The appearance of spots on potato plants and tubers is a universal problem for both commercial growers and home gardeners. “Potato spot” is a blanket term for multiple distinct diseases and physiological disorders, not a single ailment. Accurate identification of the root cause is essential for effective treatment and management, as the corrective action for a fungal disease is completely different from that for a nutrient deficiency or physical damage. Understanding the specific origins of these imperfections protects the health and quality of the potato crop.
Principal Causes of Potato Spotting
The origins of potato spotting fall into three main categories: fungal and oomycete pathogens, bacterial organisms, and non-living environmental or physiological factors. Fungal diseases depend heavily on specific climatic conditions for development and spread. Early Blight, caused by the fungus Alternaria solani, is favored by warm temperatures (75–84°F), high humidity, and leaf wetness. This disease often targets older or stressed plants later in the growing season, surviving winter on infected crop debris and in the soil.
Late Blight is caused by the oomycete Phytophthora infestans, a water mold that thrives in cooler, very moist environments. It requires free moisture and moderate temperatures (night temperatures 50–60°F, daytime 60–70°F) for rapid development. Under these conditions, Late Blight spreads rapidly, infecting foliage and moving into tubers to cause a wet rot. The pathogen survives between seasons in infected seed tubers, cull piles, and volunteer plants.
Common Scab is caused by the soil-borne bacterium Streptomyces scabies. This widespread organism survives in most soils but prefers low soil moisture during the critical period of tuber initiation (the first few weeks after tubers start to form). Common Scab is significantly more severe when the soil pH is neutral or slightly alkaline (5.2 to 8.0), making soil chemistry a primary trigger.
Physiological disorders are non-pathogenic, meaning they are not caused by living organisms. Black Spot Bruise is a common internal defect resulting from mechanical impact during harvesting, handling, or storage. The damage occurs when underlying cells are crushed, releasing enzymes that form the dark pigment melanin, causing a dark gray to black discoloration beneath the intact skin. Tubers that are dehydrated or harvested with low pulp temperatures are particularly susceptible.
Internal Brown Spot is another physiological issue, characterized by irregular necrotic spots scattered within the tuber flesh. The primary cause is a localized deficiency of calcium in the rapidly growing areas of the tuber. This deficiency is exacerbated by irregular growth patterns, extreme fluctuations in soil moisture, or high temperatures, which interfere with calcium uptake. The spots are groups of dead cells, free of bacterial or fungal infection.
Visual Diagnosis Identifying Specific Spots
Differentiating potato spots requires close attention to the lesion appearance on both foliage and tubers. Early Blight first appears on older, lower leaves as small, dark, circular-to-oval spots. The defining characteristic is the presence of concentric rings, creating a distinct “bullseye” or “target-board effect” within the necrotic tissue.
Late Blight symptoms on foliage are visually distinct from Early Blight, starting as small, irregularly shaped, water-soaked spots that look dark green or greasy. These spots expand quickly, turning large sections of the leaf brown or purplish-black, and are not confined by leaf veins. Under humid conditions, a diagnostic sign is the appearance of white, downy fungal growth (the pathogen sporulating), typically visible on the underside of the leaf at the lesion edge.
On the tuber itself, spots caused by disease and physical injury present very differently. Common Scab manifests as lesions on the tuber surface that can be shallow and superficial (roughened, corky texture) or deeply pitted (dark, crater-like indentations). The lesion type varies depending on the specific bacterial strain and environmental conditions during tuber development.
Internal Brown Spot and Black Spot Bruise are diagnosed only by cutting open the tuber, as they rarely show external symptoms. Internal Brown Spot appears as dry, rust-brown, irregularly shaped necrotic areas scattered throughout the flesh, usually concentrated toward the bud end. Black Spot Bruise presents as a dark gray to black discoloration of the tissue just beneath the skin, forming where the physical impact occurred.
Comprehensive Management and Prevention
Effective control relies on cultural practices and targeted interventions based on the identified cause. A fundamental preventive measure for soil-borne issues like Common Scab is practicing long crop rotations (ideally three to five years) to reduce the pathogen population. Planting potato varieties with known resistance to specific diseases, such as blights or scab, also significantly lowers the risk of infection.
For Common Scab, the most critical control is managing soil pH, as the bacterium is suppressed in highly acidic conditions. Maintaining the soil pH between 5.0 and 5.2 is effective; growers can use acid-producing fertilizers like ammonium sulfate to maintain this range. Consistent soil moisture is also necessary during the first few weeks after tuber initiation, as dry soil during this period greatly increases scab severity.
Managing fungal and oomycete diseases like Early and Late Blight involves controlling moisture and using chemical treatments. Avoiding overhead irrigation, especially at night, helps keep foliage dry, which is less favorable for spore germination and spread. Fungicides are applied preventatively or upon the first appearance of symptoms, requiring a regular schedule to protect the crop once the disease is established.
Preventing physiological disorders requires careful attention to nutrition and post-harvest handling. Internal Brown Spot is significantly reduced by ensuring an adequate supply of calcium, applied to the soil near developing tubers at the time of tuber initiation. Since calcium is not easily redistributed in the plant, it must be available to new cells as they form.
To prevent Black Spot Bruise, minimizing physical impact during harvest and handling is necessary. Harvesting when the tuber pulp temperature is optimal and ensuring tubers are adequately hydrated reduces susceptibility to bruising. After harvest, proper curing and storage with controlled humidity and temperature are essential to allow minor wounds to heal and prevent pressure bruises.