Annona cherimola

Cherimoya, chirimoya

Origin

This has long been thought to be the highlands of Ecuador & Peru but recent molecular evidence suggests a meso-American origin. It is now grown in several sub-tropical countries, with Spain currently the world’s largest producer.

Climate

Sub-tropical regions and tropical highlands (1000-2300m asl) are preferred, with rainfall of 800-1200mm pa and average annual temperatures of 14-24°C. Coastal regions with higher relative humidity (RH, 70-80%) in spring and summer fare better than drier inland areas. Cherimoya is the most cold tolerant of the commercial Annona species; mature plants can withstand some brief frost periods but they will kill younger ones. It requires 50-100 chill hours before flowering and can also tolerate some drought periods; excessive precipitation is detrimental.

Plant Description

They are small slow growing trees (4-8m high) with a shallow root system, low spreading branches, a dense canopy, an indeterminate growth habit, and are semi-deciduous just before the spring flush. The alternate chartaceous ovate-elliptic simple distichous leaves (6-20 X 4-10cm) on 6-12mm petioles have a blunt point at the apex and a rounded base, and are slightly furry or glabrous on the underside with short hairs on the upper. Axillary sub-petiolar buds are compound (3 vegetative and 2 flowering). Most fruiting species of Annona are diploid with 2n =14 (pond apple, A glabra, is tetraploid), but up to a third of cherimoya plants can be triploid depending on prevailing environmental conditions.

Relatives

It is in the large Annonaceae family, one of the oldest (ancestry of about 95 million years) in the flowering plants, that contains about 130 genera and 2400 species. There are approximately 175 species in the genus Annona which diverged about 25 million years ago; several of these plus one hybrid (atemoya) are grown commercially around the world for fruit. Some of the common ones are sugar apple, soursop, mountain soursop, pond apple, bullock’s heart and rollinia. Atemoya is a hybrid between A. cherimola and A squamosa and molecular genetics shows these two are the closest relatives amongst the fruiting species.

Soils

A wide range of soils is tolerated from sandy to clay loams with a pH of 6-7. Soil moisture should be maintained but good drainage is essential to avoid root rot.

Propogation

This is often by seeds which are orthodox. They show reduced ability to germinate after 6 mths storage so are best sown as soon as possible, with 30°C giving best results. Seedling plants are highly heterogeneous and best used only as root stocks. Grafting and budding are the preferred means of propagation, with up to 70% take possible with suitable pairing. To induce swelling of the sub-petiolar buds, foliage should be removed 1-2 weeks before grafting. There are some graft incompatibilities across the Annona fruiting species, but named variety cherimoya scions can be successfully grafted onto atemoya, sugar apple and of course self-grafted, and can also serve as root stocks for sugar apple. Other vegetative propagation techniques are less successful and not recommended.

Cultivars

Globally there are many named varieties, but as the species is not common in Australia, sourcing many of them can prove difficult. None has all the desirable pomological attributes and represent compromises. Fino de Jete, an early fruiting cv, dominates the Spanish market (>90%) and also worldwide. Some others include Nata, Burton’s Favourite, Bronceada, White, and Sofia. There is frequent confusion with different species and cultivars amongst those not familiar with the genus. Little scientific breeding work has been undertaken to develop improved cherimoya varieties, and to date most of those currently available have been through selection.

Flowering and Pollination

Axillary pendulous inflorescences are 1-3 flowered on current season’s growth. The hermaphrodite flowers on short peduncles are fragrant and have 3 small ovate sepals (2-4mm) and 3 main fleshy petals (2-3cm long), keel-shaped distally, green-yellowish on the outside and yellow-whitish on the inside. There are numerous spirally-arranged carpels (200-300), one ovule per carpel, with short styles on a conical receptacle, and many short filament stamens (150-200, 1-1.5mm long) helically arranged at the base. Some chilling and dry periods before anthesis promotes flowering, which under favourable conditions may continue for 3-6mths. Ideal conditions for flowering are 25-28°C and >70% RH. Flowers exhibit synchronous protogynous dichogamy, where mature flowers first open partially in the morning of one day, exposing receptive female wet type stigmas, followed by complete petal opening with anthers shedding pollen, either late the same day or more usually the following. High RH and moderate temperature slows stigma desiccation and prolongs receptivity, and genetic paternity analysis indicates there is then substantial self-pollination. Temperatures  above 30°C dry out stigmas; in dry climates like ours, humidity can be improved by misting or under-tree watering. Beetles are the natural pollinators attracted mainly by odours emitted during the female stage. Wind and other insects play a much smaller role (1-2%); a major limitation with wind dispersal is due to pollen being released in heavy clumps of 4 tetrads rather than monads. The male pollen source will influence fruit properties (metaxenia). In most countries hand-pollination is necessary for good yields and fruit shape. Pollen can be collected in the evening when pollen sacs turn from white to cream, stored in the frig overnight, and then used next day.

