Читать книгу Twelve Diseases that Changed Our World - Irwin W. Sherman - Страница 18

The perils of a single-crop economy have seldom been better illustrated than in Ireland in 1845 to 1849, for at that time without the potato the Irish economy could not survive for very long. While other regions of Europe may have been able to turn to alternative food sources, this was not possible for the Irish. The potato blight was an ecological disaster compounded by the failure of government. Some consider it to be equal to the holocaust. Although theories as to the cause of late blight were many, including an act of God, introduction of the steam locomotive, and excessive uptake of soil water that the potato could not expel, it was the Reverend Miles J. Berkeley who in 1846, after making careful microscopic examination of diseased plants and seeing a whitish felt on the leaf surface (resembling that in moldy bread), proposed that it was none of these but instead was a fungus. Berkeley was mocked, and his contention gained little support. Indeed, the prevailing opinion of the time was that a cold and damp miasma resulted in blight.

A critical question regarding blight was: which came first, decay followed by fungus or fungus and then decay? In 1861 the great German biologist Anton de Bary clearly showed that late blight was caused by the fungus he named Phytophthora infestans, the “plant destroyer.” To confirm the role of the fungus, de Bary did a simple experiment: he grew healthy potato plants in pots, divided them into two groups, and deliberately dusted spores from the plants with blight onto the moistened leaves of a group of healthy plants; he left the other group (“controls”) alone, making certain that spores could not reach them. Both groups were exposed to a cool, moist environment where the miasma could do its work. In a few days the telltale sign of blight—spots of decay—appeared on the leaves of the fungus-inoculated plants. The control group showed no sign of disease. Clearly, potato plants did not rot because of a miasma or because they took up too much water. de Bary suggested that the microscopic spores ride on the stormy winds and that blight results when rain splashes them on to the leaves. In this way the infection spreads from plant to plant, field to field, and country to country. The significance of de Bary’s work led to a novel understanding of sickness: parasites can be the cause of a disease. Today de Bary and Berkeley are rarely recognized for their pioneering work, yet their experiments anticipated Louis Pasteur’s germ theory of disease by nearly a quarter of a century.

In late blight, the signs of impending disaster first appear on the leaves in the form of brown-black spots. Under moist conditions the spots enlarge quickly and the plant has a pungent odor. A white fuzzy growth, barely visible, appears in the spots; under the microscope, these are seen to contain the long tube-like threads first seen by Berkeley. The threads (called hyphae) divide and twist like snakes to form an extensive network; they penetrate the plant tissues and act like “soda straws,” allowing the fungus to “drink” the rich nourishing sap of the potato until the leaf and stem are literally sucked dry. This takes only 3 to 5 days. At the tip of each filament a swelling develops; within the swelling, microscopic spores are produced. Millions of spores can be produced on an infected leaf, and each spore is so tiny that 500 of them would be no larger than the period at the end of this sentence. The spores germinate, giving rise to either tube-like threads or swimming spores that can also form threads. The tubes enter the leaf through its microscopic pores (stomata), or the leaf tissue is eroded by digestive enzymes released by the hyphae. The cycle of spore formation and germination is favored by moist, cool conditions. But how does blight on the leaf produce rotten potatoes? de Bary buried healthy potatoes in the soil, shook the spores from blighted leaves on the soil surface, and gently watered them as if it were raining. The spores washed down, and when the potatoes were dug up, they too were blighted. Clearly at harvest time millions of spores are washed from the leaves so that the fleshy tuber itself becomes infected. Its skin is discolored with brown-purple blotches resembling bruises, and as the microscopic threads of the fungus penetrate deeper, the tuber begins to rot. Dry rot of tubers by Phytophthora is followed by wet rot of the potato due to other microbes in the soil.

One of the mysteries concerning the late-blight fungus was how it was able to survive the cold of winter. Did it overwinter in the soil or in the tuber? Although observations of the hyphae under freezing conditions showed that they were too fragile to withstand low temperatures, within the tuber itself the fungus was protected and able to survive very low temperatures. Since only the tuber was kept through the winter, the blighted potatoes provided the source of infection for the next season’s crop. Because in a single growing season it is possible to get many cycles of late blight, it is considered to be a compound-interest disease with great powers of amplification that can lead to an explosive outbreak. As a result, an entire potato crop can be quickly destroyed. Indeed, one infected tuber per 2.5 acres can cause an epidemic of late blight, especially when the weather is right: cool, with high rainfall and humidity.

Where did the “plant destroyer” come from in the first place? Most likely P. infestans was introduced with tubers brought to Belgium from Peru. The disease was first reported in Belgium in July 1845, and it soon spread throughout Europe. Once introduced, it became a menace. Although today topical fungicides such as Bordeaux mixture (made from copper sulfate and lime and developed in the 1890s) may be sprayed on leaves before the disease begins or Ridomil, a systemic fungicide developed in the 1970s, may be applied to the soil, none of the fungicides eradicate blight—they simply reduce the amount of defoliation so that tubers can be harvested in respectable quantities. However, in 1845 to 1849, none of these were available.