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CORVALLIS, Ore. - Researchers have discovered that many plants, including some of the most important food crops in the world, are often infected by several pathogens at once, and that efforts to control plant disease epidemics need to better consider and understand the effects of the multiple attacks.

It appears that some plant disease outbreaks are most serious when these combined attacks overwhelm a plant's immune response more than any one disease might individually. Plants have less ability to adapt to specific threats than the more sophisticated immune responses of many animals.

"Most of the time we've looked at plant diseases separately as if one had nothing to do with the other," said Eric Seabloom, an assistant professor of zoology at Oregon State University. "This is one of the first times we've considered multiple pathogens over larger spatial scales in natural systems, and it's clearly showing that plant disease is an issue that must be considered in a whole community of pathogens and disease vectors."

The study, which was just published in The American Naturalist, was done by researchers from OSU, Princeton University and Cornell University. It was funded by the National Science Foundation and National Institutes of Health.

There is ample evidence in human medicine, Seabloom said, of people getting more than one infection or disease at a time, with the multiple infections compounding the seriousness of the problem - the immune system can only deal with just so much at once. Infections with two strains of HIV, or HIV and malaria, can be much more serious than being infected by a single pathogen.

In natural plant communities, very little has been done so far to study this phenomenon, although plant diseases are common, widespread and costly. One of the pathogens involved in this research, the barley yellow dwarf virus, can attack 150 grass species, including such crops as oats, wheat and rye - and in an epidemic can cause up to 80 percent losses in some of them, particularly barley. Previous OSU research also showed this virus to be the critical cause of the invasion of more than 22 million acres of native grasslands in California.

In this study, scientists developed a mathematical model to show relationships between plant pathogens and disease. They found a very high co-infection rate, much more than expected, which strongly affected pathogen infection and virulence. The co-infected hosts might also act as "superspreaders" of plant disease, the study said.

One key player appears to be the vector that carries the diseases - aphids in this study, but often other sucking insects such as leafhoppers in plants, and mosquitoes and ticks in humans and other animals. The most serious infections were observed when a plant was infected by a vector that carried more than one disease, and there was little or no cross protection against the diseases.

"This suggests that a better understanding of what insect vectors can carry which diseases may help us better address these problems," Seabloom said. "Researchers have tried breeding plants for resistance to this disease with only limited success. If we hope to better control diseases, we need to know more about the insects that are carrying multiple pathogens, and the implications of those multiple attacks."