Peter van Dijk, KeyGene
South American Leaf Blight (SALB) is a fungal disease of the rubber tree Hevea brasiliensis. It is caused by the ascomycete Microcyclus ulei. This fungus makes it impossible to produce rubber from Hevea plantations in South America. So far the fungus has not reached South East Asia, where nowadays 90% of all the natural rubber comes from. This year it is 100 years ago that the devastating effect of the fungus on South American rubber plantations became obvious for the first time. It was first observed in Surinam, which was a Dutch colony in those days. It is interesting to read the newspapers from that period. What is especially striking is how fast the fungus wiped out all plantations in Surinam. It shows what would happen if SALB would reach South East Asia. So far this has not happened, but as Wade Davis wrote in One River: “A sword of Damocles hangs over the industrial world”.
Figure 1: The development of the rubber production and prices between 1900 and 1935 in Brazil and South East Asia.
It is well known that in 1876 the British adventurer Henri Wickham smuggled 70,000 Hevea seeds from Brazil to England from where they were transported to South East Asia. Until 1907 all Hevea rubber was harvested from wild Hevea trees in Brazil (see Figure 1). Rubber tappers have to visit the trees twice a day and because of the large distances between the Hevea trees in the rain forest (about 1-2 trees per hectare) this was very inefficient. But there was a high demand for rubber and the prices for rubber were rocketing which created a lot of wealth in the North East of Brazil. The famous opera house of Manaus is exemplary for this economic boom (Figure 2).
However, from 1907 onwards the trees from the plantations in South East Asia were old enough to begin producing rubber. Because at a much higher density than the Brazilian wild trees, the production costs were much lower. At the same time the prices for rubber dropped because of the increased production.
Figure 2: The Opera house in Manaus. A relict of a wealthy rubber history
The Dutch had started rubber plantations on Java and Sumatra, based on the Wickham seed collection. So why not start rubber plantations in Surinam, which had a comparable climate and was after all, the native range of Hevea. In 1908 the first Hevea plantations were set up in Surinam. The growth of the young trees was very good and comparable to growth of such trees in the Malayan archipelago and many new trees were planted. Early 1912 about 250.000 trees of 1-4 years had been planted in Surinam.
Leaf Blight was reported in Surinam for the first time in 1910, without causing much damage. In 1911 also no alarming diseases were reported according to the newspapers. Apparently no major diseases occurred between in 1914 and 1915, since no reports can be found in the newspapers. However, in 1915 the situation changed dramatically and leaf blight became disastrous. The leaves of infected plants show many small lesions, become curly, dry out and drop (Figure 3). The canopy of the infected trees becomes less dense and finally the tree loses all its leaves. Between June and September of that year considerable damage occurred at four plantations. Between 17 and 33 percent of the 4-year old trees had died. At other plantations also trees were lost but not so many. A year later, in 1916, it is concluded that at the Slootwijk plantation due to the severe leaf blight of the 189 Ha. at least 120 Ha. was useless for rubber production. At another plantation along the Commewyne river of the 20,000 5-year old trees, only 3,900 survived that year.
In 1916 no new rubber trees were planted due to the devastating effect of the disease and the Hevea plantation culture in Surinam is abandoned. Nevertheless, a plantation of the “Compagnie des Mines d’Or" along the Lawa river was still healthy. However, in spring 1918 the leaf blight disease appeared there for the first time, apparently by infection from wild Hevea trees in neighboring forest. Within half a year one-third of the 40,000 Hevea trees died and the plantation was abandoned. In 1919 the Dutch phytopathologist Stahel concluded that the South American Leaf Blight disease made Hevea plantation cultures in South America impossible. Two expensive attempts by the Ford company (Fordlandia) became a complete failure, proving that Stahel was right.
It is already known that the rubber trees in SE Asia are highly vulnerable to SALB. In plantations the densities are very high and the genetic diversity is very low because the individuals are clonally propagated. This is the ultimate monoculture. It is well known that monocultures are highly vulnerable to pests. To prevent spread of SALB there are no direct flights between SE Asia and South American countries were SALB occurs. The FAO has developed guidelines to minimize the risk of spread of SALB. So far SALB has not yet reached South East Asia. However, there are numerous examples of outbreaks of pathogens after introduction decimating crops, for example the Irish potato disease, the Dutch elm disease, and today the banana diseases. An additional strategy to minimize the risk for the rubber industry and the consumer is to diversify the number of sources of natural rubber which is exactly what the DRIVE4EU project is aiming at.
Figure 3: SALB infected leaves with lesions. From Stahel’s 1917 report (De Zuid-Amerikaansche Hevea-Bladziekte).