Evaluation of biorational products in the management of cucumber mosaic viruses in tomato

Abstract

Plant viruses are among the most challenging plant pathogens to manage due to limited effective control options. Cucumber mosaic virus (CMV) has emerged as a major constraint to tomato production in the mid-hill regions of Nepal, particularly under high tunnel conditions during the summer rainy season. Field experiments were conducted over five consecutive years (2020–2024) to evaluate biorational and chemical treatments for CMV management. Trials in 2020–2021 were conducted under net-house conditions using a randomized complete block design, while in 2022–2024, experiments followed a split-plot design with cultivation conditions (net-house and open field) as main plots and treatments as subplots. Treatments included Anti-B, cow urine, Lentinan, Privintal, Nimbicidine, Vircon-H, Virosin, row cover, Jholmal, Rogor Plus, Plantomycin, and a water-sprayed control, with three replications. Treatments were applied weekly from transplanting up to seven weeks. Disease severity were recorded at 10-day intervals starting 21 days after transplanting, and CMV infection was confirmed using immunostrips. Across years, no consistent significant treatment effects on disease progression (AUDPC) were observed under net-house conditions. However, AUDPC values were consistently lower in tomato plants grown in the insect-proof net-house compared to the open field, indicating reduced disease pressure due to vector exclusion. Under net-house conditions, treatments such as Privental, Nimbicidin, and Lentinan showed relatively lower AUDPC and moderate percent disease reduction (PDR), although differences were not statistically significant. In contrast, open-field conditions exhibited higher disease pressure. Row cover resulted in the lowest AUDPC and highest PDR, achieving up to 47.40% disease reduction in 2024, and was the only treatment with a significant effect. Other treatments showed inconsistent performance across environments. These findings demonstrate that while biorational products provide limited suppression of CMV, physical exclusion of vectors is a more effective strategy for managing CMV in tomato.

Introduction

Cucumber mosaic virus (CMV), a member of the family Bromoviridae, is one of the most widely distributed plant viruses and is recognized among the ten most economically important plant viruses worldwide (Scholthof et al., 2011). Its remarkable adaptability is reflected in its exceptionally broad host range, infecting more than 1,200 plant species across over 100 families, including both monocots and dicots (Edwardson and Christie, 1991; Mochizuki and Ohki, 2012). CMV is transmitted in a non-persistent manner by more than 86 aphid species representing 33 genera, enabling its rapid spread and persistence in diverse agricultural and natural ecosystems (Edwardson and Christie, 1991; Scholthof et al., 2011). In addition, CMV can be seed-transmitted in some host species (Brunt et al., 1990; Yang et al., 1997; Ali and Kobayashi, 2010), although the frequency of transmission varies considerably among plant species (Palukaitis and García-Arenal, 2003).

Tomato is one of the most important hosts of CMV, and significant yield and fruit quality losses caused by CMV have been reported worldwide (Mahjabeen et al., 2012). Yield losses caused by CMV have varied considerably across years and geographic regions. For example, yield losses of 25–50% were reported in China (Tien and Wu, 1991), whereas tomato necrosis disease induced by CMV caused up to 100% yield loss in Italy and Spain during the 1987 outbreak (Gallitelli et al., 1988).

Although resistance genes have been utilized in several crop species, many confer only tolerance rather than complete resistance in tomato (Phills et al., 1977), and some resistance genes can be readily overcome by different strains of CMV. Numerous attempts have been made to introgress CMV resistance into cultivated tomato from wild relatives, including Solanum peruvianum (formerly Lycopersicon peruvianum Mill.), S. habrochaites (formerly L. hirsutum Humb. & Bonpl.), S. pimpinellifolium (formerly L. pimpinellifolium Mill.), and S. chilense. However, these efforts have generally failed to provide durable and commercially satisfactory levels of resistance (Abad et al., 2000). Therefore, reliance solely on host resistance often provides limited and unstable protection against CMV epidemics (Jacquemond, 2012). Consequently, CMV remains difficult to manage under field conditions, particularly in intensive vegetable production systems where continuous cropping and high vector pressure favor rapid disease spread.

Current management approaches largely rely on two principles: (i) reduction of primary inoculum sources and (ii) suppression of secondary spread through vector control. In practice, farmers predominantly depend on repeated insecticide applications targeting aphid vectors. However, insecticides are inherently limited in managing non-persistent viruses such as CMV, because virus acquisition and inoculation occur within seconds to minutes—well before vector mortality can be achieved (Budnik et al., 1996; Hooks and Fereres, 2006).

The use of mineral oils, vector-proof nets or row covers, and reflective mulches has been widely recommended and has been shown to provide reliable control of CMV in various crops under field conditions (Zitter and Murphy, 2009). However, the practical adoption of these management practices is often limited, particularly among resource-poor farmers in countries such as Nepal, due to issues of affordability, accessibility, and availability of these materials.

