Five-Year Trial Finds Netting, Not Sprays, Best Shields Tomatoes From Cucumber Mosaic Virus

Cucumber mosaic virus has become one of the most punishing adversaries of tomato growers in the mid-hill regions of Nepal, where summer production under plastic houses commands premium prices but coincides with peak aphid activity. A five-year field study conducted by researchers at the Nepal Agricultural Research Council’s National Plant Pathology Research Center in Lalitpur has now delivered an unusually rigorous verdict on the products many farmers spray in the hope of controlling the virus. After evaluating a dozen biorational and chemical treatments across consecutive seasons from 2020 to 2024, the team found that none of the foliar-applied products consistently suppressed disease, while simply keeping aphids away from the crop proved far more effective.

The scale of the problem motivated the study. Cucumber mosaic virus, a member of the family Bromoviridae, ranks among the ten most economically important plant viruses worldwide and infects more than 1,200 plant species across over 100 families. It is transmitted non-persistently by more than 86 aphid species, meaning the virus is acquired and inoculated within seconds to minutes during brief probing of leaf tissue. That biology defeats conventional insecticide programs, because aphids can spread the virus long before any spray kills them. Devastating cucumber mosaic virus epidemics struck the Kathmandu Valley and similar tomato-growing areas during the summers of 2023 and 2024, forcing many farmers to uproot heavily infected plantings.

Several commercially available herbal and antiviral products are marketed in Nepal for plant virus management, yet few have been scientifically validated under local conditions. Nepal’s registration system operates largely by analogy, allowing products to be registered on the basis of efficacy and safety data generated abroad, with limited mandatory local testing. Growers seeking evidence-based recommendations therefore had little independent data to guide their purchases. The researchers set out to close that gap by testing the products under realistic epidemic pressure in the subtropical mid-hill environment where the losses were occurring.

The experimental program used tomato cultivar Pusa Ruby, a widely grown variety that is highly susceptible to the virus. In 2020 and 2021, trials were conducted under insect-proof net-house conditions in a randomized complete block design. From 2022 to 2024, the design shifted to a split-plot layout with cultivation environment as the main plot, comparing net-house and open-field production, and treatments as subplots with three replications. The treatment list included Anti-B, cow urine, Lentinan, Privintal, Nimbicidine, Vircon-H, Virosin, Jholmal, Rogor Plus dimethoate, Plantomycin, a 32-mesh row cover, and a water-sprayed control. Products were applied weekly from transplanting for seven consecutive weeks at a spray volume of 300 liters per hectare.

Disease assessment began 21 days after transplanting and continued at 10-day intervals through 79 days, using a 0-to-5 ordinal severity scale ranging from no visible symptoms to severe mosaic with pronounced deformation. Virus infection was confirmed with commercial immunostrip assays on leaf samples collected from symptomatic plants, which consistently detected cucumber mosaic virus across both production environments and all seasons. Disease progression was quantified using the area under the disease progress curve, a standard epidemiological measure obtained by trapezoidal integration of severity scores over time. Yield was recorded across multiple harvests at physiological maturity.

The clearest result concerned production environment rather than any spray. In all three years in which both systems were compared, the area under the disease progress curve was significantly lower inside the insect-proof net house than in the open field, with percent disease reductions of 72.59 in 2022, 42.8 in 2023, and 36.8 in 2024, an overall reduction of 40.62 percent. Epidemic intensity also varied sharply between years: the open-field curve peaked at 1255 in 2023, whereas values in 2022 were far lower. The researchers attributed the annual differences partly to transplanting date, noting that disease pressure rose when transplanting was shifted earlier, from mid-April in 2020 and 2021 to mid-March in 2023 and 2024, exposing young plants to greater aphid activity.

Against this backdrop, the sprayed products fared poorly. Under net-house conditions, no consistent statistically significant treatment effects on disease progression were observed across the five years, although Privintal, Nimbicidine, and Lentinan occasionally produced numerically lower values. In the open field, treatment effects were likewise non-significant in 2022 and 2023. The single significant treatment effect came in 2024, when the physical row cover produced the lowest disease progress value, 561.1, corresponding to a 47.4 percent reduction relative to the untreated control. No other treatment matched that performance, and responses elsewhere were inconsistent across seasons and environments.

Yield data reinforced the picture. Mean yields ranged from 6.19 tonnes per hectare in 2020 to 24.5 tonnes per hectare in 2024 under net-house conditions, and from 19.1 to 25.1 tonnes per hectare in the open field. Treatment effects on yield were generally not significant, and in 2024 the untreated open-field control recorded the highest yield of the season. A significant but weak negative correlation between yield and the disease progress curve, with r equal to minus 0.24, indicated that greater disease severity was associated with lower yield but explained only a modest fraction of yield variation, pointing to environmental factors and unquantified insect pressure as additional influences.

The authors argue that the biology of cucumber mosaic virus largely explains the failure of the antiviral sprays. The virus establishes systemic infections rapidly after inoculation, and repeated aphid-mediated introductions throughout the cropping period may have overwhelmed any localized antiviral activity, while environmental variability in the open field likely reduced product persistence on leaf surfaces. The researchers also caution that mixed infections with other tomato viruses, such as tomato mosaic virus and tomato leaf curl virus, could not be ruled out because additional diagnostic capacity was unavailable, a limitation that may have complicated symptom interpretation.

The study’s practical conclusions are pointed. Physical exclusion of aphid vectors, whether through structural net houses or plot-scale row covers, outperformed every foliar intervention tested, consistent with earlier reports of net houses and row covers suppressing non-persistently transmitted viruses in tomato, squash, and cucumber. The team recommends that growers combine insect-proof netting with sanitation measures, including removal of weeds and volunteer plants that harbor the virus and its vectors, and that seedlings be raised under virus-free netting before transplanting. They further urge Nepal’s pesticide regulatory system to require independent efficacy validation under local field conditions before products claiming antiviral activity are approved, so that farmers are no longer paying for treatments the evidence cannot support.

Subject of Research: Field evaluation of biorational products and vector exclusion for managing cucumber mosaic virus in tomato in Nepal

Article Title: Evaluation of biorational products in the management of cucumber mosaic viruses in tomato

Article References: Khadka, R. B., Panta, B., Manandhar, S., Ghimire, T. D., Shrestha, B. L., & Baidya, S. (2026). Evaluation of biorational products in the management of cucumber mosaic viruses in tomato. Frontiers in Horticulture, 5, Article 1881524. https://doi.org/10.3389/fhort.2026.1881524

Image Credits: AI Generated

DOI: 10.3389/fhort.2026.1881524

Keywords: cucumber mosaic virus, tomato, aphid vectors, biorational products, vector exclusion, net house, Nepal, plant virology, AUDPC, row cover, integrated pest management, plant disease management