Thursday, 14 February 2019



Even when humans lived as hunters or nomads and their food consisted only of meat or leaves, fruit, and seeds, which they picked wherever they could find them, plant diseases took their toll on hunted animals and on humans. Plant diseases caused leaves and shoots to mildew and blight, and fruit and seeds to rot, thereby forcing humans to keep looking until they could find enough healthy fruit or food plants of some kind to satisfy their hunger. As humans settled down and became farmers, they began growing one or a few kinds of food plants in small plots of land and depended on these plants for their survival throughout the year. It is probable that every year, and in some years more than in others, part of the crop was lost to diseases. In such years food supplies were insufficient and hunger was common. In years when wet weather favored the development of plant diseases, most or all of the crop was destroyed and famines resulted, causing immense suffering and probably the death of many humans and animals from starvation. It is not surprising, therefore, that plant diseases are mentioned in some of the oldest books available (Homer, c. 1000 b.c., Old Testament, c. 750 b.c.) and were feared as much as human diseases and war.

Efforts to control plant diseases were similarly hampered by the lack of information on the causes of disease and by the belief that diseases were manifestations of the wrath of God. Nevertheless, some ancient writers, e.g., Homer (c. 1000 b.c.), mention the therapeutic properties of sulfur on plant diseases, and Democritus (c. 470 b.c.) recommended controlling plant blights by sprinkling plants with the olive grounds left after extraction of the olive oil. Most ancient reports, however, dealt with festivals and sacrifices to thank, please, or appease a god and to keep the god from sending the dreaded rusts, mildews, blasts, or other crop scourges. Very little information on controlling plant diseases was written anywhere for almost 2000 years.

During the two millennia of fatalism, a few important observations were made on the causes and control of plant diseases, but they were not believed by their contemporaries and were completely ignored by the generations that followed. It was not until about a.d. 1200 that a higher plant, the mistletoe, was proposed as a parasite that obtains its food from the host plant, which it makes sick. It was also noted that the host plant can be cured by pruning out the part carrying the mistletoe. Nobody, however, followed up on this important observation.

Plant diseases as the wrath of gods — Theophrastus

The climate and soil of countries around the eastern Mediterranean Sea, from where many of the first records of antiquity came to us, allow the growth and cultivation of many plants. The most important crop plants for the survival of people and of domesticated animals were seed-producing cereals, especially wheat, barley, rye, and oats; and legumes, especially beans, fava beans, chickpeas, and lentils. Fruit trees such as apple, citrus, olives, peaches, and figs, as well as grapes, melons, and squash, were also cultivated. All of these crop plants suffered losses annually due to drought, insects, diseases, and weeds. Because most families grew their own crops and depended on their produce for survival until the next crop was produced the following year, losses of any amount of crops, regardless of cause, created serious hunger and survival problems for them. Occurrences of mildews, blasts, blights on cereals, and legumes are mentioned in numerous passages of books of the Old Testament (about 750 b.c.) of the Bible. Blasts, probably the smut diseases, destroyed some or all kernels in a head by replacing them with fungal spores. Blights, probably rusts, weakened the plants and used up the nutrients and water that would fill the kernels, leaving the kernels shriveled and empty.

Mention of plant diseases is found again in the writings of the Greek philosopher Democritus, who, around 470 b.c., noted plant blights and described a way to control them. It was not, however, until another Greek philosopher, Theophrastus (300 b.c.) made plants and, to a much smaller extent, plant diseases the object of a systematic study. Theophrastus was a pupil of Aristotle and later became his successor in the school. Among others, Theophrastus wrote two books on plants. One, called “The Nature of Plants,” included chapters on the morphology and anatomy of plants and descriptions of wild and cultivated woody plants, perennial herbaceous plants, wild and cultivated vegetable plants, the cereals, which also included legumes, and medicinal plants and their saps. The other book, called “Reasons of Vegetable Growth,” included chapters on plant propagation from seeds and by grafting, the environmental changes and their effect on plants, cultural practices and their effect on plants, the origin and propagation of cereals, unnatural influences, including diseases and death of plants, and about the odor and the taste of plants. For these works, Theophrastus has been considered the “father of botany.”

The contributions of Theophrastus to the knowledge about plant diseases are quite limited and influenced by the beliefs of his times. He observed that plant diseases were much more common and severe in lowlands than on hillsides and that some diseases, e.g., rusts, were much more common and severe on cereals than on legumes. In many of the early references, plant diseases were considered to be a curse and a punishment of the people by God for wrongs and sins they had committed. This implied that plant diseases could be avoided if the people would abstain from sin. Nobody, of course, thought that farmers in the lowlands sinned more than those on the hillsides, yet Theophrastus and his contemporaries, being unable to explain plant diseases, believed that God controlled the weather that “brought about” the disease. They believed that plant diseases were a manifestation of the wrath of God and, therefore, that avoidance or control of the disease depended on people doing things that would please that same superpower.

In the fourth century b.c.; the Romans suffered so much from hunger caused by the repeated destruction of cereal crops by rusts and other diseases that they created a separate god, whom they named Robigus. To please Robigus, the Romans offered prayers and sacrifices in the belief that he would protect them from the dreaded rusts. The Romans even established a special holiday for Robigus, the Robigalia, during which they sacrificed red dogs, foxes, and cows in an attempt to please and pacify Robigus so he would not send the rusts to destroy their crops.

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