Furthermore, dependence on a single tactic increases the risk of management failure and may exacerbate environmental and resistance-related concerns (Ahmad et al., 2024; Lapidot et al., 2001). Integrated approaches have demonstrated greater effectiveness than individual strategies alone (Rahman et al., 2020), yet practical, field-validated components suitable for integration remain insufficiently developed for CMV in tomato.

Biorational products—including plant-derived extracts, mineral oils, microbial-based formulations, and resistance-inducing compounds—have shown promise in reducing aphid landing behavior, interfering with stylet-mediated transmission, or priming host defense responses.

For example, horticultural mineral oils reduce the transmission of non-persistent aphid-borne viruses through multiple mechanisms. They exhibit direct insecticidal activity against aphid vectors, interfere with virus acquisition and inoculation by disrupting virion binding and retention in the aphid stylet and foregut, induce sublethal behavioral changes that reduce probing and feeding, and may enhance the plant’s natural defense responses against virus infection (Yang et al, 2019).

Furthermore, plant-derived extracts represent an important source of antiviral compounds (Owen et al., 2022). For example, Cai et al. (2020) reported that the plant-derived compounds ursolic acid and 4-methoxycoumarin effectively suppressed tobacco mosaic virus (TMV) through protective, curative, and virus-inactivating activities in tobacco plants. More broadly, natural products have played a major role in the discovery of new bioactive compounds, with an estimated 34% of all approved new chemical entities between 2000 and 2014 being derived from plants and other natural sources (Newman and Cragg, 2016).

However, most studies have focused either on vector suppression or on controlled-environment assays, with limited evaluation under realistic epidemic pressure under natural epiphytic conditions. Moreover, comparative assessments integrating disease incidence, vector dynamics, and yield impact remain scarce, particularly in subtropical high-tunnel systems where CMV pressure is seasonally intense.

Tomato cultivation under plastic houses has become increasingly popular among growers in the mid-hill regions of Nepal because of the premium market prices and strong export potential to neighboring Indian markets during the summer season (Kafle and Shrestha, 2017). At this time of year, tomato production in the lowland plains is greatly constrained by high temperatures and flooding, creating a favorable market opportunity for tomatoes produced in the cooler mid-hill regions. However, in recent years, growers have experienced severe economic losses due to widespread outbreaks of cucumber mosaic virus (CMV) during this production season. For instance, devastating CMV epidemics were reported in the Kathmandu Valley and other similar tomato-growing areas of Nepal during the summers of 2023 and 2024, where many farmers were forced to uproot large numbers of infected plants because of extensive disease incidence.

Several biorational products claimed as herbal products are commercially available in Nepal and are marketed for the management of plant viral diseases, including CMV in tomato. Few of the commercial antiviral products have been registered by Nepal’s Plant Quarantine and Pesticide Management Centre (PQPMC) (PPQMC, 2021). However, the efficacy of most commercially available biorational products has not been scientifically validated under Nepalese production conditions, and reliable peer-reviewed evidence supporting their field performance remains limited. Although a few studies have demonstrated the antiviral potential of certain plant extracts and other biorational compounds against plant viruses, their effectiveness varies depending on the active ingredient, crop, virus species, environmental conditions, and application method (Taglienti et al., 2022). Consequently, there is insufficient evidence to support the efficacy claims of many commercially registered products available in Nepal. Therefore, growers often seek evidence-based recommendations from researchers regarding their effectiveness. This knowledge gap has created uncertainty among growers and highlights the need for independent field evaluations under local production conditions. Therefore, this study was undertaken to provide scientific evidence on the effectiveness of commercially available biorational products for the management of CMV in tomato under both open-field and plastic-house production systems in Nepal.

Materials and methods

Experimental site and plant material

Field experiments were conducted at the Nepal Agricultural Research Council (NARC)- National Plant Pathology Research Centre, Khumaltar, Lalitpur, Nepal (27.6485° N, 85.3253° E), during the tomato-growing seasons (March–August) from 2020 to 2024. The experimental site represents a subtropical mid-hill agroecological zone. The study was repeated over five consecutive years to ensure the temporal robustness of treatment effects.

Seeds of tomato (Solanum lycopersicum L.) cv. Pusa Ruby were obtained from the Directorate of Agricultural Research, Khajura, Banke, Nepal. Pusa Ruby is a widely cultivated tomato variety that is highly susceptible to cucumber mosaic virus (CMV). Therefore, it was selected for use in this study. Seeds were surface-disinfested in 1% (v/v) sodium hypochlorite for 10 min, rinsed twice in sterile distilled water, air-dried under shade for 2 h, and sown in plastic seedling trays containing a sterilized vermicompost: cocopeat substrate (1:1, v/v). Seedlings were raised for 25 days in a greenhouse under insect-proof netting to prevent early exposure to insect vectors. During the seedling growth period, the greenhouse temperature was varied between 25 and 30 °C. and grown under natural light conditions. The seedlings were drenched with a 1% urea solution once a week to maintain seedling vigor and overall health.

