3. Future threats may involve novel pathogens.
Declaring Biowar on Crops
While the development of bioweapons by the United States and Russia in past decades focused on human pathogens, there is another potential target which also represents a real dangerthe crops we eat. Cereal grains feed most Americans, and most of the people on earth. Fully one half of the calories consumed by humans are obtained from wheat, rice, and corn. A bioweapon targeted at cereal grains could, if used successfully, have a staggering impact.
Plants are certainly subject to as many diseases as humans. Roughly one-eighth of crops worldwide are lost to disease each year. There are four major groups of plant pathogens that affect crop plants:
1. Pseudomonads. These soil bacteria cause most important plant diseases.
2. Pathogenic fungi. A variety of rusts and smuts attack cereal grains.
3. Mycoplasmas. Transmitted by insects, mycoplasmas infect corn and many kinds of citrus.
4. Viruses. Over 600 plant diseases are caused by viruses, often slowing growth rather than killing.
Designing a Crop Plant Bioweapon
For maximum impact in North America, a plant bioweapon should be directed against corn or wheat. Both of these cereal grain crops are subject to serious fungal diseases.
Corn. A smut caused by the pathogenic fungus Ustilago maydis infects the cells of growing corn plants, causing the infected cells to form large growths called galls. Serious infestations often lead to the total loss of ears, so that the corn plant produces no useful food. The fungus responsible for the disease grows in the soil during the winter; in the spring, its spores are spread by the wind. Landing on the leaves sheathing corn ears, the spores germinate and infect the plant.
Wheat. A rust caused by the pathogenic fungus Puccinia graminis attacks wheat, forming reddish lesions on the stem and leaves. Puccinia spores germinate on the leaves and form hyphae that enter the plant interior through stomata (tiny pores on the underside of leaves). As the fungus grows within the wheat plant, it erupts with the reddish lesions characteristic of rusts, releasing spores that can travel 100 miles to infect other wheat plants. Puccinia is a very damaging pathogen of commercial wheat in the United states. Over one million tons of wheat annually are lost to stem rust in the United States (figure 8).
How a Puccinia epidemic starts.
The spores of either Ustilago or Puccinia spread readily on the wind. Weaponizing the spores of either fungus would employ much the same technology as used with Bacillus anthracis spores. Techniques for propagating large cultures of the fungi would need to be developed, and ways found to induce massive spore formation. Spores could be converted into easily-dispersed particles by freeze-drying, milling, and treatment with charge-neutralizing detergents, just as has been done with anthrax.
Puccinia is a particularly dangerous bioweapon candidate, as it possesses the advantages of both anthrax (its spores are lethal to its target, and the spores are easy to convert into a stable dry powder) and smallpox (infections are self-propagating, spreading from a single focus of infection to epidemic proportions). Like anthrax, many subspecies of Puccinia are knownover 200 have been collected and described. This makes it particularly difficult to breed wheat that is Puccinia-resistant; resistance to one subspecies need not confer resistance to others.
The spores of pathogenic fungi that attack corn or wheat might be effective bioweapons directed against key American crops.