RIVERSIDE, Calif. – Wild plants and crops are often neighbors, but that doesn’t mean they always get along. New research from the University of California, Riverside (UCR) demonstrates that native plants and non-native crops attract pests and spread disease in both directions.

Kerry Mauck, entomologist and associate professor of entomology at UCR, and her colleagues have dubbed this phenomenon “pathogen pollution,” a term that doesn’t place the blame fully on agriculture or wild plants.

“I feel like (the agriculture industry and native habitats) are both dealing with the same issue – we’re both dealing with these pathogens, and we need to approach the issue holistically,” she said.

Mauck has been involved in two studies that demonstrate pathogen pollution. One study investigated the movement of viral pathogens from areas of intense agriculture production to wild squash plants.

“We grow a lot of squash and melon in California, and these crops have a lot of pathogens that attack them. A lot of these pathogens are actually not native to the United States, but they’ve come in over time, either on infected plant material or carried by insects that are on infected plant material and have established themselves in these crop environments,” she said. “So, we were wondering if, after this very obvious invasion into crop environments, (the pathogens) were also invading wild plant environments.”

Mauck and her team sampled three types of perennials, including two wild squash commonly known as buffalo gourd and coyote gourd, and an ornamental flower known as sacred datura. The team found that the wild plants were infected with two dominant viruses, cucurbit aphid-borne yellows virus (CABYV) and cucurbit yellow stunting disorder virus (CYSDV). Both viruses are typically found in crop habitats, suggesting that the pathogens have spilled over from intense agricultural environments to wild plants.

CABYV inhibits the growth of plant roots, preventing infected plants from accessing and storing water. In California, wild squash provides resources to other organisms in the summer, like pollinators relying on nectar and pollen and endangered mammals that eat the plant’s seeds.

On the flip side, Mauck said that wild plants could be a source of pathogens for crops, even those that don’t cause disease in wild plants. Mauck’s team investigated bacteria known as Candidatus Liberibacter solanacearum (CLso), a pathogen that first appeared in the U.S. in the 2000s that infects nightshade plants like tomatoes and potatoes. CLso causes a potato disease called zebra chip that reduces crop yields by 90 percent by deforming the tubers and making them unmarketable. The bacteria is primarily spread by psyllids, sometimes called jumping plant lice. Psyllids are native to the U.S. but only recently began spreading CLso, so Mauck and her team wondered if the pathogen had been around longer than they realized.

By studying plant samples dating back to the 1970s, the team found a unique version of CLso that had been infecting populations of a native perennial commonly known as bluewitch nightshade decades before the pathogen began circulating in nightshade crops. Interestingly, the variant of CLso found in bluewitch nightshade doesn’t cause disease in the plants it infects, and it isn’t the same variant that is found in crops.

“Our hypothesis is that agriculture may have created the opportunity for variants of this pathogen to emerge as causal agents of disease in a crop environment,” Mauck said. “When you create a new environment (like crops), it’s a huge, uncolonized resource for the insects that transmit the pathogen. That creates an opportunity for the (bacteria) variants in the native population to select for traits for survival in (crop environments), and often that can be pathogenicity, virulence, or the ability to feed better on those crops.”

Mauck cautions producers from thinking all wild plants near their crops should be removed. She points out that not all plants support all viruses or pathogens and that producers can be savvy with what they plant next to each other to discourage pathogen pollution.

“For example, grasses have a totally different virus community than things that are not grasses, like broad-leaf plants. So, just by planting things that are dissimilar to each other in adjacent spaces, you can avoid some of that buildup (of pathogens),” she said. “We don’t want to say all crops are bad for wild plants and vice versa.”

Another option is planting annuals that aren’t around long enough to become an issue in terms of contributing to pathogen pollution. She added that while perennials encourage the long-term buildup of beneficial insect populations, they can accumulate viruses and other pathogens over time. More research is needed to understand which perennials would be the best option for providing resources for pollinators without supporting pests or pathogens.

Moving forward, Mauck wants growers and the public to be cognizant of following regulations when moving plants and organic materials.

“Anything can have pathogens in it, and we have regulations about how we need to move plants around the globe, and even between and within states,” Mauck said. “People need to know that moving plants and things that can have pathogens or insects on them incorrectly can be the start of a multibillion-dollar issue. If you’re unsure if you can move (a plant), don’t.”