Experimental design and treatment structure

The experiments were designed to identify the most effective biorational product for managing CMV in tomato. Based on the results obtained over time, the experimental design and treatments were modified accordingly. The results presented here summarize experiments conducted over five years (2020–2024) (Table 1).

The experiments were carried out under net house conditions from 2020 to 2021. In 2022 to 2024, experiments were conducted under both net house and open-field conditions (Table 2). During the initial two years, treatments were arranged in a randomized complete block design. In 2022, 2023 and 2024, the experimental layout was changed to a split-plot design with three replications.

For the 2022 to 2024 experiments, the main-plot factor consisted of two cultivation environments: (i) open-field conditions and (ii) insect-proof net house conditions. The sub-plot factor comprised five to seven biorational treatments (Table 2).

Crop establishment and agronomic management

The experimental fields were prepared according to recommended commercial practices. Land preparation involves digging the soil with a spade, removing weeds and stubbles, and making the soil into a fine tilth.

Each plot measured 2.4 m × 1.8 m and consisted of twelve plants arranged at a spacing of 60 cm between rows and 45 cm between plants. A buffer zone of 75 cm separated adjacent plots, and a meter was maintained between replications to minimize spray drift and cross-contamination.

Well-decomposed farmyard manure was incorporated at 20 t ha^-¹ at least 15 days before transplanting. Chemical fertilizers were applied as urea, diammonium phosphate (DAP), and muriate of potash (MOP) at a rate of 200:150:120 kg N:P:K ha^-¹. Half of the recommended nitrogen dose and the full recommended doses of phosphorus and potassium were applied at the time of transplanting. The remaining nitrogen was applied in two equal split doses at the first weeding and at the flowering stage. Raised beds (15 cm height) were prepared manually.

Twenty-five-day-old seedlings were transplanted into prepared beds under their respective main-plot conditions. Tomato seedlings were transplanted on 18 April 2020, 26 April 2021, 20 April 2022, 14 March 2023, and 20 March 2024.

The experimental plots were irrigated weekly to maintain adequate soil moisture throughout the growing period using the flooding method. Plants were individually supported with three bamboo stakes at 20 days after transplanting (DAT) to maintain canopy structure and reduce soil contact. All other intercultural operations were performed uniformly across treatments. No training, pruning, sucker removal, or fruit thinning was carried out during the experiment. The plots were hand-weeded starting at three weeks after transplanting at a week interval until 70 DAT. During the first weeding, the soil around the plants was loosened by hoeing, followed by earthing-up to improve plant support and promote root development. Application of biorational treatments was initiated after transplanting and continued at weekly intervals for 7 consecutive weeks. All products were applied using knapsack sprayers fitted with a Z-nozzle, maintaining a uniform spray volume of 300 L per hectare to ensure complete foliar coverage. To minimize the potential effects of spray drift between adjacent plots, appropriate precautions were taken during pesticide application.

All treatments were applied at the rate mentioned in Table 2. Jholmal and cow urine were diluted 1:10 with water before application. Fresh cow urine was collected from the NARC-National Animal Breeding Research Centre each morning before spraying. Jholmal was prepared locally as recommended (Bhusal and Udas, 2020).

Jholmal was prepared by mixing cow urine, animal dung, and water in a 1:1:1 ratio, along with 10–20 g of locally available plants possessing bitter, hot, astringent, and pungent properties, such as Artemisia vulgaris, Urtica dioica, and Lantana camara (Bhusal and Udas, 2020). Tender leaves of these plants were chopped into small pieces and mixed thoroughly with the liquid mixture. The solution was then stored in a 100-liter plastic tank, and the lid was tightly sealed to facilitate anaerobic fermentation for three weeks. After fermentation, the product was filtered through double-layered cheesecloth, diluted with water at a 1:10 ratio, and sprayed.

Disease assessment and epidemiological parameters

Disease assessments were initiated at 21 DAT and subsequently conducted at 10-day intervals up to 79 DAT. Individual plants within each sub-plot were evaluated using a 0–5 ordinal disease severity scale: 0 = no visible symptoms; 1 = mild mosaic; 2 = mosaic; 3 = mosaic with leaf deformation; 4 = mosaic with stunting; and 5 = severe mosaic with pronounced deformation, adapted from Murphy et al. (2003).

Disease progress over time was quantified using the area under disease progress curve (AUDPC), calculated according to the trapezoidal integration method:

Here,

n = number of observations/scoring made.

X_i = score of disease at i th observation.

t_i = time (number of days) at ith observation.

