Mar 9 2006
Natural rubber is currently technically unavoidable for some purposes, despite the competition from synthetic rubber. Most notably, it accounts for 75% of the rubber used in the tyre industry. However, although Hevea brasiliensis originated in the Amazon Basin, rubber growing has only really developed in Asia and Africa. In Latin America, “South American leaf disease”, caused by the fungus Microcyclus ulei, has until now totally prevented the development of rubber growing, or at least almost totally: rubber trees have become resistant to the fungus in some areas of Amazonia. However, to date, there have never been any varieties that combine parasite resistance and productivity. This could be disastrous for rubber growing worldwide were the fungus to be introduced accidentally into Asia or Africa.
Resistant and high-yielding rubber plants in quarantine at CIRAD
The threat is on the verge of being overcome. In late December 2005, plants of thirteen high-yielding rubber varieties resistant to Microcyclus were shipped to CIRAD from Michelin’s Itubera estate in Brazil. They are now in quarantine in a confinement zone, where they will remain for two years and undergo a range of tests to ensure that they do not carry any spores of the parasite.
These varieties are the first major step forward in over twenty years’ research at CIRAD, working since 1992 with Michelin, in Montpellier, French Guiana and Brazil. They are controlled crosses produced by hand pollination, and have been assessed for twelve years at Michelin’s Itubera estate. The process is continuing, crossing very high-yielding varieties that are susceptible to the disease, such as those currently grown in Asia and Africa, with resistant or highly tolerant trees from the Amazon Basin that are not so high-yielding*. Every year, during the short flowering period, researchers remove the stamens from the flowers of one variety so as to inseminate the flower by hand with the pistils of another. The resulting seeds are then germinated to produce thousands of seedlings that will grow in the presence of the disease. Those that resist strong parasite pressure in the field are selected based on their productivity after five to seven years. New plants are produced each year.
The plants that have been in the CIRAD glasshouses for the past few weeks are the first varieties produced under this research programme to reach the pre-development stage.
A new phase of tests in metropolitan France and several African and Asian countries
The latest phase of tests should provide answers to several questions. While the varieties are both resistant and high-yielding at Michelin’s estate in Bahia, will they remain so under other environmental conditions, particularly in Asia and Asia, and even in other parts of South America? CIRAD’s Microcyclus ulei fungus collection in Kourou, French Guiana, which includes several strains from different countries, could help to answer that question. There is also another question: what are the risks of the plants being susceptible to other parasites?
The first step is to ensure that the parasite was not introduced into France during transport, despite all the precautions taken. The planting material was transferred in the form of budded stumps of the thirteen genotypes involved. Before they left Brazil, they were given pesticide and fungicide treatments against Microcyclus ulei but also other parasites, as laid down in French law. On arrival, they were potted in the CIRAD glasshouses in Montpellier. They are now growing, and testing will begin in the next few months, once they have leaves. The parasite may not immediately be visible, which is why non-resistant control plants have been placed next to the test plants. If the plants pass this first test, a second round of budding should rule out any risk of “dormant” spores. If this stage is successful, the plants will be sent to partner research organizations in Africa and Asia, where they will again be placed in quarantine and undergo further tests. Trees of varieties created subsequently in Brazil will be tested in the same way each year.
Improving the breeding technique
Alongside this, researchers are working to improve the breeding technique, and have plans to use genetic markers. We now know which parts of the rubber genome are involved in resistance. However, things are not as simple as they look. Natural resistance to Microcyclus in rubber uses mechanisms that involve a large number of genes. Moreover, the fungus has already proved capable of overcoming resistance. What are the mechanisms involved? The answers that researchers manage to come up with in the coming years should shed light on how resistance develops and make it easier to breed resistant individuals.
* The rubber varieties currently grown in Asia and Africa in fact originated from a very small part of Amazonia. In the late 19th century, a British planter took tens of thousands of rubber seeds back to Kew Gardens. The resulting trees were shipped to Ceylon and Singapore, triggering the Asian commodity chain. Asia now produces 93% of the world’s natural rubber. It is Thailand that leads the market, with three million tonnes or 35% of global output. Africa accounts for 4% and South America the remaining 3%.