10.2 Water Management
10.5 Regulation of flowering for off-season production
10.6 Use of growth promoting substances
10.7 Pests and Diseases
In newly established orchards young trees should be grown as vigorously as possible for the first four years to attain the greatest tree size and bearing surface. Vigorous growth is achieved by strategic pruning, proper water and nutrient management and protection from weeds, pests and diseases.
When the trees are mature and ready to bear fruits, appropriate management in regulating vegetative and flowering cycles is required. It is important to know the phenology of each cultivar so that the vegetative, flowering and fruiting patterns for the cultivar can be monitored. For each year the tree should be limited to one or two significant vegetative flushes between harvest and the next flowering. Once flowering commences, there should be minimum flower and fruit shedding. This annual cycle can be achieved by strategic applications of water and fertilizer and appropriate timing of pruning of shoots, flowers and fruit.
In China, researchers have achieved three successive years of high and stable yields in 'Chuliang' cultivar in Gaozhou, Guangdong province, by effective cultural techniques (Huang et al., 2000). These include:
· Fostering two autumn flushes after harvest so as to form high quality fruiting shoots,
· Integrated measures for manipulation of flushing cycles so as to promote flower retention,
· Prevention of change of reproductive flush to vegetative flush, and thus increasing flowering rate,
· Proper thinning of fruit panicles to obtain good fruit quality, and
· Prevention and control of pests and diseases.
Immature young longan trees (first 3-4 years from planting) are subjected to formative pruning to obtain 'open' canopy which allows good light penetration. Only limited numbers of main branches are retained to obtain the desired structure. Surplus branches and all water shoots are removed and the skirt is maintained about 1-2 m above the ground level. In China, one strong branch is retained after every growth flush to form a natural round-shaped crown of 6 to 10 main branches. In Thailand, trees are cut to a 1.2 m trunk and 3-4 vigorous young shoots selected from the re growth to form the main framework of the tree. These laterals are forced into wide angles from the trunk with the aid of sticks. Two branches are left on each lateral shoot. Similarly, two sub-branches are left on each branch and so on. Finally, the canopy consists of 24-32 sub-branches, which may be achieved in the third or fourth year. Pruning at the immature stage produces well-formed tree canopy, strengthens fruit bearing branches, ensures annual cropping and limits insect pests and diseases. It also reduces the height of the tree to a manageable level. The flower spikes of young trees less than five years of age are normally removed in China and Thailand to encourage growth and crown expansion during summer.
In bearing trees pruning is an essential cultural practice. Harvesting itself is a form of pruning, since the entire panicle is cut. Soon after harvest this should be followed by pruning any remaining panicle or cutting out some of the subtending twigs. Cutting out these twigs completely simplifies the canopy structure and admits more light to the interior of the tree; it also removes twigs that are least likely to fruit next year, since they have fruited this year. If this is not done side shoots emerge below the cuts of the harvested panicles. These shoots make the canopy denser and come too late to initiate inflorescences for the next crop
Besides pruning, which is carried out during and immediately after fruit harvest, maintenance pruning is also practiced during the period between harvests. This involves pruning water shoots and branches that are dead or infested by pests and diseases. Weak branches, which have lost their vitality, are also pruned. However, too much pruning or removal of too much leaf and wood with the fruit panicles at harvest can reduce flowering the next season and aggravates biennial bearing.
In China, Taiwan Province of China and Thailand, prunings of flower and fruit panicles are usually practiced to overcome alternate bearing phenomenon which is common especially with trees which yield heavily and are older than ten years. Production in an 'off year is usually about 20 to 40 percent of the 'on' year (Menzel et al., 1990). In China, pruning of flower panicles involves removal of about 40 percent of the flower spikes when they are about 10-12 cm long. For pruning of fruit panicles about 30 percent of the young fruits are removed. The degree of pruning of flower and fruit panicles depends on the crop load and tree vigour.
Thinning of fruit in China not only reduces biennial bearing but also increases fruit size. Only large fruits (2.5 cm in diameter or 18 g in weight) attract a premium price. Fruits are thinned about four to six weeks after fruit set when they are of the size of a pea. Fruit thinning is essential after flower thinning because of the high rate of fruit set and greater competition for developing fruit.
In Thailand, longan growers reduce the number of flowers by half (each flower spike is retained) before fruit set in an 'on' year. After fruit set, they remove 10 percent of the fruit.
An ideal annual rainfall regime for good longan production falls between the range of 1,200 - 1,400 mm over 100-150 rainy days during the period from panicle emergence to maturation of post-harvest vegetative flush. Any rainfall outside this pattern requires proper water management in the form of irrigation. In Thailand, it is generally accepted that irrigation overrides other factors in determining yields, which are usually higher on trees growing along the rivers. In two of the main longan growing provinces of Chiang Mai and Lamphun, trees are normally irrigated during the first four years of planting. A drip irrigation system is commonly installed in the longan orchard. For bearing trees, irrigation is required from the time of panicle emergence, during flowering, fruit set and fruit development and after harvest until the maturation of the post-harvest growth flush. Irrigation is withdrawn before the next flowering so that the water stress condition allows flower initiation for the next season's crop. Water management is more easily controlled in dry areas and on light soils with low water holding capacity.