Virus detection

To confirm viral etiology, the third and fourth fully expanded apical leaves were collected from three selected plants, which represent the diversity of the symptoms in the plots. The collected samples were kept in labeled plastic bags and transported to the laboratory. Approximately 0.25 g of leaf tissue was macerated in extraction buffer using mesh extraction bags and a hand homogenizer. Immunostrip assays specific for cucumber mosaic virus (CMV) (Agdia, USA) were performed according to the manufacturer’s instructions. Strips were inserted into clarified sap extracts, and the results were recorded after 30 min.

Yield assessment

Fruits were harvested at physiological maturity over multiple pickings. Total fruit weight per plot were recorded at each harvest. Cumulative fruit yield per plot was computed across harvests and converted to yield per hectare for comparative analysis.

Weather data

Weather data, including daily maximum and minimum temperatures, relative humidity, and 24-hour cumulative precipitation, were obtained from an automatic weather station nearby the experimental field (~300 meters away from the field) for the period from March to August during 2020–2024. The weather variables were visualized using the ggplot2 package in R.

Statistical analysis

Data were analyzed using R (R version 4.5.2; R Core Team, 2025). Prior to analysis, residuals were assessed for normality (Shapiro–Wilk test) and homogeneity of variance (Bartlett’s test). Percentage data were arcsine square-root transformed where necessary, to meet assumptions of ANOVA.

Data were analyzed using a split-plot ANOVA model, with cultivation environment considered as the fixed main-plot factor and biorational treatments as the fixed subplot factor. Year was initially tested as a random effect; however, as treatment × year interactions were not significant, data were pooled across years for combined analysis. When F-tests indicated significant effects (P ≤ 0.05), treatment means were separated using Fisher’s Least Significant Difference (LSD) test implemented in the Agricolae package (Mendiburu, 2015).

Results

Pathogen confirmation

Immunostrip assays consistently confirmed the presence of cucumber mosaic virus (CMV) in leaf samples collected from experimental plots across the growing seasons (Figure 1A). Positive reactions were observed in samples from both open-field and insect-proof net house conditions, indicating widespread viral occurrence throughout the study period. In addition to serological confirmation, infected plants exhibited characteristic CMV symptoms under field conditions. Prominent symptoms included shoestring-like leaf deformation, mosaic mottling, chlorotic patches, and pronounced leaf narrowing (Figures 1B–D). In several cases, symptom progression was associated with leaf distortion and reduced lamina expansion, consistent with typical CMV infection in tomato. Symptom expression was generally evident from the early vegetative stages and intensified as the crop developed, corresponding with increasing disease incidence recorded during sequential assessments (Figure 1).

Figure 1

Disease pressure over the year

The area under the disease progress curve (AUDPC) for cucumber mosaic virus (CMV) in tomato at Khumaltar varied markedly across years and production environments (net house vs. open field) (Figures 2, 3; Supplementary Table 1).

Figure 2

In 2020, disease pressure under net house conditions was moderate, with AUDPC values 517 indicating substantial epidemic development despite protected cultivation. In 2021, AUDPC under net house declined to 339.

From 2022 onward, both production systems were evaluated concurrently. In 2022 it AUDPC to net houses declined to 98.7, and under open field conditions was slightly higher 360.

In 2023, disease intensity increased sharply in both systems. Net house plots showed moderate AUDPC values (718), whereas open-field plots recorded the highest epidemic intensity of the entire study (1255).

In 2024, disease levels remained high relative to earlier years. Net house AUDPC (763) was comparable to 2023, while open-field AUDPC (1044) remained elevated but slightly lower than the 2023 peak. Across all years in which both systems were assessed (2022–2024), AUDPC was consistently greater in open-field conditions than in net houses.

Overall, the results demonstrate (i) strong interannual variability in CMV epidemic intensity, and (ii) a consistent suppressive effect of net house cultivation on disease progress relative to open-field production.

Effect of cultivation conditions on the CMV area under the disease progress curve

The area under the cucumber mosaic virus (CMV) disease progress curve (AUDPC) differed significantly between cultivation environments in three years of evaluation (Table 3; Figure 3). In 2022, the AUDPC value was significantly lower in the tomato plants grown in insect-proof net houses compared to the open field (P <0.001). The AUDPC in open field conditions was 360.08 and in net house conditions was 98.69; the percent disease reduction was 72.59 percent compared to the open field conditions. In 2023, tomato plants grown under insect-proof net house conditions exhibited a significantly lower AUDPC compared with those grown in the open field (P <0.001). The AUDPC value in the open field was 1255.2, whereas it was reduced to 717.6 under net house conditions, representing a substantial suppression of disease development. Similarly, in 2024, CMV disease progression was significantly affected by cultivation environment (P<0.001). The AUDPC recorded in the open field was 1044, while it was 763 in plants grown inside the insect-proof net house. Based on AUDPC values, the percentage reduction in CMV disease under net house conditions was 72.59% in 2022, 42.8% in 2023, 36.8% in 2024, and overall 40.62% compared to open field conditions. These results demonstrate the strong effectiveness of insect-proof net house cultivation in limiting CMV disease progression.

