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Rubber tree or rubber plant, Hevea brasiliensis is belongs to the family Euphorbiaceae. It has soft wood, high, branching limbs and a large area of bark. These trees are generally found in low-altitude moist forests, wetlands, riparian zones, forest gaps, and disturbed areas which located in rainforests in the Amazon region of South America. It is the most beneficial, useful and economical important member of the genus Hevea because the milky white latex extracted from the tree is the primary source of natural rubber and they can be used to make a lot of products which normally used in our daily life such as boot, tyres, shoes, mattress, and so on. However, the productivity of the rubber was affected by the bacterial and fungal disease which largely reduced their quality and quantity.
Rubber trees are extremely vulnerable to the South American leaf blight which caused by the fungus, Microcyclus ulei, an ascosmycete. They attacks young leaves, stems or fruit tissue of five or more species within the genus Hevea (H. brasiliensis, H. benthamiana, H. spruceana, H. guianensis, H. camporum). (Lieberei, 2007) The symptoms caused by the infection different with the age of the infected leaves. In young leaves of susceptible clones up to 10 days of age, 3-4 days after inoculation, slightly discoloured, hypertrophic deformations are visible. 5-6 days after inoculation, greyish to olive-green masses of conidia are formed on the lower surface of the leaf. When infection rate is high, these spore-producing lesions join together, the leaves turn reddish and premature leaf were fall. The petioles, young twigs and young fruits of susceptible clones can also be infected. When conditions are favourable for disease development and in highly susceptible clones, infection and rapid re-infection of young leaflets can cause successive defoliations which lead to dieback of terminal twigs and branches and eventaully leads to the death of young trees.
For the young leaves that are older than 12-15 days, the lesions become smaller, only slightly hypertrophic and conidiospore production is low or even absent. For slightly infected young leaves or infected older leaves, no premature leaf fall is induced, instead, on the upper surface of these leaves, black stromatic areas form. The stromata contain pycnidial cavities in which conidia are formed. Later on, the stromatic areas coalesce to form ring-like structures. The leaf tissue within the rings often disintegrates, creating small holes within the rings. In these older parts of the stroma, ascospores are formed in pseudothecia.(Centre for Agricultural and Bioscience International, 2018)
Apart from that, South American leaf blight caused serious problems to the rubber plantations because the rapid distribution of spores, the high capacity for destruction and the complication in controlling the fungus. Premature leaf fall caused by M. ulei leads to dieback of trees and economic losses. Retardation of growth in seed orchards and in clone gardens reduces the number of rootstocks needed for budding as well as the amount of budwood from defined clones. Severely attacked defoliated seedlings cannot be budded and no buds can be collected from clone-defined plants because the bark is too hard and cannot be removed in the required manner. In adult plantations, continuous attacks of M. ulei cause dieback of twigs or even death of the whole plant. Furthermore, attacked plants are easily infected by other pathogens which contribute to the rapid death of trees.
In addition, the spores of Microcyclus ulei are spread widely by the rain or wind. Wind is the major factor of spreading of the spores among the plantation of the rubber. Dissemination of conidia occurs usually when the temperature is high and low relative humidity. Natural dispersal of M. ulei happens through conidia and ascospores dispersed mainly by wind although rainfall assists spore release and daytime showers can lead to large transient increases in spore concentration in the air. Spore production is decreased during dry weather. Wind may spread spores over large areas and diseased leaf debris containing the ascospore stage may also become airborne and be carried over shorter distances by the winds.
There are severe ways that can be done to prevent the spreading of the spores which eventually lead to infection of the rubber. The methods that need to carry out to prevent the further destruction caused by the fungus to the rubber are crown budding, escape areas, and biological control. First and foremost, one of the most efficient method of control is budding of very productive rubber tree stems with crowns of clones of Hevea species that are resistant to M. ulei, especially in the areas of high infection rate. Crown budding must be used as a plant management method in the high infection areas in order to replace susceptible crowns totally or at least partially. In these ‘mixed-crown’ plantations, the plants with resistant crowns will serve as a protector to the dispersal of inoculum and concurrently represent a favourable environment to natural enemies of both pathogens and phytophagous insects. In ‘mixed-crown’ plantations, the non-budded plants should consist of clones with a certain level of resistance or tolerance.
Besides, establishing plantations in ‘escape areas’ is used to control Southern American leaf blight. These areas have climatic conditions that are unfavourable to M. ulei. In various research, escape areas have been identified in Brazil, for example in the Açailândia-Maranhao state. The rubber trees have a normal production without being attacked by M. ulei, although its presence is detected in seed gardens. Plantations on the banks of the large rivers of the Amazon region and in the Sao Paulo highlands are considered to be escape areas. On-shore sea breezes were considered able to reduce the occurrence of conditions favourable for the development of the disease near coasts. In escape regions it is necessary to looking for adapted clones that change their leaves in a very short period and only during conditions which are unfavourable to the pathogen.
Furthermore, biological control also another method of control that can diminished the impact bring by South American leaf blight to the rubber plantation, the mitosporic fungus Dicyma pulvinata is found growing on conidia, forming lesions and on stromata of M. ulei. Application of D. pulvinata spores to highly susceptible monoclonal plantations does not lead to disease control in these plantations, but the fungus may serve as an important component in systems of integrated control and has been developed for potential commercial use. For integrated control measures, additional aspects such as enhancement of resistance by vesicular-arbuscular mycorrhiza have to be taken into account. In plants that are colonized by mycorrhiza, the incubation period of the leaf pathogen is prolonged, whereas sporulation and lesion diameter are diminished.(Plantwise Knowledge Bank, n.d.)
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