Pomovirus

This pathogen acts as a systemic parasite that spreads through plant tissues. Unlike many other viruses, Pomovirus relies on a biological vector to infect host plants, specifically the soil-borne plasmodiophorid fungus Spongospora subterranea.

The virus is found within the resting spores (sporosori) of the fungus, which allows it to survive in the soil for many years. This unique biological association ensures the virus can persist even in the absence of a host crop.

The systemic nature of the infection means that once a plant is infected, the virus spreads to all organs, including the tubers, facilitating its transmission through the seed supply chain.

Biologically, the virus is highly stable and can remain infectious in resting fungal spores in the soil for over a decade, making it a persistent threat in infested fields.

Symptoms of Pomovirus infection are highly variable, often depending on the host cultivar and environmental conditions. Many infected plants exhibit latent infection, showing no visible symptoms despite the presence of the virus.

Foliar symptoms typically include yellow mottling, chevron-like patterns, or V-shaped markings on the leaves. Infected plants may also exhibit severe stunting and shortened internodes, leading to a crowded, bushy appearance known as "mop-top."

Tubers often develop internal necrotic lesions, such as brown arcs, rings, or spots, commonly referred to as "spraing." These symptoms can significantly downgrade the quality of the harvest.

These internal blemishes are often more visible after cutting the tuber and can sometimes worsen during storage if temperature and humidity conditions are not strictly controlled.

• Yellow V-shaped or chevron-like patterns on leaves.
• Stunted growth and characteristic bushy habit.
• Brown necrotic rings or arcs inside the tuber flesh.
• Deformation of tubers and irregular skin surface.
• Reduced yield and poor storage quality.

The development of Pomovirus is inextricably linked to the presence of its vector, Spongospora subterranea, which causes powdery scab. The virus is introduced to the plant when zoospores from the fungus infect the roots.

Cool and moist soil conditions are essential for the movement of fungal zoospores. High soil moisture levels, especially during the early stages of tuber formation, greatly increase the probability of successful infection.

The disease spreads across fields primarily through the movement of contaminated soil attached to machinery or tools. Long-distance spread occurs predominantly through the trade and planting of infected seed potatoes.

Poorly drained, heavy soils that retain moisture for longer periods provide the most favorable environment for the vector to flourish, thereby promoting virus transmission.

Even if the fungus does not cause significant powdery scab lesions, the virus can still be transmitted to the plant, making it a covert but potent pathogen in affected regions.

The primary harm caused by Pomovirus is the reduction in marketable yield and quality. Tuber necrosis renders potatoes unsuitable for fresh market sale and high-value food processing, such as chip or fry manufacturing.

Infected crops often show reduced tuber size and uneven growth. The economic impact includes costs associated with sorting out damaged produce and the risk of entire batches being rejected by processors.

The virus compromises the overall health of the plant, potentially increasing susceptibility to secondary fungal and bacterial rot infections during storage.

For seed growers, the presence of Pomovirus is catastrophic, as it prohibits the field from being used for certified seed production due to the risk of systemic viral transmission.

Long-term infestation of fields by the virus-carrying fungus limits land use, requiring growers to invest in expensive long-term rotation strategies to mitigate the impact.

The most effective control strategy is the exclusive use of certified, virus-free seed potatoes. Testing seed lots for the presence of the virus is a critical step in preventing the introduction of the pathogen.

Implementing long crop rotation cycles, typically at least 4 to 6 years, can help reduce the soil inoculum levels of the vector. Avoiding fields with a known history of powdery scab is highly recommended.

Strict sanitation protocols for farm equipment are necessary to prevent the spread of contaminated soil between different production areas. Thorough cleaning and disinfection are mandatory after working in infested soil.

Improving field drainage to prevent waterlogging is a key cultural practice to limit the activity of the fungal vector. Managing soil pH can also influence the survival of Spongospora subterranea.

While direct chemical control of the virus is not feasible, managing the powdery scab fungus through soil applications can indirectly reduce the incidence of virus transmission to the potato roots.