Figure 3

Effect of different biorational product treatments in area under the cucumber mosaic virus severity progress curve (AUDPC)

Across the experiments conducted from 2020 to 2024, treatment effects on CMV disease progression (AUDPC) were generally not significant under either insect-proof net-house or open-field conditions (Figure 4; Supplementary Table 2). Under net-house conditions, AUDPC ranged from 464.6–575.7 in 2020 (P = 0.7503), with Nimbicidin recording the lowest value (464.6). In 2021 (P = 0.4456), AUDPC ranged from 274.3–406.2, with the lowest values observed for cow urine (274.3), Lentinan (274.6), and Privintal (301.1). In 2022 (P = 0.9432), AUDPC varied from 91.0–108.8, with Privintal recording the lowest value (91.0). In 2023, treatment effects were marginally significant (P = 0.0798), with AUDPC ranging from 541.2–863.3. Privintal produced the lowest AUDPC (541.2), representing a 16.3% reduction compared with the control (646.3), whereas Anti-B recorded the highest AUDPC (863.3). In 2024 (P = 0.3285), AUDPC ranged from 740.0–801.7, with Vircon-H and Plantomycin recording the lowest values (740.0), although they did not differ significantly from the control.

Figure 4

Under open-field conditions, treatment effects were not significant in 2022 (P = 0.2602) or 2023 (P = 0.1918). In 2022, AUDPC ranged from 253.7–570.0, with Lentinan (253.7), Anti-B (283.1), and cow urine (318.7) showing the lowest disease progress. In 2023, AUDPC ranged from 1125.2–1412.3, with Anti-B (1125.2), Privintal (1184.6), and Vircon-H (1195.7) recording the lowest values. A significant treatment effect was detected only in 2024 (P = 0.0299), when AUDPC ranged from 561.1–1266.7. Row cover produced the lowest AUDPC (561.1), followed by Rogor Plus (961.1) and Jholmal (1133.3), corresponding to a 47.4% reduction in disease compared with the control (1066.7). Overall, AUDPC values were lower under insect-proof net-house conditions than under open-field conditions. Except for the lower AUDPC recorded with Privintal under net-house conditions in 2023 and row cover under open-field conditions in 2024, none of the treatments consistently reduced CMV disease progression across years.

Effect of year and cultivation conditions on tomato yield

Tomato yield varied considerably among years and production conditions (Figure 5; Supplementary Table 3). Under net-house conditions, the mean yield ranged from 6.19 t ha^-¹ in 2020 to 24.5 t ha^-¹ in 2024. The lowest yield was recorded in 2020, followed by 2021 (10.9 t ha^-¹) and 2023 (12.6 t ha^-¹), whereas substantially higher yields were obtained in 2024. Under open-field conditions, mean yield was 25.1 t ha^-¹ in 2022 and 19.1 t ha^-¹ in both 2023 and 2024. Yield variability also differed among years, with the greatest variation observed in the 2022 open-field trial (SD = 12.2 t ha^-¹; range = 0–43.8 t ha^-¹), while the lowest variability occurred in the 2020 net-house experiment (SD = 2.51 t ha^-¹; range = 1.78–10.2 t ha^-¹).

Figure 5

Boxplots represent the distribution of tomato yield values for each year under net-house and open-field conditions. The box indicates the interquartile range, the horizontal line within the box represents the median, and whiskers extend to the range of the data excluding outliers. Red dots indicate mean AUDPC values.

Effect of treatments on tomato yield under CMV pressure

Tomato yield under CMV pressure varied considerably among years and production environments, but treatment effects were generally not significant (Figure 6; Supplementary Table 4). Under insect-proof net-house conditions, no significant treatment effects on yield were detected in 2020 (P = 0.9167), 2021 (P = 0.1800), 2022 (P = 0.6645), or 2024 (P = 0.1701), whereas treatment effects approached significance in 2023 (P = 0.0760). In 2020, yield ranged from 4.6–7.2 t ha^-¹, with Nimbicidine producing the highest yield (7.2 t ha^-¹). In 2021, yields ranged from 5.7–16.0 t ha^-¹, with Vircon-H recording the highest yield (16.0 t ha^-¹). In 2022, yields varied from 17.2–33.9 t ha^-¹, and Vircon-H again produced the highest yield (33.9 t ha^-¹). In 2023, yield ranged from 8.9–18.8 t ha^-¹, with Lentinan (18.8 t ha^-¹) and Anti-B (17.7 t ha^-¹) producing the highest yields. In 2024, yields ranged from 13.4–28.5 t ha^-¹, with Plantomycin recording the highest yield (28.5 t ha^-¹), although all treatments yielded less than the untreated control.