Mulching is recommended to reduce water loss from the soil and increase soil organic matter and structure, reduce extremes of soil temperature and encourage growth of feeder roots. If applied correctly, under-tree mulching also assists weed control (Menzel et al., 1990).
During the immature stage, a combination of organic and inorganic fertilizers may be used. Organic fertilizer such as animal manure is recommended at the rate of about 10 kg/tree/year, applied about 3-4 times in a year. The organic manure may be supplemented by inorganic fertilizer such as 15:15:15 (N:P2O5:K2O ratio) at the rate of 5-10 kg/tree/year.
Fertilizing bearing trees is directed at manipulating the crop cycle, especially towards promoting panicle growth, fruiting and vegetative flushing after cropping. The recommended fertilizer schedule for longan production in Thailand is as follows:
· The first application is carried out at two weeks after harvesting to encourage new growth flush. Inorganic fertilizer of 20:10:10 ratio is applied at the rate of 1 kg per tree together with organic manure at 6-10 kg per tree. Calcium nitrate may be added as a supplementary fertilizer.The inorganic fertilizer is applied by making a small trench of 20 - 30 cm around the canopy and applying the fertilizer in the trench which is then covered with soil to be followed by watering.
· The second application is applied when the panicle is about 5 cm long. The recommended inorganic fertilizer is 16:11:14 or 15:15:15 ratio at the rate of 1 kg per tree. This is to help in fruit setting.
· The third application is done at 2 weeks after fruit set by repeating the second application. This is to help in fruit development.
· The final application is done at the seed colouration stage by applying inorganic fertilizer of 14:14:21 ratio at the rate of 2 - 3 kg per tree.
In China, fertilizers are applied to bearing trees at a frequency of five to six times a year (Liu and Ma, 2000). Application of fertilizer at N:P:K ratios of 1:0.5:1 or 1:1:4 has been reported to increase yield significantly.
In Taiwan Province of China, fertilizer is recommended to be applied three rounds per year for immature trees while only one round per year is applied for bearing tree, particularly after flower bud formation.
In Australia, fertilizer is recommended to be applied four times during the crop cycle: (i) panicle emergence in July to August (Southern Queensland), (ii) one month before fruit set in September to October, (iii) one month after fruit set in December to January, and (iv) two weeks after harvest in March to April. These times will be one to two months earlier in north Queensland. Suggested rates which have proved reliable for well grown high yielding five year old trees under southern Queensland conditions are 625 g N, 150 g P and 800 g K increasing by 20 to 30 percent per year to 1,250 g N, 300 g P and 1,600 g K at year ten. Micronutrients, including zinc, boron, iron and copper, are also applied every two to three years (Menzel et al., 1990)
It was reported in Thailand that cinturing (of branches and stem) can induce dormancy and give rise to better flowering, fruiting and production. However results have been too inconsistent to justify the recommendation for commercial application. The easy to flower cultivar 'Phetsakon' can be induced to produce early and uniform flowering by cinturing of branches or stems (Subhadrabandhu and Yapwattanaphun, 2000a). The cincturing knife used for lychee can be used for longan (Figure 7).
Figure 7. Knife used for cincturing of lychee tree.
The technique of producing off-season longan has been practiced by Thai growers (Subhadrabandhu and Yapwattanaphun, 2000b). Under normal conditions, the commercial longan will require a period of cool and dry climatic conditions for induction of flowering. The Thai longan growers, however, have discovered an alternative method of floral induction in the absence of suitable climatic conditions. This is by the use of potassium chlorate which is applied to produce off-season longan. With off-season production the growers are able to obtain a better price for their product. There are many methods of applying potassium chlorate to induce flowering in longan. Many Thai growers apply the chemical as a soil drench and the rate of application is dependent on the tree size. Other factors which need to be taken into consideration are soil type, availability of water supply, the general health of the tree and the management of the orchard. Trees grown on a sandy soil respond better to potassium chlorate than those on a heavy soil type. Watering is essential in an area with a long dry spell, and the tree must be healthy and dormant in vegetative growth at the time of chemical application. Different cultivars are found to require different rates of potassium chlorate for induction of 100 percent of flowering. Cultivar 'Daw' requires 8 g/m² while 'Chompoo' needs 1 - 4 g/m² of potassium chlorate as soil drench. The suitable time for good flower induction by potassium chlorate is from October to February where the average temperature is rather cool with relatively dry condition. Besides soil drenching, the chemical can also be applied as a foliar spray at about 1,000 ppm or as trunk injection at 0.25 g per 1 cm diameter of stem size less than 10 - 15 cm in diameter. Sodium chlorate may be able to substitute potassium chlorate for the induction of flowering in longan.
