In Colorado, alfalfa mosaic virus is causing major losses in chile peppers, transmitted by transient aphids that need only pick it up on their mouthparts to carry it from plant to plant. New findings highlight resistant pepper varieties and integrated pest management strategies that offer promising paths to reduce disease impact. Here, a single alate aphid rests on a pepper plant. (Photo by Lara Amiri-Kazaz)

By Lara Amiri-Kazaz, Kadie Britt, Ph.D., and Adrianna Szczepaniec, Ph.D.

It’s no secret that aphids are at the center of many agricultural challenges we face today. Many species are known for causing economic damage to crops, making them a nuisance for many, from commercial agriculture settings to home gardens. While most are likely familiar with aphid injury to plants and the copious amounts of honeydew they excrete, aphids have the ability to spread some of the most damaging plant pathogens, as they transmit nearly 50% of insect-borne plant viruses.

Ada Szczepaniec, Ph.D.

Kadie Britt, Ph.D.

Lara Amiri-Kazaz

Among the long list of pathogens aphids transmit are non-persistent viruses: These can be acquired within seconds to minutes of feeding and transmitted to another plant just as quickly. Virus remnants persist on the mouthpart and do not enter the salivary glands or gut, a process akin to what we call “dirty needle” transmission.

Non-persistent viruses are almost exclusively transmitted by aphids, and many economically important crop viruses (such as cucumber mosaic virus and potato virus Y) fall into this group. This combination of rapid transmission and vectors that may not necessarily colonize the focal plant creates the perfect storm for management, and is an issue that Colorado chile peppers are currently facing with alfalfa mosaic virus.

Chile peppers are a key specialty crop in Colorado, but since 2019 the emergence of alfalfa mosaic virus in the Arkansas Valley has led to substantial yield losses and ongoing challenges for pepper farmers. In a 2025 survey, growers reported 50% or higher reduction in yield and quality of peppers due to alfalfa mosaic virus—an impact of $5.6 to $8.4 million in lost revenue each year. Once infected, common symptoms include mosaic chlorosis, deformed leaf veins, leaf curling and deformation, and necrosis, leading to overall plant stunting and reduced photosynthetic capabilities. Fruits can be misshapen and blotchy, rendering them unmarketable.

Alfalfa mosaic virus is a non-persistently transmitted virus, and several aphid species, such as the spotted alfalfa aphid (Therioaphis maculata) and green peach aphid (Myzus persicae), are vectors. These species do not habitually feed on peppers; combined with the non-persistent transmission mode, this means management is especially difficult.

In an article published in June 2025 in the open-access Journal of Integrated Pest Management, our team at Colorado State University, along with collaborators from the University of California Cooperative Extension, discuss the status of alfalfa mosaic virus in chile peppers, current management prospects, and research needs. A related article, published in October 2025 Frontiers in Agronomy, highlights several commercially available pepper cultivars and their tolerance to alfalfa mosaic virus under field conditions. Host-plant resistance is a strong first line of defense, but it’ll take a bit more effort to manage this type of disease.

In most cases, the aphid species that transmit non-persistent viruses are not pests of the crops they infect, and a lot of non-persistently transmitted viruses are transmitted by non-colonizing, transiently moving aphids. This is frequently what we see with peppers: fields inundated with alfalfa mosaic virus (hereafter, AMV) but not a single aphid in sight. Pepper farmers have little success using both contact and systemic pesticides to manage aphids, as they do not work fast enough to prevent transmission. Non-persistent viruses may also be transmitted via mechanical transmission, grafting, and seed transmission.

In Colorado, alfalfa mosaic virus is causing major losses in chile peppers, transmitted by transient aphids that need only pick it up on their mouthparts to carry it from plant to plant. New findings highlight resistant pepper varieties and integrated pest management strategies that offer promising paths to reduce disease impact. Here, a pepper plant exhibits chlorosis caused by alfalfa mosaic virus. (Photo by Lara Amiri-Kazaz)

Early evidence shows that proximity to neighboring AMV hosts increases the vulnerability of chile peppers to AMV. Alfalfa and chile peppers are widely grown in the same region of Colorado; alfalfa acts as a reservoir for AMV and a host for several aphid vector species. As a forage crop, alfalfa is harvested several times throughout the season, likely triggering the movement of aphids into neighboring crops; the first cut typically occurs when peppers are in their stage most vulnerable to disease. Pepper fields located further away from alfalfa and other AMV hosts can help reduce disease infection in neighboring fields, though this is not always possible, as growers don’t really get a say in who their neighbors are.

There are currently limited options for managing AMV. The leading strategy shown to decrease disease prevalence in the field is host-plant resistance. We have found several cultivars of peppers, such as ‘Masivo,’ ‘Mosco,’ and ‘Revolution,’ that exhibit tolerance to AMV on commercial farms, with symptom severity and number of virus-positive plants to be much lower than that of other, more susceptible varieties. These results are promising. Another proven management strategy is planting-date modification: Early planting does not decrease disease prevalence, but it does allow peppers to produce more fruit, which compensates for virus yield loss.

Several other avenues can potentially help reduce vector movement and virus infection while also working in tandem with the pest management strategies we currently employ. Biological control is easily combined with host plant resistance, though most field-based research focuses on management through consumption of colonizing aphids, which our chile pepper system lacks. For this reason, turning to the potential non-consumptive effects of natural enemies may be just the thing we need.

Early evidence suggests that the introduction of a generalist predator can deter aphids by eliciting avoidant behaviors, preventing infection of peppers. Using a generalist predator is key, as they can feed on a variety of other pests in our system while simultaneously scaring aphids away. Non-consumptive effects have been demonstrated widely in laboratory assays, and now there is a need to further test generalist predator effects under field conditions against virus infection and overall effect on yield.

Companion planting has potential in our system. One strategy is trap crops: The goal is to attract aphids to a crop they prefer to colonize, allowing farmers enough time to effectively use chemical control or providing a distraction for the aphids, reducing the likelihood they will colonize the focal crop. Another strategy is barrier crops: Usually consisting of tall plants such as corn or sorghum, these plants are grown on farm edges to physically interfere with aphid movement into chile pepper fields. Both strategies can also result in aphids “losing” the virus upon feeding on trap or barrier crops since non-persistently transmitted pathogens do not remain on the stylets for an extended period. Even if the aphids still end up in chile pepper fields, after passing through these companion plantings, they are less likely to still carry the virus.

Understanding the source of infection also plays a key role. Treating alfalfa fields with aphicides before harvest could potentially reduce the number of aphids moving to neighboring crops. Moreover, as AMV infects a wide range of hosts, a lot of common weed species may act as virus reservoirs if not properly managed. However, this threat is still unclear with AMV and needs further research. Managing virus reservoirs and implementing tactics in neighboring crops can be a promising tactic for AMV in peppers but would require extensive research and intense coordination to develop an area-wide approach with multiple growers.

The management of non-persistent viruses is an ongoing area of research and requires greater attention to landscape effects and creative strategies not yet widely employed in the field. However, our team at Colorado State and others are starting to show with preliminary data that a wide range of tactics used in tandem may provide promising results. With this in mind, we continue to work closely with farmers in Colorado’s Arkansas Valley to deepen our understanding of non-persistent virus management and help reduce yield losses cause by our tiny (but mighty!) aphid foes.

Lara Amiri-Kazaz is a doctoral student in the lab of Ada Szczepaniec, Ph.D., associate professor of horticultural entomology at Colorado State University in Fort Collins, Colorado. Kadie Britt, Ph.D., is the integrated pest management program manager at Colorado State University. Email: [email protected], [email protected], [email protected].