Viruses, Bacteria, and Fungi

Including a section on viruses, bacteria, and fungi in a chapter on plants and human welfare is contrary to a five-kingdom system, wherein none of these are considered plants. But because many of these significantly affect human welfare, they are included here for the want of a better place.

Viruses are known chiefly for their capacity to produce disease. A number of human ailments are caused by viruses; polio, influenza, chicken pox, smallpox, measles, mumps, yellow fever, warts, and the common cold are among these. Much evidence points to viruses as also causing some cancers.

A number of viral diseases affect both wild and domestic animals: the foot and mouth disease of cattle; rabies; distemper; hog cholera; psittacosis (parrot fever); and Newcastle disease (which infects fowl).

Nearly one thousand kinds of plant diseases are caused by viruses. Mosaic diseases tend to inhibit chlorophyll production in a definite pattern, often along the veins. There are mosaic diseases that affect beans, cucumbers, peas, peaches, sugar beets, sugar cane, tobacco, turnips, rice, potatoes, tomatoes, and wheat. Rice necrosis stunts rice plants. Aster yellows, potato paracrinkle, potato leaf role, sugar beet curly top, and tomato bushy stunt are all mosaic diseases. Such diseases are generally transmitted from one plant to another by leaf hoppers and aphids. Nematodes also sometimes transmit viral diseases from plant to plant. Viruses do no harm to these insect vectors. Viral diseases can also be transmitted mechanically, by injury, or grafting.

Although viruses have a bad reputation because of the diseases they cause, viruses sometimes have desired effects. For example, the rice necrosis mosaic virus, which stunts rice plants, inexplicably improves growth of jute plants (used in making burlap sacks and rope). And the highly esteemed Rembrandt tulip derives its distinctive markings from a virus.

Bacteria play a significant role in human events. Being decomposer, they are involved in the recycling of organic materials. They are able to convert atmospheric nitrogen into compounds usable by plants. Bacteria are used in the production of cheese, yogurt, sauerkraut, and pickles. They also participate in making vinegar and acetic acid. Bacteria also deserve praise for their role in making useful antibiotics such as bacitracin, tyrothrycin, subtilin, and polymixin.

At the same time that bacteria are essential to our existence, they can also present hazards to human welfare. Tuberculosis, cholera, anthrax, diphtheria, gonorrhea, and tetanus are all caused by bacteria. Food poisoning is caused by the bacterial organisms Salmonella, Staphylococcus, and Clostridium. Rickettsiae, minute bacteria that dwell in the bodies of arthropods, cause two serious diseases: Rocky Mountain Spotted Fever and Tsutsugamuchi Fever.

Many plant diseases can be traced to bacteria, including fire blight, soft rot, wilts, and crown gall. They cause the death of young apple and pear trees and destroy vegetables in storage. Citrus canker is caused by the bacterium Xanthomonas.

Fungi, like viruses and bacteria, are both friend and foe to humans. They are just as necessary for the continued existence of human life as are the green plants. They are decomposer that unlock carbon dioxide and restore it to the atmosphere, where it can again be used in photosynthesis. If fungi were selective in what they decompose, they might be less scorned. But just as they decompose old leaves, dead trees, and garbage, they also decompose cloth, paint, leather, waxes, jet fuel, wire insulation, cartons, photographic film, and timber. They also are enemies of food producers, growing on bread, fruits, vegetables, and meat. Ustilago zeae, in particular, has taken a great toll on corn crops.

As recounted in Phycomycetes, the tragic Irish Potato Famine, which was caused by the fungus Phytophthora infestans, took millions of human lives. Ergotism, caused by the fungus Clauiceps purpurea, was a scourge of the Middle Ages.

While there are 20,000 species of pathogenic fungi, there are also many valuable fungi forms. Perhaps as many as 5,000 species are economically important. Yeasts play a role in the manufacture of bread and the production of alcohol. The mycorrhiza associated with plant roots are necessary for the welfare of certain trees. And some fungi are esteemed as food.

Cyclosporine, a so-called “wonder drug,” is isolated from a soil-dwelling fungus. This drug suppresses undesirable immune-system responses to organ transplants. While other drugs do this as well, they have the undesirable side effect of also killing bone marrow cells, thereby leading to leukemia. Cyclosporine is thus of enormous benefit in organ transplants.