Microcyclus ulei (P. Henn.) V. Arx.
Fusicladium macrosporum P. Henn.
Aphosphaeria ulei Kuyper

Distribution

South American leaf blight (SALB) occurs in all the natural habitats of Hevea species. Currently, SALB areas stretches from El Palmar in Mexico (15oN) to the state of Sao Paulo, Brazil (24oS). IRRDB surveys show that this disease is very severe in the Amazon Basin (Brazil, Colombia, Peru) whereas it is not so troublesome in Mexico (and on the Pacific Coast of Guatemala). Results of the 1998 Survey.

Symptoms

The symptoms vary with the age of the leaves at the time of infection. On young leaves up to 10 days old from susceptible clones, slightly coloured hypertrophic deformations are visible three to four days after inoculation. Five to six days after inoculation, greyish to olive coloured masses of conidia are present on the lower leaf surface. When infection density is high, the spore producing lesions may coaleace. The leaves shrivel, crinkled turn blackish in colour and fall prematurely.

Young leaves which are lightly infected, leaves which are infected at the more mature stage and leaves from less susceptible clones do not suffer from premature leaf fall when infected by SALB. On the upper surface of these leaves, immediately above the lesions on the lower leaf surface are developed small spherical dark structures called the pycnidia. The pycnidia produce another spore called the pycnidiospores. The spherical structures enlarge and form small circles especially around the edges of the lesions. These dark coloured raised structures called the perithecia produce another spore called the ascospores. As the leaves mature, the leaf tissues in the middle of the circle die, turn whitish and eventually tear off leaving a hole.

The normal conidium is shaped like a baseball bat and possesses two cells. The conidium is about 23-65 x 5-10um in sizes. An important feature is that the conidium is characteristically twisted.

The pycnidiospores are bell-shaped (6-10um long) and of no significance in spreading the disease. The ascospores are also two celled and the size of the ascospores are about 12-20 x 2-5um.

Hosts

M. ulei infects only Hevea species. Among the nine Hevea species tested against several isolates of M. ulei, conidia were produced on H. camargoana, H. guianensis var. marginata, H. spruceana, H. camporum, H. benthamiana and H. brasiliensis but not on H. nitida, H. guianensis and H. rigidifolia. When H. pauciflora was inoculated in the laboratory, only clones PA 31 and CNSG 114 produced a little conidia.

Disease spread

Plants older than four to five years normally change leaves once a year at the onset of the dry season. This leaf change behaviour is very important for epidemic of M. ulei because leaves are only susceptible when they are less than 10 to 15 days old. The spores of M. ulei are disseminated mainly by rain splash or wind. Conidia are in the air mainly between 9.00 and 14.00 o'clock. Ascospores are disseminated in the night, with the peak between 06.00 and 08.00 o'clock. The ascospores are liberated when leaves containing stroma are wetted and kept in the dark at 13 to 16oC. The conidia mainly occur during wet weather especially during the refoliation period while the ascospores are prevalent throughout the year.

Disease control

The disease is threatening the rubber cultivation in the tropical regions of the globe. Until now the disease is restricted to the tropical Latin American countries, Trinidad and Tobago, and Haiti. The disease control has international as well as national components.

Quarantine

Quarantine regulations in Malaysia, Thailand and some other natural rubber producing countries have been strengthened to prevent an accidental introduction of SALB into these countries. The importation of Hevea planting materials from the American tropics is prohibited except for research purposes. Passengers especially those arriving on direct flights are required to fill up plant quarantine declaration card and their baggages are subjected to quarantine treatments e.g. exposure to ultra violet light irradiation.

Resistance

The existence of SALB resistant rubber clones especially of Hevea brasiliensis, H. benthamiana and their hybrids have been reported, but until now this resistance has been overcome by the further development of the pathogen especially the occurrence of new races. Currently, there are no high yielding and SALB resistant clones being commercially planted. Most of the clones are resistant only to some physiological races of M. ulei.

Crown budding

Top-budding of SALB resistant crowns onto high yielding trunks is an important control method for SALB especially in areas of high disease incidence e.g. the Amazon basin. Some clones are recommended for use in crown budding. Based on research and observations in commercial plantations, the clones PA 31 (H. panciflora), IAN 7388, IAN 6486, IAN 6545 and Px should be used especially in the Amazon region.

Escape areas

Siting of plantations in disease "escape areas", defined as areas which experience climatic conditions unfavourable to M. ulei but are suitable for economical rubber tree growth, is the alternative method recommended to control SALB. Escape areas have been identified in Brazil, for example in the Acailandia - Maranhao State and in the Sao Paulo highlands. The absence of SALB in a plantation solely is not sufficient to characterize the region as an escape region. Besides the climatic factors, various other components are associated with the disease. The severity of SALB depends on the climate, the virulence of the pathogen, the susceptibility and density of the host, the occurence of the inoculum, the topography of the plantation and on the tree wintering pattern.

Even in escape regions, it is necessary to plant clones which change their leaves in a very short period and only during conditions not suitable for disease (disease escape). When leaf fall occurs two times per year or in the wet weather conditions, the disease will be favoured.

Chemical control

Effective fungcides for SALB control are benomyl (0.5 g/l), thiophanate methyl (1.0g/l), triadimephon (0.3g/l), mancozeb (3.2g/l), fenarimol (0.024g/l), propiconazole (0.075g/l), triadimenol (0.075g/l, chlorothalonil (3.15g/l), carbendazin (0.9g/l) and triforine (0.15g/l). The presence of benomyl resistant M. ulei strains in Bahia State, Brazil, has recently been shown.

In nurseries, seed and clonal gardens, and young developing plantations, terrestrial sprayers can be used. For plantations up to 7m high, it is possible to use a portable motorized sprayer with centrifuge pumps and with simple adaptations, such as the elongation of the lance of sprayer or of the duct of air flow. The tractor-mounted pneumatic sprayers or atomizers can be used with efficiency in young developing plantations where the terrains are flat. In producing rubber plantations, chemical control is difficult as the trees are 15 to 25m high and the conventional spraying equipments do not reach the top of the canopy. Aerial spraying equipments can be used for mature rubber plantations instead.

Seed and clonal gardens situated in endemic areas must be sprayed weekly in the rainy season and at two weeks intervals during the dry season. For mature and immature plantations spraying if possible, must be done weekly during the refoliation period until the leaves reach their mature state.

Integrated control

Initial studies indicated that the mycoparasite Dicyma pulvinata (Berk & Curt) Arx (Hansfordia pulvinata (Berk & Curt) Hugues) has potential to assist in the control of SALB. Mycorrhiza also increases the resistance of the leaves to M. ulei. Both can be important factors in the integrated plant protection system of SALB. Crown budding involving some or all the trees in a plantation, mixed cropping systems, the use of mycorrhizal fungi and hyperparasites along with well designed plant management measures, are the proposed middle or long term complex system of integrated plant protection in Brazilian rubber cultivation mainly in the hot and wet regions of the Amazon.