Potatoes are propagated vegetatively through seed tubers, which creates an inherent risk for transmitting pathogens from one season to the next. Disease presence can lead to significant yield losses, tuber rot in storage, and reduced quality and marketability of the harvest. Understanding the specific nature of these infectious agents is the first step toward effective crop management.
Categorizing Potato Pathogens
Potato diseases are broadly classified based on the type of infectious organism. Fungal and oomycete diseases represent a large portion of potato issues, exemplified by the late blight pathogen, Phytophthora infestans. This organism causes rapid foliar dieback and tuber rot. Alternaria solani is responsible for early blight, which typically affects older foliage first. Other fungal agents, such as Rhizoctonia solani, cause black scurf on tubers and cankers on stems, interfering with normal plant growth.
Bacterial diseases cause soft rots and vascular wilts. Pectobacterium species are the causal agents for blackleg and bacterial soft rot, known for rapidly macerating plant tissue. The bacterium Streptomyces scabiei causes common scab, which manifests as corky lesions on the tuber surface, reducing cosmetic quality. These pathogens are often soil-borne and can enter the potato through wounds or natural openings like lenticels.
Viral and viroid diseases use the plant’s cellular machinery to reproduce. Viruses like Potato leafroll virus (PLRV) and Potato virus Y (PVY) cause substantial yield reductions worldwide. PLRV causes characteristic symptoms such as leaf rolling, while PVY can lead to mosaic patterns, leaf drop, and necrosis. These systemic infections cannot be cured and remain within the plant and its daughter tubers, making them problematic for seed production.
Recognizing Common Symptoms
On the foliage, late blight often first appears as dark, water-soaked lesions that quickly expand across the leaves. Under moist conditions, a faint, white, downy growth of mold may be visible on the underside of these lesions. In contrast, early blight presents as dark, circular spots, often showing distinct concentric rings that create a target-like pattern.
Blackleg, caused by Pectobacterium species, is characterized by a slimy, inky-black decay that girdles the stem base near the soil line. This rot disrupts water and nutrient flow, causing the upper canopy to wilt and turn yellow. Viral infections, such as PLRV, often result in a stiff, upright, and stunted plant habit. Affected lower leaves become leathery and rolled upward along the midrib.
Late blight infection causes sunken, purplish-brown areas on the tuber surface with a granular, reddish-brown dry rot underneath the skin. Bacterial soft rot, frequently a secondary invader, turns the tuber tissue into a mushy, cream-to-tan-colored mass that often develops a foul odor. Common scab lesions are superficial or slightly sunken, corky blemishes that range from raised bumps to deep pits, primarily damaging the tuber’s skin.
Environmental Triggers and Disease Spread
Late blight thrives in cool, wet weather. Optimal infection occurs when temperatures are between 15 and 25 degrees Celsius and relative humidity remains above 90% for a minimum of 15 hours. This high moisture enables the pathogen to produce mobile spores that can swim in water films on the leaf surface. Early blight, however, is favored by alternating wet and dry periods with warmer temperatures, with optimal conditions around 28 to 30 degrees Celsius.
Common scab is significantly suppressed in acidic soils with a pH of 5.2 or lower. Conversely, the pathogen Streptomyces scabiei is favored by dry soil conditions during the critical six-week period when new tubers are forming. Careful water management can therefore mitigate the risk of this disease.
Infected seed tubers act as the primary source of initial inoculum for many diseases, including late blight and blackleg. Fungal and oomycete spores are frequently dispersed over short distances by the splash effect of rain hitting infected leaves and soil. Viral diseases, such as PLRV and PVY, are often spread by insect vectors. The green peach aphid is a particularly efficient carrier that transmits the virus between plants.
Integrated Strategies for Disease Control
Disease control relies on an integrated approach, starting with high-quality planting material. Using certified seed potatoes is a foundational practice, as these tubers are inspected and tested to ensure they are virtually free of harmful pathogens, including viruses and bacteria. This minimizes the introduction of primary inoculum into the field, reducing initial disease pressure. Choosing varieties with known genetic resistance or tolerance to specific local diseases also provides a cost-effective line of defense.
Cultural practices create an environment unfavorable for pathogen development and spread. Implementing a long-term crop rotation, ideally three to four years without planting potatoes or other susceptible crops like tomatoes, reduces the survival of soil-borne pathogens. Proper hilling, or mounding soil around the stems, physically protects developing tubers from surface-borne spores washing down from the foliage. Irrigation management is also adjusted to minimize the duration of leaf wetness, which is essential for fungal spore germination and infection.
Chemical controls, such as fungicides and bactericides, are most effective when applied preventively, before symptoms appear. For late blight, applications are often timed using predictive models that track temperature and humidity to forecast when conditions are most favorable for infection. Systemic fungicides are absorbed by the plant for internal protection, while contact fungicides remain on the leaf surface, requiring thorough coverage to be effective.
Tubers must be harvested carefully to minimize wounds, as these entry points allow bacteria and fungi to penetrate the skin. A curing period of about two weeks at temperatures between 10 and 13 degrees Celsius and high humidity promotes wound healing before the tubers are cooled for long-term storage. Maintaining cool storage temperatures, typically 3 to 7 degrees Celsius, along with continuous, high airflow, significantly slows the progression of any remaining disease and prevents the spread of rots.