The entry says that the leaves spoke every season. This piece is about what they said, in the words of the pathologists who learned to read them, from the first collection on the Juruá in 1900 to the paper of 2014 that gave the fungus its fourth name.
Four names
In August 1900 the German collector Ernst Ule, working on the Juruá in the upper Amazon, picked a rubber leaf with a fungus on it. He collected the same thing on the Juruá Mirim in Acre in 1901 and at Iquitos in Peru in 1902, and in 1904 Paul Hennings in Berlin described the sexual stage from Ule’s material as Dothidella ulei and the pycnidial stage, which he took for a separate organism, as Aposphaeria ulei. He noted, Weir records, that it might be an important parasite; nobody else thought so. When it turned up in the new rubber plantings of British and Dutch Guiana in 1907 it was regarded as a nuisance of the nursery. Kuyper in Surinam described a third form in 1911 as Fusicladium macrosporum and said the injury to mature trees was negligible. Bancroft in British Guiana sent material to Kew in 1913, where it was named Passalora heveae. It took until 1917 for Gerold Stahel in Surinam to trace the hyphae through the leaf tissue and show that the four were one fungus with three kinds of spore, which he renamed Melanopsammopsis ulei. By then it had destroyed the plantations he was studying.
The name went back to Hennings’s Dothidella ulei for Weir’s bulletin of 1926, became Microcyclus ulei in 1962 when Müller and von Arx revised the genus, and stayed so for fifty years, which is why the Ford literature and the backlog of this journal call it that. In 2014 a group at the Federal University of Viçosa in Minas Gerais sequenced it and found that it belonged, unmistakably, among the Pseudocercospora, a large genus of leaf-spot fungi; they introduced the new combination Pseudocercospora ulei under the title “Erasing the Past”, and confirmed by experiment what Stahel had inferred by looking, that the pycnidial spores Hennings had named as a second fungus do not infect at all but serve as spermatia in the fungus’s sex. The organism had been named four times in 110 years, and each name was attached to a stage of a life that only made sense whole.
What it does
The fungus lives on the young leaf. A rubber tree’s new leaves come out soft, reddish and drooping and take about two weeks to harden; it is in those two weeks that the spores, carried on wind, rain, fog and dew, germinate on the surface and the hyphae enter between the cells. Weir’s plate of 1926 shows the sequence: the olive-green velvet of the conidial stage on a young leaf, blackened and curled; the black pycnidia on the mature leaf; the lesions on stem and petiole; the fruit attacked; and on the old leaf the shot-holes and rings of the perithecial stage, where the sexual spores form and are shot into the air to start again. An attacked leaf is shed. A tree can stand that once. In a plantation it does not happen once. Each flush is infected from the spores on the neighbouring trees’ last flush, so that, in Weir’s words, the shoots “terminate in naked stems” and “each successive elongation of the shoot becomes smaller and smaller”; secondary fungi, Diplodia above all, take the weakened branches; and a tree that is not killed is dwarfed and gives nothing.
Two things about the tree make the plantation a trap. The first is that Hevea “winters”: once a year, in the dry season, it drops its leaves and puts out a whole new crown within a few weeks. In a plantation every tree of the same clone does this together, so that for a fortnight an entire estate is nothing but susceptible leaves. The Department of Agriculture’s report of 1955 gave this as the reason Belterra stayed clean for its first four or five years and then did not: the young trees “had not generally reached the age for ‘wintering,’ or annual leaf change, when the predominance of highly susceptible young leaves makes conditions most conducive for epidemic spread of the disease.” The second is that the tree is a creature of the climax forest, slow, long-lived, and not easily moved: it had lived with this fungus for as long as both had existed, and the forest’s arrangement, two hundred feet or more between trees with a hundred other species in between, was the settlement they had reached.
The Guianas
The first people to plant Hevea in rows on its own continent were the British and the Dutch, in their Guiana colonies, from about 1907. Weir’s bulletin gives the sequence. Until 1913 every observer thought the fungus serious only in the nursery. Rorer in Trinidad reported in 1916 that it had been on the island for years and became epidemic in July, August and September, “owing to certain favorable weather conditions”, and that given its record in Dutch and British Guiana it “must now be considered the most serious trouble of Hevea in the American Tropics.” Bancroft in British Guiana found it less common on the coast, “but prevalent in areas of large plantations”, and in 1917 wrote that unless a means of control were found the cultivation of Hevea in Dutch Guiana “would soon prove unremunerative”. Grandin records a Surinam plantation of twenty thousand two-year-old trees destroyed in a single year. Stahel published the most complete account of the organism, and a treatment, in 1917, and the treatment was Bordeaux mixture sprayed on nursery stock, which no one could spray on a tall tree over hundreds of acres. By the end of the war estate rubber in the Guianas was finished, and Weir, surveying the Amazon in 1923, could report the ruins as the clearest proof of his proposition that a fungus does in a pure stand what it cannot do in a forest.