Figure 6

Under open-field conditions, treatment effects were also not significant in 2024 (P = 0.4063) and were only marginally significant in 2023 (P = 0.0530). In 2023, yields ranged from 12.9–23.2 t ha^-¹, with cow urine (23.2 t ha^-¹) and Virosin (23.1 t ha^-¹) producing the highest yields. In 2024, yields ranged from 14.6–23.0 t ha^-¹, with the untreated control recording the highest yield (23.0 t ha^-¹), followed by Rogor Plus (21.9 t ha^-¹), Anti B (21.1 t ha^-¹), and Jholmal (20.1 t ha^-¹). Although several treatments produced numerically higher yields in individual years, these responses were inconsistent across seasons and environments. Overall, repeated applications of the evaluated biorational products did not consistently improve tomato yield under CMV pressure, suggesting that seasonal environmental conditions and disease pressure had a greater influence on yield than the treatments.

Correlation between tomato yield and area under the disease progress curve for cucumber mosaic virus

A significant but weak negative correlation was observed between yield and the area under the disease progress curve (AUDPC) (r = −0.24, p = 0.02, n=148), suggesting that although increased CMV severity was associated with reduced yield, the magnitude of this relationship was limited. This indicates that a substantial proportion of yield variation was influenced by other factors, particularly environmental conditions.

Weather conditions during the experiments

Daily weather conditions varied among the experimental years (2020–2024) but followed a similar seasonal pattern from March to July (Figure 7). Maximum air temperature gradually increased from approximately 22–25 °C in March to 28–32 °C during June and July, while minimum temperature increased from 6–10 °C in early spring to approximately 18–22 °C by mid-summer. Relative humidity remained relatively high throughout the growing season, generally ranging from 70 to 95%, with a gradual increase from March to July. Rainfall was sporadic during March and April but increased markedly from late May onward, with frequent precipitation events during June and July. Although the overall seasonal trends were similar among years, 2020 and 2024 experienced more frequent and heavier rainfall events during the latter part of the growing season, whereas rainfall was comparatively lower in 2022 and 2023 (Figure 7).

Figure 7

Discussion

Across five consecutive years, the area under the disease progress curve (AUDPC) for cucumber mosaic virus (CMV) was consistently lower in tomato grown under net house conditions than in the open field, irrespective of treatment. This pattern indicates a dominant effect of production environment on CMV epidemics and underscores the importance of vector exclusion in disease suppression. Net houses provide partial physical barriers against insect vectors, buffer extreme temperature fluctuations, and reduce wind-driven vector dispersal. Because CMV is transmitted in a non-persistent manner by aphids, even brief probing events can initiate infection; thus, limiting vector access is critical (Carr et al., 2020).

The weather conditions were generally favorable for the development of CMV epidemics. Warm temperatures between approximately 20 and 30 °C accelerate aphid development, shorten generation time, and increase population growth, resulting in greater opportunities for virus acquisition and transmission. High relative humidity and periodic rainfall also promote vigorous vegetative growth, producing young, succulent tissues that serve as preferred feeding sites for winged aphids (Subba, 2013). Consequently, the combination of increasing temperature, high humidity, and frequent rainfall from late spring onwards likely enhanced secondary spread of CMV within tomato fields.

Despite broadly similar seasonal weather patterns among years, CMV epidemics differed significantly between years, indicating that weather alone did not determine disease severity. The highest AUDPC values were recorded in 2023, particularly under open-field cultivation, whereas disease pressure was substantially lower in 2022. These differences suggest that additional epidemiological factors, including the abundance and timing of aphid immigration, the level of primary inoculum in surrounding vegetation, and early-season vector pressure, also influenced epidemic development. Variation in CMV disease pressure among the experimental years may have been influenced, at least in part, by differences in transplanting dates. Tomato seedlings were transplanted on 18 April 2020, 26 April 2021, 20 April 2022, 14 March 2023, and 20 March 2024. Disease pressure consistently increased as transplanting was advanced to earlier dates. For example, the highest CMV disease pressure was observed in 2024, when transplanting occurred on 20 March, whereas the lowest disease pressure was recorded in 2020 and 2021, when transplanting was delayed until the third week of April. Earlier transplanting likely exposed young plants to higher aphid populations and more favorable environmental conditions for CMV transmission during the early growing season.

This variation in transplanting dates represents a limitation of the study because annual differences in disease pressure may have influenced the observed efficacy of the evaluated treatments. Maintaining a consistent transplanting date across years would have reduced this source of variation and allowed a more direct comparison of treatment performance under similar disease pressure. Therefore, relatively small differences in aphid movement among years can produce substantial differences in disease incidence.

Differences in yield among years were also likely influenced by the timing of transplanting, which determined the environmental conditions experienced during crop establishment and subsequent growth. The crops established in 2023 and 2024 were transplanted approximately one month earlier than those in 2020–2022, exposing plants to cooler temperatures during the early growth stage and extending the production period before the onset of heavy rainfall and high temperatures.

