Across the United States, potato growers are facing a quiet but increasingly costly threat: the growing prevalence of Potato mop-top virus (PMTV) and its close partner in damage, powdery scab disease. Together, they form a disease complex that is steadily reshaping risk management strategies in potato production regions nationwide.
PMTV is particularly troubling because of how it manifests. The virus infects tubers and causes internal necrotic lesions, dark, corky arcs or concentric rings that appear beneath skin. This symptom, commonly known as “spraing,” may not be visible from the outside, yet it can reduce quality and make potatoes unmarketable. In both processing markets and the pre-packed supermarket trade, even a small percentage of infected tubers can result in the rejection of an entire shipment. For growers, that means significant financial loss from a problem that is often hidden until harvest or grading.
While spraing is the hallmark symptom, PMTV can also produce foliar and external tuber symptoms in some cases. Primary infections occur when virus-free seed potatoes are planted into PMTV-infested soil. In these situations, symptoms are largely confined to the tubers. Secondary infections are more impactful though. Affected plants may show distorted leaves, yellow mosaic patterns, V-shaped flecks, and shortened internodes, giving the crop a stunted, unhealthy appearance.
One of the most challenging aspects of PMTV is its persistence though. The virus can remain infective in soil for 10 to 12 years, even in the absence of potato crops. This long-term survival makes crop rotation alone insufficient for eradication and complicates field management decisions.
Scientifically, PMTV is a three-part single-stranded RNA virus with two primary strains: the S (severe) strain and the M (mild) strain. Both of these strains differ by only seven amino acids. Yet that small genetic difference appears to have significant consequences. The S-type strain predominates in North America, Europe, and Asia, while the M-type strain is more common in the Andean region. The global dominance of the S strain suggests that those seven amino acid differences may enhance its efficiency in being acquired and transmitted by its vector.
That vector is Spongospora subterranea f. sp. subterranea (Sss), the soilborne pathogen responsible for powdery scab. Sss not only spreads PMTV but also directly damages crops. As a plasmodiophorid pathogen, it infects roots, stolons, and tubers, producing root galls and scab-like lesions. On tubers, these raised, corky lesions reduce cosmetic quality and marketability. More importantly, infected tubers used as seed become a source of inoculum, perpetuating both powdery scab and PMTV in subsequent plantings.
Environmental conditions strongly influence the incidence and distribution of both the virus and its vector. Cool, damp soils favour Sss development, which in turn enhances virus transmission. High soil moisture, frequent rainfall, and moderate temperatures create ideal conditions for infection. As climate variability increases, understanding these environmental drivers becomes even more critical for growers.
Currently, management of PMTV relies heavily on controlling its vector. Although breeders have identified potato cultivars with varying levels of resistance to Spongospora subterranea, no variety is completely resistant to powdery scab. Chemical seed treatments including formaldehyde, quintozene, mercury compounds, maneb, zinc oxide, and fluazinam have shown varying degrees of effectiveness in reducing disease severity and PMTV-induced tuber necrosis. However, chemical control alone is not a guaranteed solution.
In the end, prevention remains the most effective strategy. Planting certified disease-free seed and avoiding fields known to be infested with Sss are critical first steps. Given the virus’s long-term persistence in soil and the economic consequences of tuber rejection, careful field selection and seed sourcing are indispensable components of a successful management plan.
As PMTV and powdery scab continue to expand across U.S. potato-growing regions, vigilance, research, and integrated management strategies will be essential to protect crop quality and safeguard grower profitability
For detailed information on PMTV and its economic impact, read the full article by Rushton and Nalam, here- https://doi.org/10.1564/v32_feb_04
Photo Credit: Kalyandurg, P. B., Swedish University of Agricultural Sciences.
