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ANAHEIM, Calif. - The resistance of human pathogens to antibiotics may get the headlines, but a similar and perhaps even more serious problem exists with crops losing their genetic resistance to plant pathogens.

This problem gets far less attention but threatens the global food supply, said an expert who spoke Sunday at the annual meeting of the American Association for the Advancement of Science.

"Plant pathogens are continuing to evolve and overcome the natural resistance found in some of our most important food crops around the world, including wheat and rice," said Christopher Mundt, a professor of botany and plant pathology at Oregon State University.

"It's a constant race to develop new crop varieties that can produce the quality and yield we want while still being resistant to plant diseases," Mundt said. "But it's a race we can't afford to lose."

The problem, he said, is that so much remains unknown about the fundamental genetic processes involved in plant pathogens that mutate, evolve and overcome the resistance of various crop varieties. A better understanding of those processes would make the battle easier, he said.

At this point, researchers are trying to understand the nature of genetic mutations that cause pathogens to become more virulent and the effect that such mutations have on the fitness of pathogens, Mundt said.

Scientists used to think that they could explain pathogen evolution by understanding a relatively small number of genes. Now, it is becoming apparent that there may be a larger number of genes, some with individually small effects, that interact to determine pathogen evolution.

"Unfortunately, scientists cannot afford to find out exactly what is going on before taking action," Mundt said. "What we have to do is work on the basic science at the same time we apply the best techniques we have available at the moment to deal with the problem."

There have been some success stories, Mundt said. In the early 1950s stem rust of wheat sometimes destroyed up to half of the wheat crop in the central United States, but some new varieties were developed that have provided genetic resistance for a long time.

But the fight never really ends.

In Oregon, Mundt said, some wheat diseases still cause 50 percent crop losses, and reductions of yield in the 10-20 percent range are common.

Blast, a pathogen of rice, can form new pathogens the rice plant is unable to deal with in as little as two to three years, Mundt said. And other self-pollinated crops such as this which have less genetic variability continue to challenge scientists and crop breeders.

Progress is being made on several fronts, Mundt said. New genes are being identified which offer improved resistance. Sometimes mixtures of different crops and crop rotation patterns can slow the evolution of pathogens. And a concept that shows promise is more use of "quantitative resistance," in which several genes are utilized which may each only offer only a partial solution to the attack of a plant pathogen, but in combination work fairly well and last longer than reliance upon a single gene.

"These genetic systems are just far more complex than we originally thought," Mundt said. "We see very rapid loss of pathogen resistance in some cases. Some varieties can lose their resistance to pathogens in as little as five years, and then we need a new type of crop to grow."

The challenges are particularly acute, Mundt said, on comparatively low-value crops such as wheat and corn, where the routine application of chemicals to control plant pathogens is prohibitively expensive.