Avocado (Persea Americana Miller) is an indigenous plant of South America. It is widely grown in Mexico, Guatemala and the islands of the West Indies (Whiley, 1991). In Thailand, avocado plants were introduced by missionaries and were planted initially in Nan province in northern Thailand about 90 years ago. Some seedlings from these introductions are currently found around this area. In 1965, the Department of Horticulture, Kasetsart University, introduced some avocado cultivars which included Kanoe, Ruehle, Kampong and Monk and planted them at Pak Chong Research Station, Nakhon Ratchasima province. Some avocado cultivars were also introduced by interested growers. In 1975, the Department of Horticulture, Kasetsart University, with support from the World Bank introduced 11 cultivars of avocado from Florida, USA, and established them at Pak Chong Research Station. The introduced cultivars were Buccaneer, Booth 7, Booth 8, Catalina, Choquette, Hall, Lula, Peterson, Queen, Taylor and Waldin. The Hass, Bacon and Reed cultivars were introduced also from California in 1993.
The purpose of introducing avocado cultivars into Thailand was to evaluate their suitability under local conditions. Due to their high nutritive value and wide utilization possibilities, avocado fruits are becoming more popular in Thailand. Apart from being consumed as fresh fruits for their palatability, food value with high protein, vitamin E and digestible fat content, their oil is used also in the cosmetic industry.
PRODUCTION OF PLANTING MATERIAL
The Pak Chong Research Station and the Royal Project Foundation are the two main production sources of grafted and budded avocado plants. These two institutions produce more than 4,000 plants per year for the growers.
Seedlings are raised in the nursery under 50% shade. The seeds are cleaned and soaked in fungicide solution for about 20 minutes. They are germinated in 8 x 15 cm PVC bags. The seedlings can be budded or grafted after 4-6 months.
Patch budding or modified chip budding are commonly used depending on the rootstock. If the bark cannot be peeled easily from the stem of the rootstock, modified chip budding must be used. Patch-budding can be used when the bark can be peeled off easily.
Bark-grafting is used when the bark could be peeled off easily from the stem, and this is commonly seen when the rootstock is over six months old. Modified side-veneer-grafting is used for the rootstock that cannot peel off the bark easily, usually when the rootstock is 4-6 months old. Modified side-veneer-inarching is also used for rootstocks that are 4-6 months old.
4. ESTABLISHMENT OF ORCHARDS
4.1 Land preparation
Where hardpan is evident, a sub-soiler is used for breaking the hardpan. The land is ploughed and holes of 60 x 60 x 60 cm are dug. The planting medium is a mixture of soil, animal manure and organic matter at the ratio of 2:1:1. Generally, land preparation is carried out before the rainy season.
Spacing of avocado is dependent on the cultivars and the soil conditions of each site. In Thailand, a spacing of 8 x 6 metres is used for Ruehle, Peterson, Hass and Buccanaer, whereas wider spacings of 8 x 8 metres, 8 x 10 metres, and 10 x 10 metres are used for Booth 7, Booth 8 and Hall cultivars.
Planting is carried out at the beginning of the rainy season (May-June). The plants should have no sign of insect pests and diseases on the stems and leaves and they should be subjected to hardening under full sunlight before transferring to the field. The plants are brought to the prepared holes and are removed from bags or containers. They are then placed in the hole, with soil mixture filled and firmed to ensure good contact with roots. They are then watered regularly to ensure availability of sufficient moisture until establishment. Insecticides are sometimes applied to protect from termite damage every three months and also fungicide, often by using 100 grams of metalexyl (Ridomil 50 g) granules around each plant. The plants are protected from wind by planting windbreak trees as well as by using bamboo stakes for supporting newly planted trees.
Plastic sheets can be used as mulching material but this is not seen commonly in Thailand. Often, organic materials such as straw, rice husk and wood chips are commonly used as mulching material because these materials can later decay and become available to plants as organic fertilizer.
5. CARE AND MANAGEMENT OF ORCHARDS
5.1 Training and pruning
Avocado trees are normally trained under a modified leader or open centre system. When plant height of newly established plants is about 70 cm, the growing tips are pinched off to allow development of more side shoots to form a round-shaped tree. After harvesting, the trees should be pruned. The upright branches, water sprouts, dead wood, infected branches, and the branches that are not exposed to the sun should be pruned off.
5.2 Fertilizer application
A mixed fertilizer of 46-0-0 and 15-15-15 at 250 grams/tree is applied two months after planting and the same rate is repeated again three months later. The same formula with a slightly higher rate of 300 grams/tree is applied in the second year at the beginning of the rainy season with repeated applications every three months. From the third year onwards, as well as for bearing trees, the amount of applied fertilizer depends on the radius of the tree canopy, i.e. at every one-meter radius, one kilogram of mixed fertilizer of 46-0-0 and 15-15-15 is applied.
The weeds are often cut, as ploughing to control weeds is not recommended because of the shallow root system of the trees. Some growers use contact or systemic herbicides for chemical weed control.
In Thailand, where the climate is very warm, organic materials are rapidly decayed, so more organic mulching materials are recommended to be applied to the trees.
Supplementary irrigation during the dry periods appears to be needed for young as well as mature avocado trees.
Avocado root rot is the most serious disease that affects avocado trees in Thailand. The disease is caused by the fungus Phytophthora cinnamomi. This soil borne disease is commonly found in acid soils with poor drainage. It is recommended to use resistant rootstocks such as Duke-7 in the areas where the disease is prevalent. A well-drained soil is needed to prevent the spread of the disease. No ploughing underneath the tree canopy is advised. For avocado seedlings in the nursery, 20 ppm Dexon, applied as soil drench, is recommended to control Phytophthora in the nursery.
Other diseases that have been recorded are anthracnose on leaves and fruits which is controlled by spraying with mancozeb or carbendazim fungicides. Algal spot was found on leaves also, and can be controlled effectively by spraying with copper oxychloride or, alternatively, by pruning the trees to provide good ventilation.