Although earlier transplanting generally provides a longer growing season and can improve fruit production, its benefits depend on disease pressure and prevailing environmental conditions. In this study, the earliest transplanting in 2024 resulted in the highest yield under net-house conditions (24.5 t ha^-¹), suggesting that the protected environment enabled plants to capitalize on the extended growing period while reducing virus transmission. In contrast, under open-field conditions, the highest mean yield was obtained in 2022 (25.1 t ha^-¹), despite later transplanting, whereas yields declined to 19.1 t ha^-¹ in both 2023 and 2024. These findings indicate that transplanting date alone did not determine crop performance.

The lower yield in open field conditions in 2023 and 2024 are consistent with greater disease pressure observed during those seasons. Earlier transplanting may have coincided with periods of increased aphid activity and prolonged exposure to cucumber mosaic virus (CMV), allowing more time for primary and secondary virus spread before harvest. In contrast, the net-house reduced vector access and likely mitigated virus transmission, enabling plants transplanted earlier in 2024 to achieve substantially higher yields than in previous protected-cropping seasons. Therefore, the results suggest that the interaction among transplanting date, seasonal weather conditions, vector abundance, and the level of CMV infection collectively determined tomato yield rather than transplanting date alone.

This interpretation is consistent with the disease progression results, which showed that years with higher CMV severity generally produced lower yields, particularly under open-field conditions where plants were continuously exposed to aphid-mediated virus transmission.

Although CMV infection was confirmed using CMV immunostrips, there remains a possibility of mixed infections under field conditions. Tomato plants are frequently infected by multiple viruses, including tomato mosaic virus (ToMV) and tomato leaf curl virus (ToLCV), which may occur simultaneously with CMV. While the observed symptoms were predominantly characteristic of CMV, co-infection cannot be ruled out. Mixed infections are known to increase disease severity and may influence symptom expression, making it difficult to attribute all observed symptoms solely to CMV (Wang et al, 2002). Unfortunately, due to the lack of additional immunostrips and serological diagnostic facilities to detect other viruses, co-infections could not be investigated. This should be considered a limitation of the present study.

The reduced AUDPC observed under protected cultivation strongly suggests that decreased aphid ingress and altered microclimatic conditions collectively constrained virus spread.

The role of vector exclusion was further substantiated by the 2024 open-field experiment, in which individual plots covered with white insect-proof nets exhibited significantly lower CMV incidence and AUDPC compared with uncovered controls. This finding confirms that physical barriers at both structural (net house) and plot scales can effectively suppress CMV epidemics by minimizing plant–vector contact. Collectively, these results demonstrate that vector management, rather than direct antiviral intervention, was the most effective strategy for reducing CMV pressure under the conditions evaluated.

The use of row covers for insect exclusion has been widely reported as an effective strategy to suppress virus incidence and severity in several crops affected by vector-borne viruses. For instance, D'Amore et al. (1999) reported reduced cucumber mosaic virus (CMV) infection in tomato when rows were protected with low tunnels made of non-woven polyester fabric. Similarly, Walters (2003) observed a significant reduction in aphid landings on plants, which consequently lowered the incidence of watermelon mosaic virus in Illinois, USA. In another study, Kumar et al. (2021) reported complete suppression of CMV infection (0% incidence) in tomato grown inside a net house, whereas plants grown in open field conditions exhibited up to 58% disease incidence in India. All these findings highlight the effectiveness of physical exclusion barriers in limiting vector access to host plants, thereby reducing virus transmission in susceptible crops.

In contrast, none of the evaluated biorational or antiviral products-including antiviral formulations, Vircon-H, Lentinan, Privintal, or Virosin-produced consistent or statistically significant reductions in AUDPC across environments and years. The lack of treatment efficacy over five seasons suggests that foliar-applied antiviral or disinfectant products are insufficient to limit CMV progression once vector-mediated transmission has occurred. Several factors may explain this outcome. First, CMV establishes systemic infections rapidly after inoculation (Carr et al., 2020), thereby reducing the potential effectiveness of topical applications. Second, repeated aphid-mediated introductions of the virus throughout the cropping period may have overwhelmed any localized antiviral activity. Third, environmental variability in open-field conditions may have reduced product stability or persistence on leaf surfaces.

There are very few published studies demonstrating effective suppression of plant viruses through exogenous application of antiviral compounds under field conditions. For example, Fan et al. (2022) reported suppression of papaya ringspot virus (PRSV) in papaya following the application of chitosan, lentinan, and ningnanmycin. The authors proposed that these compounds did not directly inactivate the virus but rather induced host resistance, thereby reducing disease severity. Similarly, Shabbir et al. (2020) reported reduced severity of cucumber mosaic virus (CMV) in cucumber following the application of neem and eucalyptus leaf extracts. However, the study did not clarify whether the observed reduction resulted from direct antiviral activity, induction of plant defense responses, or indirect effects through the repellence or suppression of aphid vectors.

