, thus continuing the life cycle.

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Plant damage:
Damage occurs when large number of aphids feed on a single plant, causing yellowing, wilting, leaf curling/galling and puckering. New growth is distorted and shoot growth stunted. In serious cases, plants wither and eventually die. Aphid saliva injected into plants while feeding contains toxins that cause leaf foliage to curl. Additionally, by transmitting the motley dwarf virus, aphids can stunt carrot growth at an early stage, causing leaf stalks to twist and foliage to turn yellow or red. Nitrogen and phosphorus deficiency of carrots causes similar symptoms to those of motley dwarf virus. (The western celery mosaic virus, which distorts leaves and stunts celery plants, is also thought to be transmitted by aphids.)

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IGM considerations for controlling the carrot aphid:

 

  • Inspect regularly for aphids and signs of damage because they vary each year in severity. However, plants can withstand low to moderate populations of aphids, and the best way to reduce plant vulnerability and the aphid population in your home garden to a tolerable level is to focus on maintaining plant heath, and to use physical and biological controls as needed. As a good rule of thumb, if leaves curl, shoot growth is stunted or leaves yellow on 25% of the plant's surface it is time to take action. If the damage is greater than 50% or more, then the best time to act has past because the damage is too great and control is difficult.
  • Plant wisely. Avoid planting cultivars that are known to attract aphids, such as spruce trees, parsnips, willows and wild carrots.
  • Use aphid-tolerant cultivars such as Top Weight and Red Heart.
  • Sow late in the season (autumn or winter), not in early spring, to avoid aphid migration flights.
  • Conserve natural predators, such as ladybirds, spiders, syrphid fly larvae, and hover fly larvae, that can help control aphid populations during summer. The very familiar red two-spotted ladybird ( Adalia bipunctata ) and orange eleven-spotted ladybird ( Coccinella undecimpunctata ) are the most common ladybirds in New Zealand. Both the larvae and adult beetles of these insects eat around 50 aphids each day. The larvae of the lacewing, the most common being the brown Tasmanian lacewing ( Micromus tasmaniae ), prey on aphids whereas the adults feed on honeydew. Repeated application of broad-spectrum pesticides kills beneficial insects as well as pests. But since aphids recover faster than beneficial insects after insecticide applications, an imbalance occurs that may worsen the problem. Only use pesticides if they are absolutely necessary.
  • Later in the season other forms of enemies such as parasites (e.g. Hymenopterous braconid wasps) and pathogens usually control aphids.
  • Use compost, composted manures, fish emulsion and seaweed in liquid form, or urea-based fertilisers that slowly release nitrogen instead of highly soluble nitrogen fertilisers. Fertilise by dividing the recommended dose of fertiliser into smaller, frequent doses (e.g. a monthly dose split into quarters and used weekly), because aphids tend to reproduce more rapidly on plants with increased levels of nitrogen. Using slow releasing fertilisers, such as seaweed, can help plants build up resistance to fight off viral attacks.
  • Physically remove aphids by wiping, clipping, pruning or washing plants with a strong stream of water (when washing it is better to wash the plant in the morning to reduce the risk of fungal disease). After clipping or pruning, put the infested plant parts in a bucket of soapy water or burn them to kill the aphids.
  • Use row covers.
  • Spot treat or spray the whole plant with soapy water or mix commercial vegetable cooking oil (1 cup) such as sunflower, coconut and soybean with common liquid dishwashing detergents (1 tablespoon), and dilute two teaspoons in one cup of water. Test a small part on the plant first to ensure the mixture is not damaging.
  • Use insecticides; drench the foliage with spray when a threshold of 25% plant damage is exceeded.
  • Grow plants that deter aphides such as nasturtiums, which are orange and produce a mustard oil secretion that aphids dislike.
  • Grow companion plants, such as chives, which improve the flavour and growth of carrots but deter aphids.
  • Pull out and burn virus-infected plants.

