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Is bioterrorism a threat to agriculture?

"We can't get inside the heads of terrorists because we apparently can't 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 going to realistically protect American agriculture, we're going to have to acknowledge our incredible vulnerabilities. Our fields are not under lock and key, and they often are 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 bio-terrorism. Once any risks are identified, the magnitude of these risks must be assessed, and then the probability of each risk 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 anywhere between 50 and 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 of more than 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 non-exportable," 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."

According to the USDA Animal and Plant Health Inspection Service (APHIS), the disease was first reported in 1931 in the Indian State of Haryana, and has since been found in India, Pakistan, Iraq, Afghanistan and Mexico.

Isolated cases have also been reported in some counties in Texas, California, and Arizona. These areas have since been quarantined by USDA to prevent a spread of the disease.

Spread mainly by the planting of infected seed, Karnal bunt cannot be easily detected in infected plants because diagnosis of the disease requires the grain be removed from the head and examined.

"When checking crops for Karnal bunt, wheat growers should look for bunted kernels that are fragile, dark in color, and fishy smelling," APHIS reports.

"The kernel usually remains whole, although part of the germ may be eroded.

Cracks in the surface reveal a black powdery spore mass within the endosperm at the embryo end of the kernel or along the kernel groove."

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, or Globodera rostochiensis, and Gray leaf spot disease, also known as 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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