Citrus maxima (Burm.) Merr.

Tree, 5-10(-15) m tall, low-branching; branches spreading, spiny (seed propagation) or spineless (vegetative propagation), spines up to 5 cm long; young parts manifestly pubescent. Leaves ovate to elliptical, 5-10(-20) cm x 2-5(-12) cm, base rounded to subcordate, margin entire to shallowly crenate, apex obtusely acute, glandular dotted; petiole broadly winged, up to 7 cm wide obcordate. Inflorescences axillary, with a cluster of a few flowers or a single flower; flowers large, 2-3 cm long in bud, 3-5 cm wide when fully expanded, pentamerous, puberulous; petals creamy-white; stamens 20-25(-35); ovary with 11-16 loculi. Fruit a subglobose to pyriform berry, 10-20(-30) cm in diameter, greenish-yellow, densely glandular dotted; peel 1-3(-4) cm thick; segments with large, pale yellow or pink pulp-vesicles, filled with sweetish juice. Seeds usually few, large, plump, ridged, yellowish, monoembryonic. The closely related grapefruit (Citrus paradisi Macf.) can be distinguished from the pummelo by the following characteristics: fruits usually smaller, subglobose (pummelo often pyriform); the pulp-vesicles are coherent and the juice copious (in pummelo easily separable and juice rather scanty); petioles broadly winged, but not subcordate as in pummelo; seeds white and polyembrionic (in pummelo yellowish and monoembryonic). The top pummelo cultivars of Thailand are in a league of their own and fairly distinct: small trees with a spreading habit; fruit with thin rind and firm, crisp flesh, not very juicy; fruit often seedless or with shrivelled seed. Examples of widely-known cultivars: 'Kao Hawm', 'Kao Nam Pheung', 'Kao Paen', 'Kao Phuang', and 'Kao Thong Dee'. 'Kao Thong Dee' is pink-fleshed, the others white-fleshed; 'Kao Nam Pheung' and 'Kao Phuang' are pear-shaped, the latter with a prominent neck, whereas the others are more rounded or flattened ('Kao Paen'). The group also includes excellent cultivars which are typical for a single locality, e.g. 'Kao Teang Kwaa' (Chinat Province), 'Som Tha Koi' (Phichit Province) and 'Som Thubthim' (Nakhon Prathom Province). Several of these outstanding cultivars have been introduced to other countries, the names (e.g. 'Siamese', 'Bangkok') indicating their origin. However, fruit quality does not seem to be as good as in Thailand where top quality fruit is also largely limited to the 3 provinces mentioned above. The numerous lesser cultivars in the tropics have more in common with the seedling trees: large, tall or spreading trees bearing large fruit with thick rind and quality that range from good to poor. Names often refer to a locality rather than strictly denoting a cultivar, e.g. Tambun (Malaysia), jeruk Bali and jeruk Cikoneng (Indonesia). The subtropics have their own cultivars, adapted to lower temperatures. In China the leading cultivars are 'Shatianyou' (in Guangxi) and 'Wendangyou' (in Zhejiang and Fujian).

In the tropics the trees flower 2-4 times per year, mainly in conjunction with shoot growth flushes. The main flowering period follows the onset of the monsoon rains, unless it is brought forward by irrigation as in Thailand (bloom in January-February). A load of growing fruit limits fruit set from subsequent blossom periods. As a consequence there is usually little overlap of crops in high-yielding trees, whereas such overlaps are common in shy bearers. Fruit matures 7-10 months after flowering, in Thailand mainly from August to October. Some fruit is available throughout the year. In Java, the main harvest months are form April-June, following blossoming in September-October. In Malaysia there are harvest peaks in July-October and in January-February. The anthers lie against the surface of the stigma and release their pollen before the flower bud opens. Nevertheless most cultivars are self-sterile to a high degree. In the presence of pollinators most cultivars produce seeded fruit. It has been suggested that seediness may also be the result of selfing, provided favourable conditions prevail, e.g. a temperature (25-30 deg. C) high enough for fast pollen tube growth and low enough to delay wilting of the pistils. During the main season fruits contain more seed than those produced out-of-season; the latter are often virtually seedless.