The treatments evaluated in the present study were selected based on manufacturers’ claims of antiviral activity or their reported potential to repel insect vectors. For example, cow urine and Jholmal are widely used by Nepalese farmers as traditional for insect management because of their strong odor, which is believed to repel insect pests. Likewise, the remaining commercial products are registered and widely marketed in Nepal for plant virus management. However, none of the evaluated treatments provided satisfactory suppression of CMV incidence or disease severity under the five-year study. These findings indicate that the antiviral efficacy claimed by manufacturers was not consistently supported under practical field conditions.

According to the manufacturer, lentinan, a polysaccharide extracted from shiitake mushroom (Lentinula edodes), possesses immunomodulatory and antiviral properties that can directly inactivate viruses, inhibit viral replication, and induce systemic resistance in plants. However, reliable peer-reviewed evidence supporting these claims against plant viruses, particularly under field conditions, remains limited. Therefore, the effectiveness of such products should be interpreted with caution until independent scientific validation is available.

Nepal follows a pesticide registration by analogy (Food and Agriculture Organization of the United Nations (FAO), 2015) whereby products can be registered primarily based on efficacy and safety data generated in other countries, with limited or no mandatory validation under Nepalese agroecological conditions. Consequently, products with unverified efficacy may enter the market, and growers often rely on manufacturers’ promotional claims rather than independently validated scientific evidence. This situation may lead to the widespread use of ineffective products, resulting in unnecessary production costs and poor disease management.

A notable example is Virkon^® S (Ali et al, 2023) (LANXESS Deutschland GmbH, Cologne, Germany) (Lewandowski et al., 2010), a globally recognized disinfectant with demonstrated antiviral properties that is widely used for biosecurity and sanitation. In contrast, a product marketed as Vircon-H has been commercially available in Nepal for many years despite limited publicly available information verifying its authenticity, formulation, or efficacy. The similarity in product names may create confusion among growers and undermine confidence in registered plant protection products. Therefore, based on the findings of this five-year study, it is strongly recommended that Nepal’s pesticide registration system require independent efficacy validation under local field conditions before products intended for plant virus management are approved. Such a validation system would improve the credibility of registered products, protect farmers from misleading claims, promote evidence-based disease management, and strengthen confidence in the national pesticide regulatory system.

The biology of CMV further complicates chemical suppression. As a highly adaptable, multipartite RNA virus with a broad host range, CMV is difficult to manage solely through external antiviral agents. Rapid and repeated transmission by aphids, together with complex virus–host–vector interactions, likely limited the practical effectiveness of the tested compounds. Although some products showed numerical reductions in specific seasons, these responses were inconsistent and not reproducible across years, highlighting the limited reliability of these interventions under epidemic field conditions.

A significant but weak negative correlation between yield and AUDPC (r = −0.24, p = 0.02) indicates that increased disease severity was associated with yield decline, but that CMV intensity explained only a modest proportion of total yield variation. This suggests that other factors—particularly environmental variability and potential insect pressure—played substantial roles in determining productivity. Because aphid populations were not quantified during the experiments, it is plausible that yield reductions were influenced not only by CMV infection but also by direct feeding injury and associated physiological stress. The absence of vector population data represents a limitation and warrants inclusion in future studies.

These findings collectively emphasize that physical exclusion of aphid vectors is more effective than repeated application of antiviral or biorational products for CMV management in tomato. Integrated pest management (IPM) strategies that combine vector-proof netting, targeted aphid monitoring, and optimized agronomic practices are likely to provide more sustainable control than reliance on foliar antiviral sprays. Future research should prioritize (i) quantification of vector dynamics, (ii) refinement of barrier technologies for smallholder systems, and (iii) development of host resistance.

In Nepal, tomato is commonly cultivated under open wall plastic houses primarily to protect the crop from rainfall. However, the present study demonstrated that CMV incidence can be significantly reduced when tomatoes are grown under netted plastic houses, which restrict aphid entry and consequently reduce virus transmission. To maximize the benefits of protected cultivation, growers should adopt good sanitation practices before transplanting, including the complete removal of weeds and volunteer plants that may serve as alternative hosts for CMV and its aphid vectors, as well as the destruction of crop residues from previous planting cycles. Combining insect-proof netting with clean cultivation practices could offers an effective and practical strategy for reducing CMV incidence and improving tomato yield under Nepalese production systems.

In summary, the study reinforces the importance of environmental control in disease management, particularly through the use of net houses to reduce the spread of vector-borne diseases like CMV. However, the lack of significant effects from the applied treatments suggests that additional strategies, including integrated approaches combining physical, chemical, and biological control methods, are necessary for effective CMV management in tomato crops. Future research should explore alternative treatment options, optimize application strategies, and further investigate environmental factors that influence disease dynamics in both controlled and open field conditions. Furthermore, clean planting techniques such as raising seedlings under a virus-free net house and row cover with an appropriate net may be effective for managing CMV during the summer rainy season, especially in mid-hill regions of Nepal.

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