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Bioterrorist threat to farms

Protecting our food supply against potential bioterrorist threats is something that prior to six months ago few people thought about, but now is likely to make headlines on the nightly news.

“We can't get inside the heads of terrorists because we can't apparently think like they can. Clearly someone can think of things we thought were unthinkable prior to Sept. 11 though, and a potential adversary would have to look at America as a target-rich society,” says Mark Weaver, a plant pathologist with USDA's Agriculture Research Service in Stoneville, Miss.

“If we're to protect American agriculture, we're going to have to acknowledge our incredible vulnerabilities. Our fields are not under lock and key, and they are often located in remote areas. To make matters worse, we've certainly helped our enemies by giving them a huge target,” he says.

Weaver, who considers American agriculture “highly vulnerable,” suggests focusing on probable targets instead of protecting everything at any cost.

To do that, he says, the agricultural industry must first identify any potential risks of bioterrorism. Once any risks are identified, the magnitude of these risks must be assessed, and then the probability of each risk must be determined.

At the top of Weaver's list of potential risks is the introduction of yield-robbing pathogens into American agricultural production. Other possible candidates to watch for include nematodes, funguses and mycotoxins.

One exotic pathogen that could substantially reduce soybean yields if introduced into the United States is soybean rust, also known as Phakopsora pachyrhizi.

“Soybean rust is the major pathogen limiting soybean production in Asia. It is a real threat because it could effectively limit soybean production in the United States,” Weaver says.

Originating in Asia and Australia, soybean rust is a fungus which causes yield losses of 50 to 100 percent. Infection under cool, wet conditions usually takes place within six hours, with rust symptoms appearing within 10 days. Reported symptoms are water spots on lower leaves progressing into reddish-brown or rust-colored lesions.

“Yield reductions of 10 to 30 percent are typical where the Asiatic strain is found, and yield reductions over 90 percent have been reported,” reports Ohio State University plant pathologist L.V. Madden. “The pathogen has been in Puerto Rico for some time, but the strain is less aggressive than the Asiatic one. However, the Asiatic strain is now in South America.”

Another disease caused by an exotic pathogen is Tilletia indica, which causes Karnal bunt disease in wheat.

A fungal disease, Karnal bunt can cause infected plants to produce less grain and can lessen the quality of any grain produced. “This is a small grain-quality issue, but it is a major regulatory issue because it makes wheat nonexportable,” Weaver says.

Madden adds, “Karnal bunt is an interesting case because the disease has virtually no impact on crop production, yet is important because of international trade restrictions on wheat from areas where the disease occurs.”

Other chronic yield-robbers with the potential to harm U.S. crop production if introduced by terrorists include the difficult-to-control Golden cyst nematode (Globodera rostochiensis) and Gray leaf spot disease (Cercospora zeae-madis).

A potential threat also exists from mycotoxins, which can cause inflammation, chronic lesions, nausea, and vomiting in humans. Mycotoxins have been suspected by some as the cause of the “yellow rains” reported during the Vietnam War.

Despite the potential for bioterrorism, Weaver says, “All is not doom and gloom. Trying to do this clandestinely would be very difficult. We've got good defenses, and even the worst-case scenario likely would be caught quickly and controlled effectively.”

Standing in the way of terrorist threats is the scientific expertise necessary to produce the inoculum, and then the secondary metabolite, or toxins, necessary to infect an agricultural crop. “This is not a simple system. It requires a certain amount of finesse and expertise, in addition to the required rapid and efficient deployment of the inoculum,” Weaver says.


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