Cultivation

A full sun position is best. Vegetative growth with new plants should be encouraged for the first 3 years to stimulate growth and canopy shape, and then they can be brought into flowering and fruiting. Overdoing the fertilising, particularly N, does not have the same negative impact as happens with determinate species, but if markedly in excess then vegetative growth will be stimulated rather than reproductive. Cherimoya is not really suitable for growing in pots; dry soil can lead to yellow foliage which can easily be misinterpreted as a nutritional problem. Yield and fruit quality will diminish with other plants in close proximity. Organic mulches are beneficial, as are mycorrhizal associations (VAM) that can be formed.

Wind Tolerance

They have soft wood which predisposes to wind damage. Wind may also cause fruit rubbing and damage to the thin fruit skin.

Pruning

Young plants should be topped at about 90cm to promote lateral branches which then form the main scaffold for an open vase shape. Without an open form in the dense canopy, sunlight penetration may be only a few percent. Mature trees are best pruned in spring while still dormant without much bud growth; low hanging branches should be skirted and any dead wood removed.

The Fruit

Fruit is a green-skinned syncarp with varying morphology depending on the variety and cultural conditions (even on the same tree) but usually are ovoid heart-shaped, 10-20 X 10-15cm and 150-600g, sometimes much larger. The skin can be smooth or have varying protruberances that derive from separate but loosely fused carpels. These protruberances have been used to broadly classify fruit into 5 types – smooth, fingerprint, tuberculate, mammillate and umbonate. The sweet white flesh has a pleasing aroma and sub acid flavour and contains numerous brown-black bean-shaped seeds, 1-2cm long. Fruit with less than ideal pollination (fewer seeds) are smaller, misshapen and have less flavour. Fruit flesh represents 65-80% of fruit weight. The carbohydrate level can exceed 20%, mainly as glucose and sucrose, and the major acids are citric and malic. Usually as fruit ripen flesh acidity decreases, but with cherimoya it increases. The seeds should not be eaten.

Fruit Production and Harvesting

The juvenility period for seedlings is 3-4 years, about half this time for grafted plants. Yield will then gradually ramp up to the fully mature level in 7-10 years (300-400 fruit/tree if well managed), with timing influenced by seedling or root stock heterogeneity. Fruit follows a double sigmoidal growth curve and may take 16-24 weeks to mature from fruit set; skin colour changes from immature greyish-green to mature yellow-green when ready for picking. Fruit russeting can develop during the tree maturation period if temperature and/or RH are less than 13°C and 60% resp. Harvest is usually through winter. Cherimoya is a climacteric fruit and when it ripens, fruit contours become smoother. Handicaps in cherimoya ever becoming a major commercial fruit are it’s soft skin susceptibility to bruising, and high perishability, which is only 4-7 days at ambient temperature, 6-9 days at 17°C. Storage below 13°C causes chilling injuries, including skin darkening and cell membrane breakdown, desiccation and a lack of full flavour development.

Fruit Uses

It is usually eaten and enjoyed fresh by spooning out as dessert fruit, but is also used in ice-creams, flavoured yoghurts, cakes, custards, pies etc. The pulp can be frozen for later use.

Pests and Diseases

Some pests and diseases found in Australia include mealy bugs, scale insects, anthracnose, root rot, black canker, bacterial wilt and Phytophthora infections. Snails and weeds may also need controlling. Good orchard hygiene and tree management help minimise some of these problems.

Comments

Cherimoya has been called one of the ‘Lost Crops of the Incas’. It’s a small tree that can be grown in typical backyards. Atemoya is the most commonly grown and best known custard apple in Australia, but cherimoya, as one of its parents, has similar taste and flavour and the advantage that it’s more suited to our cooler winters. Good fruit yield and quality will probably be dependent on hand pollination. Seedling variability is such that they really have value only as rootstocks.