Stage 4: Wider reading :

Unit 4: Plants And Microorganisms

Plants And Microorganisms

In "Famine on the Wind", Carefoot and Sprott (1) present a detailed description of the profound influence the important fungus diseases, and a few of the viruses, have had upon the history of many countries, operating by their effects on agricultural productivity.

Man's increasing demands for food and for other valuable vegetable products, like rubber, and timber have frequently been frustrated by insect pests, fungi, bacteria or viruses.

Given the correct seasonal conditions, these organisms can work havoc upon agricultural productivity.

There are numerous examples, such as: ergot of rye, wheat rust, the grain smuts, potato blight, downy mildew of grapes, coffee rust and banana diseases;

the bacterial diseases that attack silk worms, vegetables, citrus and trees;

and viruses that cause systemic diseases such as barley yellow dwarf or leaf roll of potato.

Many are carried by wind and others by secondary agents, such as the aphids that transmit the tobacco mosaic virus, although these too may be borne on the wind.

Three fungal diseases may be chosen as examples of historic interest, ergot of rye, mildew of grapes and potato blight.

The conditions which favour the development of the purple cockspurs on rye are wet, heavy morning fogs in the spring time.

When the prevailing climatic conditions are of the right pattem, the fungus (Claviceps purpurea) spreads explosively through the fields, until in some bad-ergot years half or more of the rye heads carry the ergot.

In the past, it was customary for black rye bread to be the staple food of the common people in southern France, and in inclement years the prevalence of the ergot led to a serious poisoning, termed 'Holy Fire" or "St. Anthony's Fire"

The fever attacks the central nervous system.

It tore away men's reason and turned them into screaming, gibbering brutes, or into hopeless cripples as their fingers, toes, arms and legs blackened and corroded from a dry gangrene.

Only death gave relief.

Holy Fire was first reported from the Rhine Valley in the Kingdom of the Franks in +857. Another epidemic spread through France in 1039, and led a certain Gaston de la Valloire, to establish hospitals for the victims.

These were dedicated to St. Anthony and hence the alternate name.

More and more hospitals were established to treat the victims as the disease raged on whenever the spring time was moist and foggy.

The scourge was greatly feared, and its cause remained unrecognised until 1670 when a French doctor found it was associated with the eating of rye bread.

It has been found that only 2% ergot contamination of the bread is likely to cause an epidemic.

The disease even had significance historically, for Peter the Great failed in a bid to open a route to the Black Sea because his army was badly hit by the Holy Fire, when it entered the Volga Delta and affected horses and men.

The cause of the sickness has now been traced to powerful alkaloid drugs, one of which is lysergic acid diethylamide or the LSD of modern drug addiction fame.

Downy mildew of grapes is a disease of more recent origin, but the growing of grapes has an ancient history.

The grape is an ancient plant found in fossil form from Alaska to China, but in greatest abundance around the Caspian Sea.

Its origin is placed at somewhere in the valleys of the Caucasus.

From thence it spread westward through Asia Minor into Thrace and eastward into Turkestan.

The main species still grow wild in the forests around the Caspian.

There are about 50 species, but the most useful is Vitis vinifera, whose sweet, succulent berries, charged with bouquets and flavours are carefully tended on every continent of the world.

Wine from grapes is mentioned in the histories of the Near East as far back as 4000 years ago.

It was in -500 that the grape first came to France.

In 1845, mildew was observed on grapes in England and then, in 1848, it was seen on vines near Versailles.

Within five years it had reached every vineyard in Europe, North Africa, and the islands of the Mediterranean.

By that time there were 7 million hectares of vines in Europe, and financial losses were therefore enormous.

At first, the mildew was controlled with a lime sulphur spray, but later it was found in Bordeaux that a mixture of copper sulphate and lime gave the most effective control.

The best control, however, is a climatic one.