IntroductionThe control of South American leaf blight is extremely difficult, because the plants grow up to 25 m high, the pathogen reveals a high physiological variability and, at present, there are no high-producing clones with satisfactory horizontal resistance. In the humid tropical region of the Amazon there is a chance to establish new rubber plantations using crown budding of high-producing clones with crowns of Hevea species that are resistant to M. ulei.
Host-Plant ResistanceThe most efficient method of control is to use resistant productive clones, but clones with a sufficiently high resistance (race non-specific resistance) and good production have been difficult to find so far. The resistance of new selections is generally broken within a few years by the high variability of the pathogen. For example, Clone IAN 6158 was considered to be resistant up to 1990, but in 1991, after 700 ha had been planted in Amazonas state, the trees were severely attacked and the plantations were abandoned (
Gasparotto et al., 1992). Promising clones have recently been selected in Sao Paulo state (de
Souza et al., 2000).
Rivano (1997b) found that Hevea clones from South America possessed the highest levels of resistance whereas those from Asia were most susceptible. Evaluation of resistance showed that smaller lesion size and lower sporulation were the main components and rates of accumulation of a phytoalexin, scopoletin and lignin were strongly correlated with resistance (Garcia at al., 1999). Quantitative trait loci (QTLs) have been identified in Hevea germplasm which impart resistance to the disease (
Lespinasse et al., 2000).
Crown BuddingBudding of very productive rubber tree stems with crowns of clones of Hevea species that are resistant to M. ulei is an important control method, especially in areas of high disease incidence such as the Amazon basin. According to
Gasparotto et al. (1995), crown budding must be used as a plant management method in the humid Amazon area in order to substitute susceptible crowns totally or at least partially. In these 'mixed-crown' plantations, the plants with resistant crowns will serve as a barrier 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.
In Brazil, at EMBRAPA/CPAA (Manaus, Amazonas), researchers are attempting to identify those combinations of disease-resistant crowns with high-producing stems that reveal the lowest depression of latex production. Such depression in latex production partly results from a low number of latex vessel rings in clones used as crowns. A test system based on the structural anatomical aspect has been developed for Hevea pauciflora plants, which are generally suitable for crown budding (VHF Moraes, EMBRAPA/CPAA, personal communication, 1995). The depression of latex flow can be partially overcome by stimulation with ethephon and the addition of magnesium also enhances latex production. The availability of magnesium in the stem latex vessels seems to be reduced when a H. pauciflora crown is combined with the stem of a high-producing clone (
Moraes, 1997).
Escape AreasEstablishing plantations in 'escape areas' is used to control Southern American leaf blight. These areas have climatic conditions that are unfavourable to M. ulei but suitable for economic rubber tree growth. In various studies, 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, though its presence is detected in seed gardens (
Pinheiro et al., 1982). Plantations on the banks of the large rivers of the Amazon region (
Bastos and Diniz, 1980) and in the Sao Paulo highlands (
Camargo et al., 1967) are considered to be escape areas. On-shore sea breezes were considered to reduce the occurrence of conditions favourable for the development of the disease near coasts (
Pezzopane et al., 1996).
In escape regions it is necessary to search for adapted clones that change their leaves in a very short period and only during conditions which are unfavourable to the pathogen. When leaf fall occurs two times per year or in the 'wrong' weather conditions the disease will be favoured.
In Brazil, rubber plantations cover about 200,000 ha. About 90% of the plantations are located in disease escape areas, especially in the states of Sao Paulo, Mato Grosso and Espírito Santo.
Chemical Control
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Biological ControlThe 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 (
Junqueira and Gasparotto, 1991), but the fungus may serve as an important component in systems of integrated control and has been developed for potential commercial use (
Bettiol, 1996).
For integrated control measures, additional aspects such as enhancement of resistance by vesicular-arbuscular mycorrhiza have to be taken into account (
Feldmann, 1991). In plants that are colonized by mycorrhiza, the incubation period of the leaf pathogen is prolonged, whereas sporulation and lesion diameter are diminished.