Viruses have a significant impact on the tomato industry in South Africa, affecting different aspects of production, economic viability and food security.
South Africa is well-known for its diverse agricultural landscape with producers having to face numerous challenges in safeguarding their crops against various diseases daily. Among these challenges, viruses stand out as a significant aspect that has impacted the country’s tomato industry. The most common viruses to occur are TMV (Tobacco mosaic virus), TSWV (Tomato spotted wilt virus) and TYLCV (Tomato yellow leaf curl virus) in South Africa also referred to as ToCSV (Tomato Curly Stunt Virus). Not so common viruses, but appearing sporadically are PVY (Potato virus Y), AMV (Alfalfa mosaic virus) and ToCV (Tomato chlorosis virus).
Key impacts viruses have:
Reduced yields
Viral infections can lead to a decrease in tomato yields, especially when the plant is infected in the early stages of its life cycle. Infected plants can show symptoms such as stunted growth, reduced fruit size, and poor fruit quality. This reduction in yield and quality directly affects the overall productivity and profitability of tomato farming.
Marketability issues
Viral infections can render tomatoes unmarketable due to visual symptoms and reduced quality. Although virus infected fruit do not affect humans, tomatoes showing signs of viral infection are rejected by consumers, leading to market losses for the producer. This can have cascading effects on supply chains and market dynamics.
Economic losses
Lower yields and decreased market value of infected produce result in financial challenges for both the producer and the broader tomato industry. The cost of implementing control measures and managing viral infections adds to the economic burden. It is critical to note that control measures need to be done at the correct stage before the vectors transmit the virus. Once a plant has been infected with a virus no other measure will prevent the economic losses. Virus symptoms are often only noticed two to four weeks after infection has taken place. At this stage no insect vectors are visible, but infarction has already taken place. Regular and accurate scouting for insects is therefore a very critical part of any management strategy.
Increased production cost
Managing viral infections requires additional resources, including the use of pesticides, preventative measures, and the removal and disposal of infected plants. These increased production costs add financial strain on tomato producers.
Impact on export markets
South Africa is a significant exporter of tomatoes and viral infections can jeopardize the country’s reputation and competitiveness in international markets. Strict phytosanitary regulations in importing countries may lead to restrictions or bans on the export of infected tomatoes.
Environmental impact of pesticide use
In attempts to control viral infections, growers may resort to increased pesticide use. This can have environmental consequences, affecting non-target organisms, soil health and water quality.
Long-term sustainability issues
The continued presence of viruses in the tomato industry raises long-term sustainability concerns. Sustainable farming practices, such as Integrated Pest Management (IPM) and the development of resistant varieties, are essential for securing the industry’s future.
Challenges for smallholder growers
Smallholder growers, who often lack resources for advanced disease management, can be particularly vulnerable to the impacts of viral infections. Their livelihood may be disproportionately affected, contributing to food insecurity at household level. Many smallholder growers still rely on open pollinated varieties with no virus resistance, this makes economic production very difficult.
To address these challenges, a multi-faceted approach that combines research, education and regulatory measures is crucial. Once a plant is infected, it will remain infected for its life cycle, therefore it is important to keep the disease away from other plants, as far as possible. Most of the viruses that affect tomatoes are transmitted through vectors, but not all. Table 1 shows how the different viruses are being transmitted.
Table 1: How different viruses are transmitted
| Virus | Transmitted by |
| TMV, ToMV | Contact |
| TSWV | Thrips |
| TYLCV (ToCSV), ToCV | Whiteflies TYLCV (Bemisia tabaci) ToCV (Trialeurodes vaporariorum ) |
| PVY, AMV | Aphids |
To combat these viruses, different approaches should be considered, depending on the way the virus is transmitted. Viruses being transmitted by contact will mainly be combated by means of sanitary precautions, while viruses that are being transmitted through vectors (aphids, thrips and whiteflies) will mainly focus on controlling the vector. For both viruses being transmitted either by contact or through a vector, the first step will always be to remove the diseased plants.
Breeding for resistance
Breeding for virus resistance is highly successful in modern agriculture. Tomato varieties have been bred to have resistance against TMV, ToMV, TSWV, TYLCV and a combination of these viruses. Breeding for resistance is also the first line of defence to ensure that a tomato cannot be infected by viruses. Unfortunately, it is also true that breeding a tomato variety to have resistance against all viruses is very difficult and highly unlikely, therefore the use of phytosanitary precautions and IPM are still very important.
Phytosanitary precautions
Implementing effective sanitary precautions is crucial to combat viruses, especially the ones that are transmitted through contact. Sanitary measures that should be in place are:
- Hand hygiene – workers need to wash their hands with disinfecting soap before entering tomato fields
- Footwear sanitation
- The use of personal protective equipment, such as gloves and the proper disposal thereof
- Sanitation of tools and equipment
- Restricted access, to limit unnecessary movement
Integrated Pest Management
IPM is a holistic approach that combines various strategies to control pests while minimizing the impact on the environment, human health and beneficial organisms. The following IPM strategies can be used to combat thrips, aphids and whiteflies:
- The use of biological control – by using natural predators’ beneficial plants that harbor beneficial insects
- Cultural practices – by weeding and removing alternative host plants, that harbor these vectors. TSWV has over 925 host plants, some of which will carry the virus without expressing symptoms
- Selective pesticide use, especially rotating between different chemical class groups to prevent the vectors from building resistance
- Yellow and blue sticky traps, in the case of whiteflies and thrips
It is important to keep in mind that IPM requires regular monitoring, timely interventions and an understanding of the specific pests and beneficial organisms in a tomato production system. Integrating these strategies collectively enhances the overall resilience of the tomato crop against thrips, aphids and whiteflies.
As South Africa continues to face various challenges with viruses, ongoing research remains crucial for developing sustainable and resilient solutions. By implementing phytosanitary measurements, adopting Integrated Pest Management practices and planting the correct tomato varieties, producers will be able to manage these viruses and grow a healthy crop that will provide for the country.