One might think the realization that biodiversity protects plant health is a new one, given that it wasn’t that long ago that biodiverse farming became a rare practice. But in fact, scientists and farmers have recognized this connection for at least centuries, and probably longer, said evolutionary biologist Amanda Gibson from the University of Virginia.

The basic concept is simple enough: A typical pathogen can only infect certain plant species. When that pathogen ends up on a species it can’t infect, that plant acts like a sinkhole. The pathogen can’t reproduce, so it’s neutralized, and nearby plants are spared.

Disease-resistant plants can also alter airflow in ways that keep plants dry and healthy and create physical barriers that block pathogen movement. Especially if they’re tall, resistant plants can act like fences that diseases have to hop over. “Somebody did a nice experiment taking dead corn stalks and just plopping them in the bean field,” said plant pathologist Gregory Gilbert from the University of California, Santa Cruz. “And that works, too, because it’s just keeping things from moving around.”

In nature, this dynamic between plants and pathogens can be part of healthy ecosystems. Pathogens spread easily between stands of the same species, killing off plants that are too close to their relatives and making sure landscapes have a healthy degree of biodiversity. As “social distancing” is restored between susceptible hosts, the disease dies down.

In monocultures, there are no sinkholes or natural fences to stem the spread of pathogens. Instead, when a disease takes hold in a crop field, it’s poised to burn through the entire thing. “We create amplification rather than dilution,” said Altieri.

New technology has driven home these old lessons: Over the last decade, it’s become possible for scientists to isolate a broad swath of the microbes found within a particular niche — like an ear of corn or a stalk of wheat — and use DNA sequencing to create a censuslike list of everything that lives there.

The results have been unsettling, but not always unexpected. Plants in cultivated lands carry a significantly larger variety of viruses than those in adjacent biodiversity hotspots, plant and microbial ecologist Carolyn Malmstrom from Michigan State University and her colleagues found in one study. 

Conversely, they later found that some fields of barley and wheat were largely devoid of viruses, but that could also be a sign of problems to come. Pesticides may be keeping virus levels low: “So we might think, OK, yay, we’re protecting our crops,” Malmstrom said. But not all microbes are bad. 

“By pulling our crop systems out into a virus-free situation, we may also be removing them from some of the richness of the biodiversity of microbes that’s beneficial,” she added.

The bigger the farm, the more serious the disease problems, at least in the case of a pathogen called Potato virus Y, which leads to low potato yields. When researchers looked at the amount of simplified cropland surrounding a potato plant, they found that the prevalence of the pathogen went up steadily as the percentage of surrounding area covered in cropland increased. Unmanaged fields and forests, on the other hand — carrying wild mixes of plants — seemed to have a protective effect.

In natural landscapes, increasing biodiversity lowers the number of virus species present. But increasing biodiversity along the edges of crop fields doesn’t seem to have the same effect, plant ecologist Hanna Susi from the University of Helsinki found. 

Fertilizers and other chemicals leached from the crops might affect the susceptibility of nearby plants to infection, Susi and her coauthor postulated. Beneficial microbes found on wild plants may be keeping many of these viruses from causing disease, but if the same viruses get into crops that lack that protection, “We don’t know what may happen,” she said. Farmers could find themselves dealing with new kinds of crop diseases.

On Altieri’s farm in the Colombian state of Antioquia, he mixes many plants — corn with squash, pineapples with legumes — and said, “We don’t have the diseases that neighbors have, that have monocultures.” 

The results of recent DNA-sequencing experiments are familiar to him because traditional Latin American farmers have long used biodiversity to protect their crops. “These papers are good ecological research,” he said. “But actually, they’re basically reinventing the wheel.”

This old wheel does have to get over a new hill, however. Climate change is redistributing pathogens, bringing them into contact with new crops, and changing weather patterns in ways that foster disease.

Already, Liebman has seen the effects of climate change firsthand in Iowa, where tar spot disease — an infection that kills the leaves on corn plants — is on the rise. “We have warmer nights and more humid days,” he said. The tar spot pathogen loves the new weather.

Predicting exactly how much climate change will increase crop disease is difficult, said Singh. But there are some general conclusions he can draw.

Rising temperatures will likely favor certain pathogens that cause disease in major crops. A wheat-infecting fungus called Fusarium culmorum, for example, is likely to be replaced by its more aggressive and heat-tolerant relative, Fusarium graminearum. That could spell bad news for Nordic countries, where wheat crops could suffer.

Hotter temperatures will likely knock back other pathogens. A fungus that infects the herb meadowsweet, for example, has already begun dying out on islands off the coast of Sweden. In general, however, Singh thinks regions that are currently cold or temperate will likely see increases in crop disease as they warm.

For regions that are already warm, rising humidity could cause trouble. For example, parts of Africa and South America are among the regions that will probably see increases in funguslike pathogens called Phytophthora. Food insecurity is already prevalent in some of these areas, and if nothing’s done to stop disease spread, that’s likely to get worse. “We need a lot more information,” Singh said. “But I agree that that is one of the scenarios that is a possibility.”