9.4 Viral diseases

Plant viruses are microscopic parasites that infect and replicate within living plant cells, causing diseases that can devastate crops. These pathogens spread through various means, including insect vectors, mechanical transmission, and infected seeds, leading to significant economic losses in agriculture.

Viral infections in plants manifest through diverse symptoms like leaf mottling, curling, yellowing, and stunting. Diagnosis involves serological and molecular techniques, while control strategies include developing resistant varieties, managing vectors, and implementing cultural practices to minimize virus spread and impact.

Viruses as plant pathogens

• Viruses are obligate intracellular parasites that require living host cells to replicate and cause disease in plants
• Plant viruses are responsible for significant economic losses in agriculture by reducing crop yields and quality
• Viruses can infect a wide range of plant species, including important food crops (wheat, rice, potatoes) and ornamental plants

Viral infection process

Virus attachment to host

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• Viruses initiate infection by attaching to specific receptors on the surface of host plant cells
• Attachment is mediated by viral coat proteins that recognize and bind to host cell receptors
• Successful attachment allows the virus to penetrate the host cell and initiate the infection process

Viral entry into cells

• Following attachment, viruses enter host cells through various mechanisms (endocytosis, membrane fusion)
• Some viruses inject their genetic material directly into the host cell cytoplasm
• Others enter the cell intact and undergo uncoating to release their genetic material

Virus replication in host

• Once inside the host cell, viruses hijack the cell's machinery to replicate their genetic material and produce viral proteins
• Viral replication occurs in specific cellular compartments (nucleus, cytoplasm) depending on the virus type
• Replication often leads to the formation of viral inclusion bodies or factories within infected cells

Virus assembly and release

• Newly synthesized viral components are assembled into complete virus particles within the host cell
• Assembly may occur in the cytoplasm, nucleus, or specific cellular membranes
• Mature viruses are released from the host cell through cell lysis or budding, allowing them to infect neighboring cells and spread throughout the plant

Types of plant viruses

DNA viruses vs RNA viruses

• Plant viruses can have genomes composed of either DNA or RNA
• DNA viruses (Caulimoviridae, Geminiviridae) replicate using DNA polymerases and typically have double-stranded DNA genomes
• RNA viruses (Potyviridae, Bromoviridae) replicate using RNA-dependent RNA polymerases and may have single-stranded or double-stranded RNA genomes

Single-stranded vs double-stranded viruses

• Single-stranded viruses have genomes consisting of a single strand of nucleic acid (DNA or RNA)
  • Examples include Potyviridae (ssRNA) and Geminiviridae (ssDNA)
• Double-stranded viruses have genomes composed of two complementary strands of nucleic acid
  • Examples include Reoviridae (dsRNA) and Caulimoviridae (dsDNA)

Positive-sense vs negative-sense RNA viruses

• Positive-sense RNA viruses have genomes that can directly serve as mRNA for protein synthesis
  • Examples include Potyviridae and Bromoviridae
• Negative-sense RNA viruses have genomes that must be transcribed into positive-sense RNA before translation can occur
  • Examples include Rhabdoviridae and Bunyaviridae

Major plant viral diseases

Mosaic diseases

• Caused by viruses that induce mottling, discoloration, and mosaic patterns on leaves
• Examples include Tobacco mosaic virus (TMV) and Cucumber mosaic virus (CMV)
• Mosaic diseases can affect a wide range of crops (tomatoes, peppers, cucurbits) and reduce yield and fruit quality

Leaf curl diseases

• Characterized by upward or downward curling and distortion of leaves
• Commonly caused by viruses in the Geminiviridae family (Tomato yellow leaf curl virus, Cotton leaf curl virus)
• Leaf curl diseases can stunt plant growth and reduce photosynthetic efficiency

Yellowing diseases

• Viruses that induce yellowing, chlorosis, and vein clearing in infected leaves
• Examples include Beet yellows virus (BYV) and Barley yellow dwarf virus (BYDV)
• Yellowing diseases can weaken plants, reduce yield, and make them more susceptible to other stresses

Stunting diseases

• Caused by viruses that inhibit plant growth, resulting in dwarfed or stunted plants
• Examples include Rice tungro bacilliform virus (RTBV) and Wheat dwarf virus (WDV)
• Stunting diseases can significantly reduce crop yield and quality

Viral disease transmission

Vector-mediated transmission

• Many plant viruses are transmitted by insect vectors (aphids, whiteflies, leafhoppers)
• Vectors acquire the virus while feeding on infected plants and transmit it to healthy plants during subsequent feeding
• Some viruses can replicate within the vector, while others are only carried on the vector's mouthparts

Mechanical transmission

• Viruses can spread through physical contact between infected and healthy plants
• Transmission can occur through contaminated tools, equipment, or human handling
• Mechanical transmission is common for stable viruses like TMV

