Potato scab is a globally widespread disease affecting potato tubers, causing significant economic losses for commercial producers and frustration for home gardeners. The disease primarily diminishes the aesthetic appeal and marketability of the crop, creating unsightly blemishes on the potato skin. Though this condition does not typically affect the potato’s internal quality, its presence can cause substantial downgrading or rejection of the harvest.
Symptoms and the Causal Agent
The symptoms of potato scab manifest exclusively on the surface of developing tubers, never affecting the above-ground foliage. The lesions are variable, presenting as superficial, russeted areas, slightly raised corky patches, or deep, dark pits that penetrate the skin. The final appearance depends on the potato variety, the specific strain of the pathogen, and environmental conditions during tuber formation.
This disease is caused by the soil-dwelling bacterium, Streptomyces scabies, which belongs to the Streptomyces genus. This filamentous bacterium survives indefinitely in the soil as a saprophyte, feeding on decaying organic matter. The bacterium produces a toxin called thaxtomin, which causes potato cells to die and respond by forming the characteristic corky, scabby tissue.
Soil Conditions that Promote Scab Development
The severity of potato scab depends on specific environmental factors in the soil, particularly moisture and pH levels. Streptomyces scabies is favored by neutral to alkaline soil conditions. The disease is greatly suppressed when the soil pH is maintained below 5.2, but severity increases as the pH rises toward 7.0 and above, often seen in soils where lime has been recently applied.
In addition to soil chemistry, the moisture level during the initial stages of tuber development plays a determining role in infection. Scab is much more severe under warm, dry soil conditions, particularly during the first four to six weeks after the tubers begin to form. Low soil moisture during this period allows the Streptomyces bacteria to successfully infect young tubers through natural openings called lenticels.
Cultural Practices for Management and Prevention
Selecting potato varieties with known resistance or tolerance is often the most straightforward management strategy. While no variety is completely immune, options like ‘Superior,’ ‘Norland,’ and ‘Russet Burbank’ are known to be less susceptible to common scab.
Soil Modification
Soil modification techniques focus on lowering the pH, which is the primary method for long-term control of common scab. Applying acidifying fertilizers, such as ammonium sulfate, can help gradually reduce the soil pH over time. For small garden plots, incorporating elemental sulfur into the soil well in advance of planting allows time for the microbial breakdown process that generates the necessary acidity.
Water Management
Precise water management during the tuberization period is an effective cultural practice. Maintaining consistently high soil moisture, ideally around 80–85% of the soil’s water-holding capacity, for the four to six weeks following tuber initiation significantly reduces scab incidence. This consistent moisture helps suppress the Streptomyces population and prevents the dry conditions that favor infection. Once tubers develop a mature skin, they are no longer susceptible to new infection, and watering frequency can be reduced.
Crop Rotation
A long-term strategy involves practicing crop rotation, which helps reduce the overall pathogen load in the soil. Planting potatoes in the same area should be avoided for at least three to four years, rotating instead with non-susceptible crops like grains, legumes, or leafy greens. Using certified, disease-free seed potatoes is also important, as infected seed pieces can introduce the bacteria into clean soil.
Effects on Edibility and Storage
Potatoes affected by common scab are safe to eat, as the disease is purely cosmetic and does not penetrate deeply into the flesh of the tuber. The lesions can simply be removed by peeling before the potato is prepared for consumption. In cases of deep-pitted lesions, however, significant peeling may be required, which increases waste and reduces the usable portion of the harvest.
The presence of scab lesions affects the post-harvest storage potential of the tubers. The damaged, corky tissue can lead to increased moisture loss, causing potatoes to shrink more rapidly in storage. Additionally, these lesions act as entry points for secondary organisms, such as soft-rotting bacteria, which can cause the entire tuber to decay. Scabby potatoes should be stored in a cool, dark, and dry environment to minimize the chance of secondary rot development.