Weir’s chapter on control is the chapter the Ford plan did not read, and it is worth seeing what it offered. Protection: sites on high, windy, well-drained ground, where the leaves dried fast; fertile soil, so that a tree could refoliate; a knowledge of the prevailing winds, so that a new estate could be put upwind of infected forest. Exclusion: planting from seed or cuttings that did not carry the fungus, in regions where it was absent, under quarantine; he noted that trees in Florida, Haiti and Cuba had shown no sign of it. Eradication: cutting out wild Hevea near the estate and clearing fallen leaves, which he thought would help locally and could not be complete. Immunisation: he observed that “very rarely individual trees standing among infected groves are entirely free from the disease”, that such trees could be selected and propagated by cuttings and grafts, and that “Rorer is of the opinion that the only hope of combating the disease is through the development of resistant types.” Then the observation about the site that the plan reversed: that all the plantations of the Guianas had been started with seed from the lower Amazon, “where the ravages of the disease were observed to be most severe”, and that Wickham’s seed, the ancestor of every plantation tree in Asia, had probably come from the susceptible white variety of the lower valley. The clones Weir brought from Sumatra in 1934 were Wickham’s trees come home, and they had never met the fungus.
Why Asia
The reason the fungus is not in Asia is that it did not travel with the seed. Wickham’s 70,000 seeds crossed the Atlantic in 1876 and the seedlings went out from Kew to Ceylon and Singapore as plants, and the fungus, which lives in living leaf tissue and is spread by spores that do not last, was left behind. That accident of 1876 is the foundation of the world’s rubber supply, and it has been guarded since by quarantine: rubber planting material from the Americas is not admitted to the Asian plantations, and the fear in every survey of the industry from Weir’s time to the paper of 2014 is of the day a spore gets through. Asia’s plantations are planted in close rows of one clone, which is precisely the arrangement that would feed the fungus best, and that is why the Viçosa authors could call it “the most serious threat to the natural rubber industry worldwide” in a continent where it has never been seen.
The three-part tree
What Fordlandia contributed, after it had lost its trees, was the one thing Weir’s chapter had set down as hope. After the epidemic of 1935 the estate’s staff noticed scattered trees that had kept their leaves, took budwood from them and established them as clones, and in 1936 Weir grafted their buds, and buds of Hevea guianensis, onto the crowns of some ten acres of seedlings and Eastern clonal trees: a susceptible, high-yielding trunk given a resistant head. The Department of Agriculture’s history of 1955 calls this the first crown-budding in the Western Hemisphere and describes the result in a photograph from Belterra, one grafted tree in full leaf among a row of eight-year-old trees killed or stunted by the blight. From it came the three-component tree: a seedling rootstock, bud-grafted near the ground with a high-yielding Eastern clone, and grafted again at about eight feet with a resistant crown. Belterra’s two million trees were cut off at seven feet and re-headed this way between 1942 and 1945, and the Department carried the technique to its stations from Mexico to Bolivia, where by 1955 the resistant crowns were themselves producing seed that grew into nurseries which did not need spraying. The method was still too complicated for a small farmer, the report admitted, and the rubber it made could not compete with Asia’s price. But it was a method that had read the leaf, and the Brazilian agronomist who took over Fordlandia called the estate, for that reason only, “the true cradle of the technique of double grafting”.
Sources
- James R. Weir, A Pathological Survey of the Para Rubber Tree (Hevea brasiliensis) in the Amazon Valley, USDA Department Bulletin 1380, 1926: the historical section on the fungus from Hennings to Stahel, Rorer and Bancroft, the sections on protection, exclusion, eradication and immunisation, the remark on the lower Amazon seed and Wickham’s white variety, and Plate XV. archive.org
- Robert D. Rands and Loren G. Polhamus, Progress Report on the Cooperative Hevea Rubber Development Program in Latin America, USDA Circular 976, 1955: wintering, the scattered survivors, Weir’s crown-budding of 1936, the pollarding at Belterra, the three-component tree and Figure 4. archive.org
- Braz Tavares da Hora Júnior, Davi Mesquita de Macedo, Robert Weingart Barreto, Harry C. Evans, Carlos Raimundo Reis Mattos, Luiz Antonio Maffia and Eduardo S. G. Mizubuti, “Erasing the Past: A New Identity for the Damoclean Pathogen Causing South American Leaf Blight of Rubber,” PLoS ONE 9 (8), e104750, 2014: the names, the phylogeny, the spermatial function of the pycnidial stage, and the threat to Asia. doi:10.1371/journal.pone.0104750
- Greg Grandin, Fordlandia, 2009, for Weir’s report of 1935 and the Surinam plantation of twenty thousand trees, at second hand. archive.org