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Carrot rust fly ( Psila rosae)

Characteristics:
The carrot rust fly (CRF) poses a serious threat to carrots throughout the year. This pest was first observed in Auckland but is now abundant in warmer, carrot-growing districts of the North Island. CRF can also be found in the Bay of Plenty, Poverty Bay, Hawke's Bay, Waikato, Manawatu, Otago, and Southland. Besides carrots, this fly attacks other host plants such as parsnips, turnips, celery, parsley, and the weed hemlock. Normally an adult carrot fly is less than 5-8 mm long, with a shiny black body, straw-yellow legs and large red eyes. The fly is a weak flier and has difficulty flourishing in windswept environments. They fly close to the ground and seek refuge in hedges. Their eggs are white, and about 0.5 mm long, while their young (larvae/maggots) are initially colourless, wedge-shaped and legless. The young eventually turn yellowish-white. Maggots mature through three growing stages (instars), to reach an average length of 8 mm. Pupae develop into flies within shiny yellowish-brown, slender, cigar-shaped, 4.5-6 mm long pupariums (toughened larval skin). Pupae can be observed in soil around taproots.

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Life cycle:
Several generations of carrot fly develop each year. The first generation female flies lay eggs from September until October, singly or in clusters, just below the soil surface or in soil cracks near young carrots or host plants. These eggs hatch within 1-2 weeks and the larvae begin feeding on both young and mature carrot plant roots for 4-6 weeks until they mature. The first instar larvae feed on taproots or on side roots. After completing all three instars the mature larvae move back into the soil to pupate and a second generation of adults emerges by late December. Winter is spent as larvae in roots, or as pupae in the soil, although some adults may survive the winter. Their ability to lay eggs from September through to May and the promptness of their life cycle allows carrot flies to complete two more generations, a third generation by February, and a partial fourth generation by May.

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Plant damage:
Larvae feed on developing carrot taproots and fine roots, interfering with water transportation from root systems. This results in leaves with a characteristic rusty or bronzed appearance. Plants wilt, are stunted and droop and the roots become smaller.Young carrot plants may also be killed when CRF maggots injure their growing tips. Larval tunnelling causes a rust-coloured etching and deforms roots, providing entry for decay organisms such as bacteria and fungi that ultimately cause roots to rot. The maggots also continue to feed in stored carrots, promoting rotting.

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IGM considerations for controlling CRF:

 

  • Follow good crop rotation.
  • Use pelleted carrot seed when possible. Seed pelleting was first developed in the 1950s in Australia and New Zealand to introduce effective rhizobia (nitrogen-fixing bacteria that form nodules on legume roots and improve plant access to nutrients) to soils. Lime and nutrients were often included in the coating and pelleting of seeds, and pesticides are now included to protect against pests and diseases.
  • Reduce weeds and other sources of shelter around the vegetable garden to minimise the recurrence of adult flies.
  • Use CRF tolerant cultivars, e.g. Egmont Gold.
  • Sow in October and harvest before the peak of third-generation flight activity (mid April-early May).
  • Use clean cultivation methods and remove members of the umbelliferous plant family (e.g. celery, parsnips, fennel, dill, and chervil) to minimise the effects of CRF.
  • Grow plants that repel CRF such as rosemary, onions, leeks, wormwood and sage. These are also good companion plants (plants that benefit each other's growth and health), improving the growth and flavour of carrots.
  • Cover seedbeds with floating row covers, Mikroclima cloth or frost cloth before seedlings emerge. Carrot flies usually fly close to the ground so barriers of fine mesh nets or clear polythene sheets, 60 cm high on either side of rows and at the end of rows, can be highly effective. Carrots can also be grown under tunnels of row covers.
  • Avoid thinning young carrot crops during the peak flights of the successive generations of adults (e.g. September-October, late December, February and May) because a disturbed soil surface will attract egg-laying females.
  • Cover soil near the carrot plants with fresh grass clippings to camouflage the scent of carrots, especially during thinning.
  • Remove and burn infested crops to stop the carry-over of pests into the next growing season.
  • Harvest soon after the carrot taproots develop, and clean up all crop residues to minimise the risk of attack.
  • Spread creosote-treated sawdust, which is free of wood treatment chemicals, near the carrot plants, to help repel CRF.
  • Mix a few crumbled mothballs into the soil around the carrot plants to deter the CRF.
  • Use traps (e.g. sticky traps) to monitor the occurrence of CRF. Sticky traps can be made by painting stiff cardboard pieces (11x14 cm) yellow/orange and coating both sides with an insect adhesive or grease. The traps are normally placed around the borders of the carrot crops, about 15 cm above plant tops, and should be observed twice a week, reapplying grease each week.
  • Only use insecticides such as diazinon (granules) when the average number of flies caught in the sticky taps exceeds 1-2 flies per trap.

