Emerging and re-emerging plant viruses are a growing concern in agriculture and ecosystems worldwide due to their potential to significantly impact crop yields, biodiversity, and food security. These viruses often arise or spread due to changes in environmental conditions, agricultural practices, global trade, and climate change.
1. Tomato Brown Rugose Fruit Virus (ToBRFV)
- Family: Virgaviridae
- Genus: Tobamovirus
- Host Plants: Primarily tomatoes and peppers, both economically important crops.
- Transmission: Mechanically transmitted via contaminated tools, human hands, and plant-to-plant contact; also seed-transmissible.
Emergence Factors
- Global Trade: ToBRFV spread internationally due to infected seeds and transplants.
- High Mutation Rates: This virus has adapted to overcome the genetic resistance in many commercial tomato varieties.
Impact and Symptoms
- Causes mosaic patterns, yellowing, and necrosis on leaves and fruit surfaces, resulting in deformed, unmarketable fruits.
- Highly resistant to control measures and can survive on surfaces for extended periods, posing significant challenges to greenhouse operations.
Control Measures
- Use of virus-free seeds and rigorous sanitation protocols in greenhouses.
- Development of new resistant tomato cultivars and strict quarantine measures to limit spread.
2. Maize Lethal Necrosis Disease (MLND)
- Causal Agents: Coinfection by Maize Chlorotic Mottle Virus (MCMV; family Tombusviridae) and a potyvirus, often Sugarcane Mosaic Virus (SCMV).
- Host Plants: Maize (corn).
- Transmission: Spread by insect vectors (e.g., thrips and beetles for MCMV, aphids for SCMV) and through infected seed and soil.
Emergence Factors
- Increased Maize Cultivation: High demand for maize has led to intensive monoculture farming, which exacerbates disease spread.
- Vector Movement: Expansion of vector ranges due to climate change has facilitated MLND spread across Africa and Asia.
Impact and Symptoms
- Causes chlorosis, necrosis, and wilting, often resulting in total crop failure.
- MLND has led to severe economic losses in maize-growing regions, particularly in sub-Saharan Africa.
Control Measures
- Integrated pest management to control vector populations.
- Use of virus-free seeds and rotation with non-host crops to reduce inoculum sources.
3. Cassava Brown Streak Virus (CBSV) and Ugandan Cassava Brown Streak Virus (UCBSV)
- Family: Potyviridae
- Genus: Ipomovirus
- Host Plants: Cassava, a staple food crop in Africa.
- Transmission: Transmitted by whiteflies (Bemisia tabaci) and potentially through cuttings used for propagation.
Emergence Factors
- Climate Change: Changing weather patterns have increased whitefly populations and altered their distribution, enhancing virus spread.
- Inadequate Sanitation: Farmers often use infected cuttings, facilitating local and regional spread.
Impact and Symptoms
- Causes root necrosis, making cassava tubers unfit for consumption or sale.
- Leads to substantial losses in cassava production, directly affecting food security in affected regions.
Control Measures
- Use of virus-resistant cassava varieties and sanitized planting material.
- Whitefly management through biological control, insecticides, and use of clean propagation practices.
4. Wheat Blast (Magnaporthe oryzae Triticum Pathotype)
- Causal Agent: A fungus, but often synergizes with viruses, notably Wheat streak mosaic virus (WSMV).
- Host Plants: Wheat.
- Transmission: Seed-borne and also spread by infected crop residues.
Emergence Factors
- Climate Variability: Increased rainfall and temperature fluctuations create conditions conducive to wheat blast outbreaks.
- Genetic Uniformity: Intensive wheat monocultures with limited genetic diversity allow rapid spread and establishment of pathogens.
Impact and Symptoms
- Causes bleached spikes, shriveled grains, and premature death of infected plants, leading to major yield losses.
- Has particularly devastated wheat production in South America and poses a threat to Asia.
Control Measures
- Use of disease-free seeds and rotation with non-host crops.
- Development of resistant wheat varieties and strict quarantine regulations.
5. Banana Bunchy Top Virus (BBTV)
- Family: Nanoviridae
- Genus: Babuvirus
- Host Plants: Banana, a crucial fruit and staple food in many tropical regions.
- Transmission: Spread by the banana aphid (Pentalonia nigronervosa) and through infected planting material.
Emergence Factors
- Global Movement of Planting Materials: Transportation of infected banana plants has spread BBTV to new regions.
- Vector Expansion: Climate change has expanded the habitat of banana aphids, facilitating wider virus spread.
Impact and Symptoms
- Causes stunting, with plants producing small, bunched leaves and failing to fruit.
- Results in significant economic losses in banana production, especially in smallholder farming communities.
Control Measures
- Use of virus-free tissue culture plantlets for propagation.
- Aphid control and removal of infected plants to prevent local spread.
6. Apple Proliferation Phytoplasma
- Family: Not applicable (phytoplasmas are bacteria-like pathogens).
- Genus: Candidatus Phytoplasma
- Host Plants: Apple trees and other rosaceous plants.
- Transmission: Transmitted by leafhoppers (Cacopsylla picta) and through infected rootstocks.
Emergence Factors
- Vector Spread: Leafhopper vectors have expanded their range due to warmer winters and increased orchard plantings.
- Increased Commercial Plantings: High-density apple orchards promote the spread of phytoplasmas.
Impact and Symptoms
- Causes witches’ broom, small fruits, and early leaf drop, significantly affecting apple quality and yield.
- Outbreaks have led to substantial economic losses in Europe.
Control Measures
- Removal of infected trees and use of resistant rootstocks.
- Vector management through insecticides and habitat modification.
Factors in the Emergence and Re-emergence of Plant Viruses
- Climate Change: Shifts in climate alter the distribution and population dynamics of insect vectors, often extending their range and increasing virus transmission rates.
- Global Trade and Movement of Plant Material: International trade spreads viruses and vectors through infected seeds, plants, and propagation material.
- Intensified Agricultural Practices: Monoculture farming and intensive planting increase susceptibility to virus outbreaks.
- Vector Adaptation: Increased virus-vector adaptability enhances virus transmission, particularly for viruses that can switch vectors or have broad vector ranges.
- Genetic Mutability of Viruses: High mutation rates in plant viruses allow them to overcome plant resistance genes, leading to frequent outbreaks of new viral strains.