Pummelo tolerates a wide range soils from coarse sand to heavy clay. However, the tree prefers deep, medium-textured, fertile soils free from injurious salts.

The pummelo thrives in lowland tropics. In the production centres of Thailand mean monthly temperatures are about 25-30 deg. C with a few cooler (and dry) months; the dry season lasts for 3-4(-5) months and annual rainfall is about 1500-1800 mm. The crop is not grown commercially above elevations of 400 m. In this connection it is noteworthy that the best orchards in the 3 major pummelo provinces in Thailand are situated on the banks of current and former river courses.

Although many trees in home gardens are raised from seed, the common propagation method in South-East Asia is air-layering. When certified virus-tested mother trees become available, budding is recommended. Pummelo seedlings of sufficiently uniform population can be used as rootstocks. In the Philippines shield budding is already the standard method, using rootstocks of calamandarin (believed to be a hybrid of calamondin and mandarin).

Plantation: Trees are spaced 8-10 m x 6-8 m, depending on vigour, on well-prepared land; they are shaded and watered frequently until they are established. The planting material is cut back, especially if bare-rooted. Husbandry (paragraph 1): In South-East Asia pummelo is grown in home gradens, in mixed citrus orchards and in pure pummelo orchards, the latter for instance in Thailand and the Philippines. Mixing citrus species complicates crop protection in the orchard. A banana intercrop can serve as wind-break, shade and source of early income. In Thailand the areca palm is also found as intercrop, or as a border plant along the ditched where the pummelos are grown on raised beds (Nakhon Prathom Province). Husbandry (paragraph 2): Young trees are pruned to leave 3 main framework branches, the lowest being at least 30-40 cm off the ground. The trees also need some prunning in later years to keep the tree interior open, to make sure that fruit on sagging branches does not touch the ground, and to remove dead wood. Trees that bear well are propped up with bamboo poles. A cover crop suppresses weeds to some extent, but in the rainy season weeds need to be slashed; early in the dry season the orchard is hoed or treated with herbicide. Mulching under the trees with rice straw or other material is strongly recommended to maintain root growth in the topsoil. Husbandry (paragraph 3): Irrigation is important from before flowering until after harvest to supplement rain. During the subsequent dry period irrigation is delayed until the trees show signs of wilting. It is customary to force early flowering by irrigating the wilting trees, provided the water supply is secure until the rainy season starts again. Forcing the trees to advance the harvest has its limitations, as it is difficult to sustain new shoot growth and flowering during the hot dry months preceding the rains. Husbandry (paragraph 4): Fertilizer requirements of citrus is also apply to pummelo, including attention to magnesium and micro-nutrient (Zn, Mn, Cu, B). An annual or biennial dressing with manure forms a good basis. In Nakhon Prathom growers are advised to apply about 5 kg NPK 16-16-16 per tree per year in bimonthly applications and foliar fertilizer for every new flush. In the last dressings before harvest potassium-rich NPK 13-13-21 is used to improve fruit taste. Elsewhere 2 fertilizer dressings are recommended, the first before flowering and the second 4-6 months later.

Diseases: Pummelo is particularly susceptible to bacterial canker, also on the fruit, following fruit fly stings. Frequent spraying with copper fungicides in Thailand does not give adequate control. Root rot, gummosis on the trunk and brown rot of the fruit, all caused by Phytophora fungi, appear to shorten the life of many trees in South-East Asia, even though pummelo is not rated as very susceptible. Pests: All the citrus pests seem to be at home on the pummelo, including the obnoxious leafminers Phyllocnistis citrella (in Java it has been recommended to protect young trees with a mosquito net!), leaf-eating caterpillars, fruit-boring caterpillar (Citripestis sp.), scales, red mites, fruit flies, nematodes and vermin (rats).

Food: The fresh juicy pulp vesicles are eaten out of the hand or in fruit salads; sometimes the juice is extracted. The white inner part of the peel can be candied after the outer peel containing oil glands has been removed. Essential oils: The aromatic flowers are used to make perfume in Vietnam. Timber: The wood is used for tool handles. Medicine: Even if the fruit is of inferior quality, the tree may still be grown for the medicinal applications of leaves, flowers, fruits and seeds, including the treatment of coughs, fevers and gastric disorders.