Seed transmission

• Certain viruses can infect seeds, allowing them to be transmitted to the next generation of plants
• Seed transmission is an important survival mechanism for some viruses (Pea seed-borne mosaic virus, Lettuce mosaic virus)
• Infected seeds can introduce viruses into new areas and perpetuate disease cycles

Grafting transmission

• Viruses can spread from infected to healthy plants through grafting
• Grafting is a common practice in horticulture and can inadvertently transmit viruses if infected plant material is used
• Viruses like Citrus tristeza virus (CTV) and Plum pox virus (PPV) can be transmitted through grafting

Symptoms of viral infections

Leaf mottling and mosaics

• Irregular patches of light and dark green or yellow and green on leaves
• Caused by uneven distribution of chlorophyll due to viral interference with chloroplast development
• Examples include TMV and CMV

Leaf curling and distortion

• Upward or downward curling of leaf margins, often accompanied by thickening and brittleness
• Induced by viruses that disrupt normal leaf development and expansion
• Common in Geminivirus infections (Tomato yellow leaf curl virus)

Yellowing and chlorosis

• Uniform or patchy yellowing of leaves due to reduced chlorophyll content
• Caused by viruses that interfere with photosynthesis or chloroplast function
• Examples include BYV and BYDV

Plant stunting and dwarfing

• Reduced plant height and overall growth due to viral interference with plant development
• Caused by viruses that disrupt hormonal balance or meristem function
• Common in infections by RTBV and WDV

Diagnosis of viral diseases

Serological techniques

• Detect viral proteins using antibodies specific to the virus
• Enzyme-linked immunosorbent assay (ELISA) is a common serological method for virus detection
• Serological techniques are rapid, specific, and can handle large numbers of samples

Molecular techniques

• Detect viral nucleic acids using PCR-based methods or nucleic acid hybridization
• Reverse transcription PCR (RT-PCR) is used for RNA viruses, while PCR is used for DNA viruses
• Molecular techniques are highly sensitive and specific, allowing for early detection and strain identification

Electron microscopy

• Visualize virus particles using high-magnification electron microscopes
• Transmission electron microscopy (TEM) can reveal virus morphology and cellular localization
• Electron microscopy is useful for confirming the presence of viruses and studying their structure

Control of viral diseases

Resistant plant varieties

• Develop and deploy plant varieties with genetic resistance to specific viruses
• Resistance can be introduced through traditional breeding or genetic engineering
• Resistant varieties are the most effective and sustainable approach to managing viral diseases

Vector control measures

• Manage insect vector populations to reduce virus transmission
• Use insecticides, biological control agents, or physical barriers to control vectors
• Integrate vector control with other management practices for effective disease suppression

Cultural control practices

• Implement cultural practices that minimize virus spread and impact
• Examples include crop rotation, intercropping, and adjusting planting dates to avoid peak vector activity
• Cultural practices can help break disease cycles and reduce inoculum levels

Sanitation and hygiene

• Remove and destroy infected plants to prevent virus spread
• Disinfect tools, equipment, and greenhouse surfaces to eliminate virus particles
• Implement strict hygiene protocols in nurseries and propagation facilities to prevent introduction and spread of viruses

Economic impact of plant viruses

Crop yield losses

• Viral diseases can significantly reduce crop yields by impairing plant growth and development
• Yield losses can range from minor to complete crop failure, depending on the virus and host combination
• Examples of yield-reducing viruses include Cassava mosaic virus and Maize streak virus

Reduced crop quality

• Viruses can affect crop quality by inducing visual defects (mottling, distortion) or altering taste and nutritional value
• Reduced quality can lower market value and consumer acceptance of infected produce
• Examples include Potato virus Y, which causes necrotic ringspots on potato tubers

Increased production costs

• Managing viral diseases requires additional inputs (pesticides, resistant varieties) and labor (scouting, roguing)
• Crop losses and reduced quality can necessitate replanting or compensatory practices, increasing overall production costs
• Quarantine measures and trade restrictions imposed due to viral diseases can further increase costs and limit market access

Emerging plant viral diseases

Factors influencing emergence

• Globalization and increased international trade facilitate the introduction of viruses into new areas
• Climate change can alter virus and vector distribution, enabling viruses to expand their range
• Intensification of agriculture and reduced genetic diversity in crops can create conducive conditions for virus emergence

Recent outbreaks and epidemics

• Cassava brown streak virus (CBSV) has emerged as a major threat to cassava production in East Africa
• Tomato brown rugose fruit virus (ToBRFV) has rapidly spread globally, causing severe fruit damage in tomatoes and peppers
• Maize lethal necrosis (MLN), caused by co-infection of maize with two viruses, has emerged as a devastating disease in East Africa

Strategies for early detection

• Develop and deploy sensitive diagnostic tools for rapid virus detection in plants and vectors
• Establish surveillance networks to monitor virus presence and distribution in crops and wild plant populations
• Utilize advanced technologies (remote sensing, genomic sequencing) to identify and characterize emerging viruses
• Foster collaboration among researchers, extension agents, and growers to facilitate early detection and reporting of new viral diseases