The combination of potassium chlorate application and use of easy to flower cultivars such as Thetsakon' has led to the expansion of longan growing areas in Thailand. These include the central, warm tropical region and also the eastern provinces such as Chanthaburi and Trat where early cultivars are been grown.
Growth promoting substance such as ethephon when used as a foliar spray has been reported to be beneficial for floral initiation and inflorescence development (Qiu et al., 2000). A single foliar spray with 400 µl/L ethephon on 'Shixia' longan cultivar was found to increase cytokinin, ABA and cytokinin/gibberellins (GA1+3) ratio in the flower buds, while inhibiting gibberellins activity. The increase in cytokinin level led to the promotion of flower differentiation and morphogenesis. The higher level of ABA exerted a positive effect on bud dormancy and morphogenesis. Also, higher cytokinin/GA1+3 ratio favoured flower initiation during the period of floral initiation and inflorescence development. In addition, higher starch content in the ethephon-sprayed trees seems to be beneficial for floral initiation and inflorescence development.
Numerous pests are found on longan. Of particular importance is the longan stink bug (Tessaratoma javanica) which can ruin bloom in a year with light flowering. The adult bug feeds on flower panicles, young fruits and newly developed shoots of longan, causing withering and later abscission of those infested organs. The infested fruit ceases to develop. In Thailand, this stink bug can be of epidemic level during March to April, which coincides with the flowering and fruit setting times of longan. The suggested control is to get rid of the eggs as well as the adults. The bugs usually mate in February, thus it is recommended that the tree be sprayed at that time with insecticide such as azodrine at the rate of 10 - 20 g chemical diluted in 20 litres of water at about 2-3 weeks intervals from February to April. At the same time the population of the bug's natural enemies in the orchard should be maintained at a satisfactory level. The natural predators of this stink bug are identified as Anastatus sp., Micropanurus sp. and Eupelmid sp.
Other insect pests found on longan include Erinose mite, scales, fruit flies, aphids, stem borers, leaf eating caterpillars, flower eating caterpillars, mealy bug, fruit spotting bug, elephant beetles and fruit piercing moth.
Of particular importance is fruit bats or flying foxes, which can devour the longan fruits during fruiting seasons. This pest has been reported to cause serious damage in Thailand and Australia. The only effective means of reducing damage is to erect a protective net around the perimeter of the orchard or over each tree (Figures 8 and 9). However, this method can be very expensive. Alternatively, a draconian control method is electrocution by a high screen of thin, parallel electric wires in the orchard.
A serious disease in longan is the rosette shoot or witches' broom. The latest report from China indicated that the disease is caused by a filamentous virus which is transmitted by vectors such as litchi stink bug (Tessaratoma papillosa) and longan psylla (Cornegenapsylla sinica). It is also transmitted through infected seeds and budwoods of longan and dodder weeds (Cuscuta campestris) (Chen et al., 2000). In Guangdong province a new species of gall mites (Eriophes dimocarpi Kung) has also been included as a vector for the transmission of the virus (He et al., 2000). Affected trees show abnormal growth with distorted mature leaves and unexpanded young leaves while the shoots on infected branches become compact clusters. Flowers are poorly developed and the deformed inflorescence lose all flowers on an infected branch (the flowerless panicle resembles a broom).
The earliest description of this disease in China was in 1941. In 17 cities of Fujian province, the percentage of trees infested by witches' broom varies from 20 to 100 percent, the higher infestation occurs in mature trees. The disease causes an average crop loss of 10-20 percent. However, in severe cases, the loss can exceed 50 percent. This disease also occurs in other provinces such as Guangdong, Guangxi, and Hainan. Beside China other longan growing countries in Asia have also reported the presence of this disease, for example, Thailand and Taiwan Province of China.
Figure 8. Prevention from birds and bats using netting over individual tree (from: Nicholls, 2000).
Figure 9. Prevention from birds and bats using netting around perimeter of orchard in Queensland, Australia (from: Nicholls, 2000).
No cure is known for witches' broom disease. However, there are cultivars which have been reported to be free from this disease. In Thailand, the cultivar 'Daw' has been reported to be free from this disease (Subhadrabandhu and Yapwattanaphun, 2000a). In China, cultivars such as 'Lidongben' and 'Shuinan No.1' in Fujian province are highly resistant to the disease. Chen et al. (2000) suggested that top grafting with scions of resistant cultivars can effectively reduced the morbidity of the disease in severely infested orchards. They have outlined an integrated management programme against the disease, viz., strict quarantine inspection; selection and use of disease-resistant cultivars; establishment of virus-free nurseries; timely control of insect vectors; removal of infected branches, inflorescence and infected seedlings in the nurseries and orchards; and improving tree vigour by judicious fertilization, irrigation and soil management. Shoot tip culture has been successfully employed to produce virus-free plantlets of longan (Wang, 2000).
Figure 10. Witches' broom disease in longan.