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Diseases

Field root knot nematode ( Meloidogyne hapla )

Characteristics:

The most common and widespread root knot nematode in New Zealand is the field root knot nematode (FRKN). This pest tolerates low soil temperatures and infects a wide range of hosts, such as carrots, parsnips, lettuce, kumara, kiwifruit, roses, and others. Maize, cereals and grasses are not affected. Nematode populations increase if carrots are cropped repeatedly on the same site, making it impossible to grow good carrots.

Both female and male FRKN are only visible under a microscope or a hand lens. They vary in appearance. Mature female nematodes are bulbous and males are long with worm-like bodies. The females are always present in a population and can be found inside galls of the host plant or attached to root surfaces. There may be several males or none at all in any particular population.

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Life cycle:
Root knot nematodes are obligate endoparasites ( parasites found inside plants ) that complete most of their life cycle within their host roots. The nematodes survive as eggs and second stage larvae in cold soils. Mature females of root knot nematodes deposit hundreds of eggs (on average about 500) within gelatinous egg sacs, which protect the eggs from dehydration. The larvae hatch from eggs and move a short distance through the soil in search of host roots. They then start to feed on the vascular root tissues. They penetrate the host roots just behind the root tip region where the cells are induced to enlarge, forming galls (knots). This often results in extensive secondary root formation and branching of the main roots of affected plants.

On a favourable host and at appropriate temperatures, the nematode life cycle may be completed in just one month, with several generations possible per year. However, in cooler weather the life cycle takes longer to complete. Nematodes can live in the soil for several years without feeding on potential host plants. They may spread through irrigation water, vegetative plant parts, and in soil infested with eggs or larvae.

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Plant damage:
Nematodes are less active in cooler weather and more likely to start feeding on host roots. Larvae normally prefer rapidly growing roots and penetrate the elongating area of the root tip. Infested plants are usually stunted, pale, and lower in yield than healthier plants. They wilt severely on warm days and may die for no apparent reason. Each nematode inserts a thin tube, called a stylet, into roots and sucks the sap, decreasing the size and efficiency of root systems. The host plants develop forked roots, irregular round galls, branched rootlets, and spindle-shaped enlargements on taproots or side roots in which the nematodes live.

IGM considerations for controlling FRKN:

 

  • Remove weeds and rotate susceptible crops with maize, cereals and grasses or avoid planting carrots for a few years. This form of repeated cultivation will kill nematodes in the upper layers of the soil by exposing them to heat and drying conditions, as well as starving them over time.
  • Remove all badly infested plants and burn them.
  • Do not move soil from an infested area to a clean area.
  • Maintain high humus content in garden soil because this increases the number of invertebrates and microbes (e.g. fungi) that can compete with the nematodes.
  • Plant cover crops such as mustard, which produces compounds that suppress root knot nematodes.
  • Plant marigolds in a border around the carrot beds to repel nematodes.
  • Provide plants with adequate water and fertiliser to help them tolerate nematode damage.
  • Harvest as soon as possible and remove all plant debris from the soil.

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Leaf blights or spots of carrot

Characteristics:
The most common fungal pathogens that cause leaf blight and leaf spot symptoms on carrots are Cercospora carotae and Alternaria dauci . Both of these are seed and soil borne, spread by wind and rain, and only infect leaves when they are wet. Cercospora carotae generally attacks younger foliage and then moves to older leaves, increasing in severity as plants grow. Alternaria is more damaging on mature foliage, but younger leaves and roots (especially after mechanical injury) may also become infected. Root lesions are irregular, dark brown to black, and can appear as shallow firm areas of decay. Identifying both these pathogens requires microscopic examination and laboratory diagnosis.

