Papaya: The Giant Herb That a Virus Almost Destroyed

What it is

Carica papaya, the papaya (or pawpaw, in some countries — not to be confused with the temperate pawpaw), a fast-growing tropical plant bearing large, melon-like orange-fleshed fruit, grown throughout the warm world.

Like the banana, the papaya is not really a tree: it is a giant herbaceous plant with a soft, hollow, unbranched, non-woody trunk topped by a crown of large, deeply-lobed leaves, growing tree-sized in a year or two and bearing fruit almost continuously from near the top of the stem. It is one of the fastest of all fruit plants — seed to fruit in under a year — which makes it a staple dooryard fruit across the tropics and a quick producer in any warm food forest.

Papain: the meat-digesting enzyme

The papaya's other great gift is chemical. The unripe green fruit, and the plant's milky latex, are rich in papain, a protein-digesting (proteolytic) enzyme.

Papain breaks down protein, which gives it a range of uses: it is the active ingredient in most commercial meat tenderisers, and green papaya has been used for centuries to tenderise tough meat (wrapping meat in papaya leaves, or cooking it with green fruit). It is used industrially in brewing (to clarify beer), in the leather and textile trades, in some digestive supplements, and in wound and burn treatment. The enzyme is why green (unripe) papaya is treated as a vegetable — grated raw into the Southeast Asian green papaya salad (som tam), or cooked — while the ripe fruit is eaten sweet. The latex is a mild skin irritant, so green papaya is handled with a little care.

Male, female, and the pollination puzzle

Papayas have a complicated sex life that matters to the grower. Plants may be male, female, or hermaphrodite, and you cannot tell which a seedling is until it flowers.

Female plants bear fruit only if pollinated by a nearby male or hermaphrodite.
Male plants bear no fruit (they only provide pollen).
Hermaphrodite plants are self-fertile and bear the uniform, elongated fruit preferred by the commercial trade.

So a home grower traditionally plants several seedlings together and thins out surplus males once the plants flower, keeping roughly one male to several females, or grows self-fertile hermaphrodite varieties. The flowers are pollinated by moths, and sometimes by hand in commercial crops. See pollinator habitat.

The virus and the rescue

Papaya holds a landmark place in the history of agriculture as the setting for one of the first great tests of genetic engineering to save a crop.

Papaya ringspot virus is a devastating disease, spread by aphids, that stunts and kills papaya plants and had crippled the crop in country after country — by the 1990s it was destroying the papaya industry of Hawaii, then a major producer, with no effective conventional control. In response, scientists developed a genetically engineered papaya ("Rainbow" and "SunUp"), which carried a gene from the virus's own coat protein that gave the plant resistance — a kind of vaccination at the genetic level.

Released in 1998, the transgenic papaya is widely credited with saving the Hawaiian papaya industry from collapse, and it remains one of the clearest, most-cited examples of a GM crop deployed to rescue a specific crop from a specific disease (rather than for herbicide tolerance or pest resistance in a commodity crop). It also became a focus of the wider GM debate, over labelling, cross-pollination with non-GM papaya, and export markets that reject GM fruit — so the papaya sits at the centre of both a scientific success story and a continuing controversy. See tree diseases and integrated pest management.

Growing it

Tropical to warm-subtropical and frost-tender — even a light frost kills or badly damages the soft-stemmed plant, which sets its cool limit — wanting heat, full sun, and warmth year-round.

Soil and site. Rich, moist, and above all very well-drained soil; the fleshy stem and roots rot quickly in wet or heavy ground, so drainage is the first priority, and papaya is often grown on mounds. It is a heavy feeder and drinker for its fast growth, but hates waterlogging.

Lifespan. Short-lived — most productive for only 3 to 4 years before the tall, top-heavy plant declines and is replaced — so, like the tamarillo, it is grown as a fast, renewable, quick-return crop rather than a long-term tree.

Propagation. Almost always from seed, which germinates fast; grow several and thin for sex, or use self-fertile hermaphrodite varieties. See propagation.