Plant damage:
Cercospora carotae produces primary lesions that are small, roughly circular, tan or grey to brown in colour, with a dead centre. They usually develop along the margins of leaves, causing them to curl. However, as the lesions increase in number and size they may join together, making large spots. These large lesions tend to shrivel, blacken, and finally kill leaves, closely resembling symptoms of Alternaria blight. In humid or hot weather, leaf blight develops rapidly and the lesions turn light grey or silver due to the production of spores (conidia).

The fungi also affect petioles (stem portion of a leaf or a leafstalk) and stems, producing lesions that have dark brown borders and tan to grey centre. Eventually these elongate to form lesions with pale centre and dark margins. The petioles may be girdled, causing the leaves to die. Carrot taproots are not directly affected.

The lesions caused by Alternaria dauci first appear as irregular, brown to black areas with pale yellow borders, near the margins of leaflets. In prolonged warm, moist weather, the enlarging lesions cause entire tops to turn yellow-brown, shrivel and die. Under these conditions, foliage appears bronzed or scorched, as if burned by fire, and these symptoms are often confused with frost damage. Research in Canada has shown that significant harvesting losses can occur when 10-20% of the leaf area of a crop is affected by blight.

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Disease cycle:
Carrot leaf blight pathogens survive on, or in the seed, and also on diseased crop debris in the soil. The fungal pathogens produce spores that become airborne and spread predominantly by wind. Moisture is essential for infection by all blight organisms because fungal spores require surface moisture and warm temperatures to germinate. The higher the temperature, the shorter the wet period required for infection. However, whenever temperatures are warm or when moisture levels are persistently high, the threat of infection and rapid spread of these organisms are high.

IGM considerations for controlling leaf blights or spots:

 

  • Rotate carrots with other crops, excluding celery, parsley and other carrot family members, for at least 2 to 3 years because the pathogens normally survive on infected plant debris.
  • Control weeds around the garden, particularly those weeds that are associated with the carrot family. E.g. Cow parsnip or hogweed and hemlocks.
  • Plant in raised and well-drained beds to reduce the soil moisture content.
  • Pull out, compost or burn all tops after harvesting to hasten the decomposition rate of plant residues and reduce fungal spores
  • Apply nitrogen to promote the growth of new foliage. Research has shown that well fertilised carrots are less likely to develop Alternaria leaf blight.
  • Only spray fungicide when disease is first evident. Drench the plants with fungicide, paying special attention to older leaves because once they are infected with spore producing lesions they become sources of infection for healthy leaves. For best results, spray in the morning or the evening, when the wind is reduced, and leaves are covered with dew.

Nutritional value
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Food science has shown that carrots are especially rich in vitamin A, a fat-soluble vitamin, that enhances eyesight, increases cell reproduction, and helps prevent cancer. It also acts as an antioxidant and immune system enhancer. One medium carrot supplies enough beta-carotene for the body to make two days worth of vitamin A. Interestingly; beta-carotene also gives carrots their orange colour. Carrots produce useful amounts of fibre, calcium, potassium, vitamin C, vitamin B, and various other antioxidants.

Uncooked vegetables, including raw carrots, have higher vitamin levels than over-cooked or boiled ones because heat destroys essential nutrients in vegetables. Therefore, avoid boiling or overcooking and try methods such as steaming, stir-frying or microwaving to soften the carrot tissue, releasing vital compounds bound to the cell walls. Furthermore, carrot skins and outer leaves have high levels of nutrients, so avoid peeling and disposing of carrot leaves whenever possible.

Recipe – Carrot salad
Ingredients
1 cup grated carrot
¼ cup desiccated coconut
2 tablespoons finely chopped onion
1 tablespoon finely chopped green pepper
½ teaspoon black pepper
½ onion finely chopped
Salt, sugar and lemon juice to taste

Method

Mix all ingredients in a bowl and serve immediately.

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